Download Impacts of native fish stocking on fish within the Murray

Impacts of native fish stocking on
fish within the Murray-Darling Basin
Bronwyn M. Gillanders
Travis S. Elsdon
Andrew R. Munro
Impacts of native fish stocking on fish within the MurrayDarling Basin
Murray-Darling Basin Commission Contract Number MD239
Bronwyn M. Gillanders, Travis S. Elsdon and Andrew R. Munro
February 2006
Copyright page
Impacts of native fish stocking on fish within the Murray-Darling Basin
Bronwyn M. Gillanders1, Travis S. Elsdon2 and Andrew R. Munro1
1
School of Earth and Environmental Sciences
University of Adelaide
SA 5005
Australia
2
Biology Department, MS 50
Woods Hole Oceanographic Institution
Woods Hole, MA 02543
USA
Murray-Darling Basin Commission Contract Number MD239
February 2006
This work is copyright. Graphical and textual information in the work may be stored,
retrieved and reproduced in whole or in part, provided the information is not sold or used for
commercial benefit and its source (Murray-Darling Basin Commission, Impacts of native fish
stocking on fish within the Murray-Darling Basin) is acknowledged. Such reproduction
includes fair dealing for the purpose of private study, research, criticism or review as
permitted under the Copyright Act 1968. Reproduction for other purposes is prohibited
without prior permission of the Murray-Darling Basin Commission.
To the extent permitted by law, the copyright holders (including its employees and
consultants) exclude all liability to any person for any consequences, including but not
limited to all losses, damages, costs, expenses and any other compensation, arising directly or
indirectly from using this report (in part or in whole) and any information or material
contained in it.
The contents of this publication do not purport to represent the position of the MurrayDarling Basin Commission. They are presented to inform discussion for improved
management of the Basin’s natural resources.
Cover photo credits:
John Pogonoski, NSW Department of Primary Industries (Fisheries) (silver perch) and
Andrew R. Munro (all other photos)
2
Table of contents
List of tables...............................................................................................................................4
List of figures.............................................................................................................................4
Acknowledgements....................................................................................................................5
Executive summary....................................................................................................................6
Introduction................................................................................................................................8
A framework for the review.......................................................................................................9
Murray-Darling river system ...................................................................................................10
Stocking programs ...................................................................................................................11
Australian Capital Territory (ACT) .....................................................................................11
New South Wales (NSW) ....................................................................................................12
Queensland (QLD)...............................................................................................................13
South Australia (SA)............................................................................................................14
Victoria (VIC)......................................................................................................................14
Nature of impacts.....................................................................................................................14
Potential impacts......................................................................................................................16
Abundance and behavioural responses to fish stocking ......................................................17
Competition......................................................................................................................17
Methods for determining competition .............................................................................20
Likely effects in Murray-Darling Basin...........................................................................21
Direct impacts of competition..........................................................................................21
Indirect impacts of competition .......................................................................................25
Behavioural changes ........................................................................................................26
Likely effects in Murray-Darling Basin...........................................................................27
Expansion of species range and displacement of wild stocks..........................................28
Likely effects in Murray-Darling Basin...........................................................................29
Predation ..........................................................................................................................29
Methods for determining predation .................................................................................31
Incidental captures ...........................................................................................................32
Genetics................................................................................................................................33
Direct effects....................................................................................................................34
Indirect Effects.................................................................................................................36
Genetic changes in hatcheries..........................................................................................37
Likely effects in the Murray-Darling Basin.....................................................................40
Measuring genetic variation.............................................................................................41
Summaries of genetic structuring in MDB native fish species........................................44
Minimising genetic impacts/knowledge gaps..................................................................49
Disease, parasites, exotic organisms....................................................................................51
Likely effects in the Murray-Darling Basin.....................................................................54
How are impacts of introduced pathogens evaluated (or controlled for/mitigated)?.......55
Ecosystem level effects........................................................................................................57
Exceeding the carrying capacity of an ecosystem ...........................................................57
Likely effects in the Murray-Darling Basin.....................................................................58
Trophic cascades/ecosystem shifts ..................................................................................58
Likely effects in the Murray-Darling Basin.....................................................................58
Extinctions .......................................................................................................................59
Conclusions..............................................................................................................................59
References................................................................................................................................85
3
List of tables
Table 1. Reasons for stocking.................................................................................................63
Table 2. Numbers of four key native species stocked into different catchments of the MDB64
Table 3. Extent of impacts associated with abundance and behavioural responses to stocking
fish, including both spatial and temporal aspects ............................................................65
Table 4. Appropriate experimental designs manipulating both intra- and inter-specific
competition between two fish species..............................................................................66
Table 5. Summary of known dietary information for fish from the Murray-Darling Basin...67
Table 6. Potential genetic effects of stocking hatchery fish on wild populations, including
causes of these effects and whether they are positive (+) or negative (-) ........................69
Table 7. Summary of studies of genetic structuring in native fish species of the MurrayDarling Basin ...................................................................................................................71
Table 8. Summary of potential impacts of commonly stocked fish (which are predatory as
adults) on different types of fish, populations and communities within the MDB..........73
Table 9. Considerations for stock enhancement programs proposed by Blankenship and
Leber (1995).....................................................................................................................75
List of figures
Figure 1. Map of Murray-Darling basin showing major river systems. .................................77
Figure 2. Numbers of native fish stocked into the Murray-Darling Basin .............................78
Figure 3. Inter- and intra-specific competition ......................................................................79
Figure 4. Dietary differences between stocked and wild brown trout in a subarctic lake ......80
Figure 5. Predation of tethered wild and hatchery-reared summer flounder ..........................81
Figure 6. Some direct and indirect genetic effects of releases of exogenous, hatchery-reared
fish on native population..................................................................................................82
Figure 7. Egg-smolt and smolt-adult survival of hatchery and wild spring chinook salmon .83
Figure 8. Piscivore biomass in relation to biomass and production of vertebrate
zooplanktivores, large herbivores and phytoplankton .....................................................84
4
Acknowledgements
We acknowledge Valerie Morris (Adelaide Research and Innovation) for project
management. Stocking information was provided by a number of people from State agencies
including Jason Higham (SA), Mark Lintermans (ACT), Greg Hayes (Victoria), Anita
Wohlsen (Queensland), and Andrew Sanger (NSW). We gratefully acknowledge the help of
Justin Rowntree for entering and manipulating stocking data, tracking down references,
scanning them and entering them into the database, as well as generally assisting with all
aspects of the review. For reviewing a draft of the document, we acknowledge Michael
Hammer (University of Adelaide), Qifeng Ye (SARDI Aquatic Sciences) and Dean Ansell
(MDBC).
5
Executive summary
•
Fish stocking (i.e. hatchery production of fish to a size or stage so that they can be
released into an area) has been practiced worldwide for centuries, but it is only recently
that the environmental and ecological risks have been recognized.
•
Stocking has been largely undertaken in the Murray-Darling Basin (MDB) either to
enhance recreational angling or to aid conservation of a species (e.g. threatened species
recovery plans).
•
Stocking usually occurs because fish numbers have been reduced (e.g. through overfishing or environmental degradation) therefore other issues may need addressing prior to
stocking.
•
The paper reviews the impacts of native fish stocking on fish within the Murray-Darling
Basin, as well as provides a summary of stocking activities within the region. Potential
impacts on abundance and behaviour, genetics, disease and ecosystem level effects are
discussed. We do not address social or economic issues of stocking, but acknowledge
that stocking may be undertaken for these reasons.
•
Changes to abundance and behaviour of fish from stocking primarily arise through
competitive influences between stocked and wild fish. Competitive effects can be either
direct (for food and habitat) or indirect (habitat alteration, behavioural changes, expansion
of species range and displacement of wild stocks). Predation and incidental capture can
also alter fish abundance and behaviour. Generally, there is a lack of research on
abundance and behavioural responses to fish stocking on native Australian species.
•
Genetic impacts of hatcheries and hatchery fish on wild populations have received a lot of
attention, but the literature is primarily theoretical in nature. Genetic effects can be direct
(e.g. hybridisation, introgression, expression of deleterious effects) and indirect (e.g.
altered selection regimes, reduction in population size caused by predation, competition
and diseases). Artificial propagation of fish also invariably alters the genetics of captivebred populations. Very little is known about the genetic structure of native fish
populations in the MDB but where it has been investigated, significant and complex
population structures exist.
•
Impacts of introducing diseases, parasites and exotic organisms unintentionally when
stocking fish have historically received little attention. The unintentional introduction of
a pathogen with the stocking of native species will most likely have a negative impact on
6
wild populations. Several examples of the spread of pathogens through stocking exist for
the MDB (e.g. epizootic haematopoietic necrosis virus, protozoa).
•
Ecosystem alteration from stocking fishes is extremely difficult to demonstrate, and has
mostly been attributed to introduced species rather than native species.
•
Targeted research on species within the MDB is needed for all potential impacts.
•
Good baseline data (although this is likely to be already altered) and good monitoring
programs are essential. A risk assessment of potential impacts is also necessary.
Stocking will likely result in changes to the system, but it is important to ensure that the
benefits outweigh the costs and ensure that the goals of the stocking program are met.
•
Given the continued increase in stocking of hatchery-reared fish and the potential for
interactions with wild fish, it is essential to take a responsible approach and to monitor
and experimentally evaluate any stocking program. Only with such an approach will the
success of stocking programs be evaluated and the risks mitigated.
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Introduction
Freshwater systems have been exploited for a variety of reasons including water abstraction
for domestic, industrial and agricultural use, effluent disposal, fisheries and aquaculture, and
navigation (Cowx & van Zyll de Jong 2004). Over the past 100 years many of these
activities have interrupted, degraded or destroyed the functioning of freshwater ecosystems
and fish stocks have subsequently declined (Collares-Pereira & Cowx 2004, Cowx & van
Zyll de Jong 2004). Thus, there has been a concerted effort to restore and rehabilitate
freshwater ecosystems. A widely practiced management option is stock enhancement
(Welcomme & Bartley 1998).
Fish stocking has been practiced worldwide for centuries, but only recently have
environmental and ecological risks been recognised (Utter 1998, Blaxter 2000, Molony et al.
2003). Stocking has largely been undertaken to enhance recreational angling or to aid
conservation of a species (but see Table 1 for list of reasons). Regardless of the objectives,
appropriate and effective management of stocking activities is required because a number of
risks, which are not always well understood, are possible. In addition, little research has
specifically addressed many of the ecological risks. However, it has been noted that stocking
(and the translocation) of fishes is a potent factor contributing to species extinction, declining
genetic diversity and homogenization of plant and animal assemblages (Levin et al. 2001,
Harris 2003). Limited research has been done to fully evaluate efficacy of stocking native
fish for conservation.
Although many people have expressed concern about the impacts of stocking, it has widely
been used to address declining fish abundance and much less emphasis has been placed on
the initial causes of decline (White et al. 1995). Despite thoughts that stocking can increase
population sizes of wild fish, fish produced in hatcheries often exhibit poor post-stocking
survival and reproduction due to morphological, physiological and behavioural problems
(White et al. 1995). In addition, stocking often occurs in conjunction with high levels of
exploitation by fishing. When population numbers are decreased by exploitation, habitat
degradation or other stressors the risk of damage to the recipient wild population is likely to
be greatest (Evans & Willox 1991).
Despite many years of native fish stocking, its overall success is still in question (Hilborn
1998). Historically, the fate of stocked fishes has rarely been assessed and early evaluations
8
were fairly haphazard (Wahl et al. 1995, Welcomme & Bartley 1998). Most studies
determine the success of a stocking program by assessing either the yield or percentage of
stocked fish in the total catch (Evans & Willox 1991, Vollestad & Hesthagen 2001). If
density-dependent mechanisms occur, these criteria may not be acceptable for measuring
success because introducing more fish into a system may lead to negative impacts on wild
conspecifics (e.g. reduced growth, increased mortality) (Vollestad & Hesthagen 2001). Even
when there are so-called positive effects of stocking, genetic changes, which are often more
difficult to document and monitor than demographic or ecological effects (Allendorf 1991),
may have long-term negative effects (Hindar et al. 1991, Utter 1998).
In Australia and indeed worldwide, the majority of research has focused on impacts from
salmonids, largely because salmonids have a longer history of stocking than native species.
Salmonids are an introduced species to Australia and comprise the majority of fish stocked in
the Murray-Darling Basin (MDB; see Fig. 1). We focus the current review on stocking of
native species, but will use salmonid literature from other areas where it is pertinent, as
salmonids are also widely stocked throughout their native range (North America, Europe,
parts of northern Asia). The overall objective of this paper is to review the impacts of native
fish stocking on fish within the Murray-Darling Basin. This review aims to synthesise major
findings, point out weaknesses of some paradigms and highlight areas for future research.
We provide a brief framework for the review, followed by information on the MurrayDarling river system and stocking programs within this region. The nature of impacts is then
discussed followed by potential impacts on abundance and behaviour, genetics, disease and
ecosystem level effects.
A framework for the review
Within research papers methods were evaluated to determine whether they were appropriate
or inappropriate and whether they were correctly or incorrectly applied. It was important to
distinguish pattern from processes. A process is the cause of the observed pattern. Most
ecological research begins with the description of patterns or observations of some kind of
relationship (Andrew & Mapstone 1987, Underwood 1990, 1997). The most comprehensive
descriptions are at a range of spatial and temporal scales.
Besides observational studies that investigate patterns and correlations, a number of
experimental studies have also investigated impacts of stocking fish. Life history and
9
population models can be used to compare the effectiveness of stocking and other
management decisions (e.g. improving habitat, decreasing fishing pressure) if performance
criteria are set (e.g. Heppell and Crowder 1998). Particular parameters can be altered while
others are held constant and the effects of changing a parameter in the model can be
investigated through sensitivity analyses. Besides determining the effectiveness of different
management decisions, models can also be used to explore potential impacts between stocked
and wild fish.
Research papers were evaluated using the following criteria: a) whether the research was
relevant to native fish stocking; b) if the design was confounded; c) if the analysis and
interpretation of data were correct and the conclusions valid. It was sometimes difficult to
evaluate these criteria because there was insufficient information. Studies that did not meet
these criteria but provided useful information to generate hypotheses were also considered.
Murray-Darling river system
The Murray-Darling is the largest river system in Australia encompassing 1.073 million km2
and is among the largest in the world (Murray River 2560 km long, Darling River 2740 km
long) (Walker 1992) (Fig. 1). In addition, the system includes 20 major rivers and distinct
sub-sections such as the Eastern Mount Lofty Ranges joining the Lower Murray. The south
and east are bounded by the Eastern Highlands, whereas vast expanses of arid land are found
to the north and west (Walker 1992). Annual discharge is comparatively small, but is
variable even under intensive flow regulation (Walker 1992). The average flow reaching the
river mouth is approximately 75% less than under natural conditions (Maheshwari et al.
1995).
River habitats and native fish populations have been affected by a number of processes.
Regulation and abstraction has changed the flow regime of the river system (e.g. altered
seasonality of flow and natural flooding and drying cycles) and waterways are subject to
waste disposal from townships, irrigation runoff and increasing salt loads. Many fish are
restricted in movement by man-made barriers (dams, weirs, diversions), which were
constructed from around the 1920’s. Habitat loss has also occurred through direct removal of
river snags (logs, branches, fallen trees), which were removed to improve navigation. Native
species are harvested recreationally and commercially by humans to varying degrees, and are
forced to interact with at least eleven alien fish species. Thus, in the last 50 years,
10
populations of native fishes in the MDB have suffered serious declines in distribution and
abundance (Cadwallader 1978, 1981, Cadwallader & Gooley 1984, Brumley 1987).
Native fish populations are now estimated at 10% of pre-European settlement levels (MurrayDarling Basin Commission 2005). Eight of the 35+ native fish species are listed nationally as
vulnerable or endangered; numerous others are listed at the State level (Morris et al. 2001).
There have also been localised extinctions of some native fish species (e.g. trout cod,
Maccullochella macquariensis, has been extirpated from ACT and South Australia, and
became extinct in NSW before restocking occurred), and most commercial fisheries no
longer operate. With such heavy declines, there is a perceived need for stocking to restore
native fish populations.
Stocking programs
Stocking in the Murray-Darling Basin has largely been done either to enhance recreational
angling or to aid conservation of a species (e.g. threatened species recovery plans). All
current stocking programs within the Basin rely on the hatchery production of stock. The
majority of fish are stocked as fingerlings, although there are some reports of stocking older
fish (e.g. 1 year Murray cod, Maccullochella peelii peelii).
Programs for stocking native fish occur in all states and territories within the MDB, with the
exception of South Australia (i.e. Queensland, New South Wales, Victoria and Australian
Capital Territory), and all relevant fisheries authorities have different policies and programs
for stocking native fish (see below). Around 30 hatcheries in NSW, Queensland and Victoria
produce between 5 and 8 million fish annually. Six major native species (golden perch,
Macquaria ambigua, Murray cod, silver perch, Bidyanus bidyanus, trout cod, freshwater
catfish, Tandanus tandanus, Macquarie perch, Macquaria australiasica) have been stocked,
but it should be noted that other translocations, illegal introductions and some conservation
stockings also occur. Over 65% of all stocked fish are golden perch (Table 2, Fig. 2).
Significant numbers of silver perch (18% of all stocked fish) and Murray cod (13% of all
stocked fish) have also been stocked. Stocking is an on-going process in the MDB.
Australian Capital Territory (ACT)
Stocking within the ACT is currently governed by Environment ACT’s fish stocking plan
(2001-2005) released in May 2000. A formal fish stocking policy for the ACT was first
11
developed in 1996, with the policy reviewed every five years (Lintermans, Environment
ACT, pers. comm.). All stocking for recreational purposes occurs into lakes within the upper
Murrumbidgee catchment, with a single conservation stocking program (trout cod) also
releasing fish into the Murrumbidgee River itself. Six native species (Murray cod, golden
perch, silver perch, freshwater catfish, and trout cod) have been stocked. Stocking of
freshwater catfish was discontinued as the species is not endemic to the Canberra region, and
silver perch have not been stocked since 1999-2000. Historically, native fish stocking
commenced in 1972-73 when around 21,500 golden perch were stocked into Lake Burley
Griffin. Golden perch together with Murray cod and trout cod continue to be stocked
regularly (Lintermans, Environment ACT, pers. comm.).
New South Wales (NSW)
The original goal of stock enhancement programs in NSW was to establish fish populations
and create recreational fisheries in areas where none existed (e.g. impoundments) (Henry
1997). Stock enhancement programs have also been done to improve fishery resources in
rivers that have been subject to environmental degradation or overfishing (Henry 1997). A
number of native fish have been introduced into NSW waters outside of their natural range
(e.g. Murray cod, trout cod, Macquarie perch, Macquaria australasica, golden perch, silver
perch and freshwater catfish). These stockings (translocations) mostly occurred prior to the
1940’s and were sponsored by NSW Fisheries or Acclimatisation societies. The NSW
government took over management of stocking around 1960. Hatchery-bred golden perch
and silver perch have been stocked following the development of production techniques by
NSW Fisheries at its Narrandera research facility in the late 1970’s. These fish were
produced to stock farm dams and funded by Treasury. Private hatcheries are now also
involved in producing these species, as well as Murray cod and freshwater catfish.
Stocking of eggs, fry or fish into NSW requires a permit under the Fisheries Management Act
1994. Stocking of native species is permitted into impoundments within their natural range,
and stocking of fish into rivers within the species natural range is considered on a case by
case basis. Funds from the Recreational Freshwater Fishing Trust are used to support local
stocking groups to stock native recreational species into NSW waterways. A dollar-for-dollar
scheme managed by NSW DPI involves matching funding to those organisations (e.g.
angling clubs, local councils) that are raising money to purchase fish from private hatcheries
to stock into public waters. Such funding is available for golden perch and Murray cod in the
12
NSW section of the MDB, but the appropriate broodstock for the waterway to be stocked
must be used to produce fry.
Trout cod currently have a conservation-stocking program implemented by NSW DPI, which
has been an on-going initiative since 1986, but this is currently being evaluated to determine
the effectiveness of past stocking activities (Murray-Darling Basin Commission 2005).
Conservation stocking of purple-spotted gudgeons in the Murrumbidgee catchment are also
undertaken by NSW DPI. Although large numbers of silver perch are stocked, there has been
a failure to detect an increase in abundance in the Murrumbidgee River and therefore these
stockings may not provide a viable recreational fishery (Gilligan 2005).
Queensland (QLD)
Within Queensland, the Recreational Fishing Enhancement Program was initiated in 1986
with the aim to develop the sport fisheries potential of Queensland’s impounded waters
(Wohlsen, Department of Primary Industries & Fisheries Queensland, pers. comm.). Prior to
this time, fish stocking was largely research driven. Stocking is undertaken by approximately
70 stocking groups who regularly stock virtually every suitable impoundment, as well as
some river systems. Stocking is only permitted by these community based stocking groups if
a general fisheries permit for stocking Crown waters has been obtained. Stocking groups
must prepare a 5-year management plan with their DPI and Fisheries regional fisheries
liaison officer. Stocking permits are then issued for 5 years indicating maximum numbers of
fish to be stocked into each water body per annum. A stakeholder based stocking and
translocation subcommittee of the Freshwater Management Advisory Committee reviews any
applications to stock previously unstocked waters. Stocking of artificially created waters on
private lands (e.g. farm dams) does not require a permit if it meets certain requirements of the
Fisheries Management Plan 1999 (e.g. fish species occurs naturally in river basin or species is
stocked regularly). Some dams are also included in the Stocked Impoundment Permit
Scheme (introduced in July 2000) where a permit is required to fish certain stocked dams. A
minimum of 75% of revenue raised from the scheme then goes back to stocking groups to
buy fingerlings to restock these dams. Only three species (golden perch, silver perch and
Murray cod) have been stocked into seven catchments of the QLD section of the MDB since
about 1984.
13
South Australia (SA)
South Australia does not currently have a formal written policy for the management of fish
stocking programs, although it is an offence under the South Australian Fisheries Act 1982
(currently being revised) to release any cultured fish (including into farm dams) without an
exemption. Although native fish cannot be stocked legally in SA, a number of illegal
introductions are likely to have occurred (Higham, Primary Industries and Resources South
Australia, pers. comm.). Downstream immigration of stocked individuals from other parts of
the MDB may also occur.
Victoria (VIC)
Victoria has developed guidelines for the translocation of live aquatic organisms (Department
of Primary Industries 2003) where translocation refers to the deliberate human-assisted
movement of a live aquatic organism. Proposals for stocking undergo a two-stage evaluation
process including an initial screening stage followed by a risk assessment. The risk
assessment investigates the pest potential, disease status, potential to introduce parasites and
diseases and possibility of affecting biodiversity. Protocols for the translocation of fish in
Victorian inland public waters have also been drafted (final draft available) where the
stocking is repetitive in nature or has similar characteristics. Five species (golden perch,
Macquarie perch, Murray cod, silver perch and trout cod) have been stocked into the MDB
section of Victoria. Most fish are stocked as fingerlings (about 1 g each), although an
increasing number of Murray cod are stocked as yearlings (about 150 g each). Stocking trials
in the Goulburn and Loddon rivers are evaluating whether there is any benefit in terms of
return to the angler from releasing yearling Murray cod as opposed to fingerlings. Results
from these trials should be available shortly (Fulton, Department of Primary Industries
Victoria, pers. comm.).
Nature of impacts
It is useful to consider stocking of native fish and the responses of organisms to stocking as
impacts. An impact is defined here as the alteration in the ecology, behaviour, or genetics of
some members of a population and/or assemblage caused by a perturbation or disturbance.
Although many people perceive the term ‘impact’ as negative, it can also be positive (i.e. an
impact is some form of actual change in the unit of interest). For example, stocking may
enhance abundance of populations and thereby aid the recovery of threatened species or it
may increase the capital value of a fishery because more and larger fish are caught. It is also
14
likely that there could be a range of impacts, both positive and negative, from a single action
or stocking.
Unless declines in wild populations can be solely explained by over harvest, stocking could
just mask overall problems leading to population decline. As a result, wild populations will
continue to decline in abundance and face the threat of being numerically replaced by stocked
fish if stocked fish are better adapted to the changed conditions (discussed in detail further
on). A key component to minimising the impacts of stocked fish is, therefore, to rectify the
cause of initial population decline.
The persistence of a perturbation can vary greatly, although two kinds of perturbation are
recognised (Bender et al. 1984). In the first type, the perturbation is made, the population or
assemblage responds and returns to pre-impact conditions quickly (pulse), whereas in the
second type the alteration or potential impact is maintained (press). The pulse perturbation is
a relatively instantaneous alteration, after which the system recovers once the potential
impact has ceased. The press perturbation, by contrast is a sustained alteration that may lead
to complete elimination of some species. Both the nature of the impact and the response of
the organisms can be either press or pulse (Glasby & Underwood 1996). Thus, a pulse
disturbance (e.g. major stocking of fish into one area after which they rapidly disperse) may
show either a short-term (pulse), or a continuous (press) response depending on how quickly
stocked fish move away from the stocking site and the level of survival. A delay in response
may also be observed, for example, genetic effects may not be observed immediately
(Allendorf 1991).
Impacts of stocking native fish may occur at the stocking site and at adjacent aquatic habitats,
therefore impacts need to be considered at both the local and larger spatial scales. The rate
and spatial extent of fish movement from a stocking site may be influenced by many factors
including dispersal ability/rates and available dispersal routes, demographic changes in the
stocked population and demographic pressures in potential source populations (Adams et al.
2001). For lotic organisms, dispersal is necessarily linear (rather than equal in all directions),
but the rate and frequency may differ in each direction and at different locations along a river
(Adams et al. 2001). For example, if stocking occurs in the headwaters of a stream, dispersal
in a downstream direction is more likely.
15
At present, stocking records do not indicate the spatial distribution of stocking other than the
location or general area of release. In general, fish may be stocked as one batch (spotplanted) or distributed evenly along the shoreline (scatter-planted). Stocking groups may,
however, try and ensure an even distribution of stocked fish throughout an area to lessen
predation by other fish and birds (NSW Fisheries 2003). Some studies have noted that the
overall recapture rate of scatter-planted fish was higher than spot-planted fish, but others have
found no significant difference between the two methods of stocking suggesting that the
results may depend on locality, stocking density and time (Vollestad & Hesthagen 2001).
The magnitudes of ecological, behavioural and genetic risks may depend on the spatial and
temporal overlap of stocked and wild fish (McMichael & Pearsons 2001). It has generally
been assumed that risks are low upstream of sites where fish are released (particularly where
the stocked species is anadromous), but several studies have noted significant upstream
movement suggesting that upstream sites are not immune to risks associated with stocking
(McMichael & Pearsons 2001).
Impacts are likely to occur at the species (e.g. increased intra-specific competition due to
increased abundance as caused by stocking fish), population, community and ecosystem
levels, and represent ecological, genetic and behavioural responses (Aprahamian et al. 2003,
Molony et al. 2003, Nickum et al. 2004, van Zyll de Jong et al. 2004). Because some species
of fish have ontogenetic dietary shifts, a myriad of possible interactions both within and
between species can lead to a number of community interactions (Heppell & Crowder 1998).
Potential impacts
Four key mechanisms exist by which stocking may affect the ecology of a system:
(1) Stocking of fish may give rise to competition and/or predation;
(2) Stocking of fish may lead to a variety of genetic-related impacts;
(3) Stocking of fish may also lead to the unintentional introduction of pathogens or other
organisms, which could negatively impact wild populations;
(4) A number of ecosystem level effects, including exceeding the carrying capacity of the
system, trophic cascades, and extinctions, are also possible.
16
Abundance and behavioural responses to fish stocking
Stocking of fish to habitats, such as those in the Murray-Darling Basin, can have wide
reaching effects on the abundance and behaviour of both stocked and wild fish populations.
Changes in abundance and behaviour primarily arise through competitive influences between
stocked and wild fish. Changes due to competition can be either direct (for food and
habitats) or indirect (habitat alteration, behavioural changes, expansion of species range and
displacement of wild stocks). In addition, several factors not directly associated with
competition, such as predation and incidental capture, can alter fish abundance and
behaviour in both positive and negative ways. Regardless of the mechanism, all of these
factors can cause impacts to both intentionally stocked and wild fish populations (see Table
3).
Generally, there is a lack of research on abundance and behavioural responses to stocking of
native Australian species, particularly from the MDB. As such, examples of potential
impacts of fish stocking will be generic in nature, often on salmonids, for which extensive
research has been done. Likely impacts in the MDB will, however, be specific to species and
habitats within Australia.
Competition
Competitive interactions between fish and their resulting impacts have overarching negative
effects on fish abundance and behaviour. Competition is defined as an interaction in which
one fish uses a resource that could be otherwise used by another (Begon et al. 1996).
Competition can be broadly segregated into two mechanistic categories: exploitation and
interference. Furthermore, competition can occur between fish of the same species (intraspecific competition), or between fish of differing species (inter-specific competition). The
concepts of competition are discussed in depth in the general ecological literature (Krebs
1978, Begon et al. 1996, Matthews 1998), but largely underpin the ecological mechanisms
behind the impacts introduced fish can have on native fish populations and vice versa and
therefore competition theory will be briefly reviewed here.
Exploitation competition refers to competitive exclusion of one individual from a shared
resource by another more dominant individual; however, individual fish never directly
interact (Begon et al. 1996). An example of exploitation competition would include two river
fish that consume the same algae, or two fish that both require limited river sand beds for
17
grazing on freshwater invertebrates. Interference competition occurs when individuals
exclude each other from shared resources through direct interactions (see Peery et al. 2004).
In this case, one individual actively and directly prevents another from gaining access to a
resource. An example of interference competition would be a fish that actively defends a
territory that could be used by another fish of the same or different species. A key element of
interference competition is that one individual will have a disproportionate share of a
resource. Interference competition can also evolve from exploitation competition, if
exploitation is severe and causes one species to evolve defences (Begon et al. 1996).
Given that species-resource requirements differ, a distinction is also required between intraand inter-specific competition. In a freshwater fish stocking scenario, intra-specific
competition would occur between the stocked fish and wild members of the same species.
Stocked freshwater fish can also compete with wild members of other species. Thus, four
possible combinations of competition occur: (i) exploitation – intra-specific, (ii) exploitation
– inter-specific, (iii) interference – intra-specific, and (iv) interference – inter-specific (Mills
et al. 2004). The stocking of freshwater fish is likely to evoke competition characteristic of i,
ii, and iv. It is possible, however, that individuals of the same species (stocked and wild)
could show interference competition (iii), given that hatchery fish may have adapted
interference competition from exploitation competition during the hatchery rearing period
(i.e. stocking densities in hatcheries may provoke aggressive behaviour over habitat and food
resources) (Olla et al. 1998). The premise of both exploitation and interference, and intraspecific and inter-specific competition will continue when examining aspects of food and
habitat, habitat alteration, behavioural changes, and expansion of species ranges and
displacement of wild stocks.
Competition generally evokes negative effects to either stocked fish or wild fish populations.
Several studies have reported strong intra- and inter-specific competition between stocked
and wild fish (e.g. Lachance & Magnan 1990b, Fjellheim et al. 1995). Competition will not
always result in negative impacts on wild fish, with several studies detailing detrimental
impacts of competition occurring to the stocked fish (e.g. Fjellheim et al. 1995). Generally,
studies that report competitive interactions among and between species do so through
changes in factors such as fish growth, survivorship and behavioural responses. Most studies
determine competition based on observational patterns, with few experimental tests of
competition having been done (but see Lachance & Magnan 1990b for an example). It should
18
be noted that observations cannot describe the existence or extent of competition (Underwood
1986); they merely highlight that competition might occur. Describing and interpreting
competition between stocked and wild fish requires manipulative experiments. Although we
have used literature that describes competitive interactions based on observations, largely
because of a lack of experimental evidence, we do place a greater emphasis on results gained
from experimental manipulations.
Competitive influences of stocked fish on wild populations are largely dependent on fish
density, relative to resource availability. Density dependent effects on competition are rarely
examined, even though density dependent effects on factors, such as fish growth, mortality,
and movements do occur (Le Cren 1973, Weiss & Schmutz 1999a, Imre et al. 2005). In
experimental investigations of competition between stocked and wild fish, we could find no
investigations that examined the impacts of density dependence on either intra- or interspecific effects. In addition, experiments that aim to determine competition between species
often do not control for differences in density that co-vary with treatments (Underwood 1986,
Weber & Fausch 2003), thus providing an incomplete view of density versus competitive
effects. Similarly, the effect of resource availability on competition has been investigated in a
number of field situations. A classical view of competition is that interactions will be greatest
under limiting resources. Thus, investigations should manipulate both density of fish and
resource availability to determine the relative effects of these on competition.
The majority of studies detailing impacts of stocked fish on wild fish populations have
examined fish survivorship or growth, as these data are presumably easier to collect
compared to observations on food and habitat resource use and behavioural responses. In a
review by Einum and Fleming (2001), fifteen of sixteen studies reported reduced
survivorship of stocked fish, compared to their wild counterparts. Mortality of stocked fish is
generally considered to be high (up to 99% in 11 months) compared to wild stocks examined
over the same time frame (Fjellheim et al. 1995). Growth of stocked and wild populations
does not seem to follow the same trends as survivorship, with studies detecting mixed results
between stocked and wild fish growth: Stocked < Wild (Weiss & Schmutz 1999b), Stocked =
Wild (Levings et al. 1986, Kellison et al. 2003), and Stocked > Wild (Weiss & Schmutz
1999a). Cases involving differences in survivorship and growth of fish are largely attributable
to competition; however, the competitive mechanism and resources that they are competing
over are rarely described.
19
An example of a typical experiment that examined competition between stocked and wild
fish comes from Lachance and Magnan (1990b), who stocked brook trout, into lakes in the
presence of intra- and inter-specific competition (native brook trout and white suckers,
Catostomous commersoni). Stocked brook trout were of three genetic origins; domestic (50
years of hatchery rearing), hybrid (male wild, female domestic from 50 years of hatchery
rearing), and wild (F1 or first generation of wild parents), thus providing an additional test of
intra-specific competition between introduced fish. In addition, all stocked fish were finclipped, allowing distinction between stocked and wild fish. The two responses examined
were fish weight and percentage recovery (change in stocked fish abundance: number finclipped fish captured/total number of fish captured × 100), which were measured two years
after the introductions. Domesticated brook trout had greater weights compared to hybrid and
wild stocked brook trout, yet, wild brook trout had greater recovery (see Fig. 3). The pattern
of weights and recoveries (e.g. domestic > hybrid > wild) were consistent between lakes
possessing intra-specific competition, and intra- and inter-specific competition. In lakes with
inter-specific competition, however, there was a decline in both weight and recoveries, such
that the white suckers negatively impacted stocked brook trout, regardless of genetic origin
(Fig. 3). The present study did, however, lack all appropriate treatments that manipulated
density of fish, which are needed to infer competition (see Methods for determining
competition).
Methods for determining competition
Determining competition between species requires an experimental design in which both
competitors are manipulated (intra- and/or inter-specific), as well as fish density.
Manipulating density is vital for determining competition, because competitive interactions
between species are likely to occur during resource limitation, as discussed earlier. Examples
of designing competition experiments can be found in general ecological literature
(Underwood 1986). A more specific fish example is that of Mills et al. (2004). It should be
noted, however, that many competition experiments done on fish do not manipulate density.
Experiments can be modified depending on the response variable in question (e.g. food and
habitat resource use, behaviour, survivorship, or growth), but in general, a suitable design
would involve manipulating stocked fish into habitats (enclosures) in the presence and
absence of competitors, at high and low fish densities (Table 4). Ideal habitats for such
20
manipulations would include small and isolated lakes or dams (as used by Lachance &
Magnan 1990b), caged areas of streams and rivers (as used by Weiss & Schmutz 1999a), or
laboratory tanks for examining behavioural and habitat choice responses (as used by Mills et
al. 2004). Establishing four replicated treatments of competitors would allow both intra- and
inter-specific competition to be determined. These treatments would be: (i) fish stocked in the
absence of competitors (control), (ii) fish stocked in the presence of wild fish of the same
species (intra-specific), (iii) fish stocked in the presence of wild fish of a different species
(inter-specific), and (iv) fish stocked in the presence of both intra- and inter-specific
competitors. Coupled to manipulations of predators, should be the manipulation of fish
density, to provide resource limited and enriched treatments.
Likely effects in Murray-Darling Basin
Specific effects of competition in the Murray-Darling Basin are dealt with under specific
impacts (see below), however, two general factors that influence whether competition will
occur between stocked and wild fish will be discussed here. The impacts of competitive
influences in the MDB habitats of rivers, lakes, wetlands, and tributaries will largely depend
on the density of fish and the abundance of resources, both of which can either be directly
influenced by stocking (increasing densities reduce resources), or in the case of resource
availability, may be naturally limited within habitats. As discussed earlier, competition will
not always occur when resources, such as habitat and food, are plentiful. Several studies have
shown that a strong correlation exists between the abundance of resources and the success of
stocking programs, both in terms of impacts to wild fish and increasing abundance or
survivorship of stocked fish (Stockner & Macisaac 1996). It is, therefore, important to assess
the ‘carrying capacity’ of habitats within the MDB, in terms of habitat availability and food
resources, as these will likely influence impacts of fish stocking (see Exceeding the carrying
capacity of an ecosystem). In resource-depleted areas within the MDB, such as land locked
lakes, competitive interactions between stocked and wild fish may exceed those of resourceenriched areas.
Direct impacts of competition
Food resources – The impact of stocking on food resources is likely to depend on fish
density, available resources and the adaptive abilities of stocked fish. The stocking of fish
will cause an increase in abundance within a given habitat, which may lead to increased intra-
21
and inter-specific competition for food. Outcomes of increased competition include reduced
growth, changes in resource use, displacement of stocks, and in extreme cases, starvation.
Possible impacts of stocking on food resources will largely depend on the food resources
used by stocked and wild fish. Kahilainen and Lehtonen (2001) examined food consumption
and dietary composition between wild and stocked brown trout, Salmo trutta, that had been
collected from similar places in Finland lakes. Newly stocked fish had reduced volume of gut
contents, compared to wild stocks of the same age. In addition, food resource use differed
between stocked and wild brown trout, although this effect was seasonal in nature. In
summer, the diet of stocked brown trout (1st year introductions) consisted of approximately
65% aquatic invertebrates and insects with only 20% of their diet consisting of whitefish,
whereas wild trout diets consisted of 45% whitefish, Coregonus sp., and only 20% aquatic
invertebrates and insects. Differences in diet between stocked and wild brown trout did,
however, change with season, and diets became similar with increased time after stocking (1,
2, and 3 years post stocking) (Fig. 4). Food resource differences have also been reported
between other populations of stocked and wild brown trout (e.g. Fjellheim et al. 1995), and
other species of fish (e.g. Smirnov et al. 1994).
Contrary to the findings of dietary differences, studies have detected no differences in diet
between stocked and wild fish, including summer flounder, Paralichthys dentatus (Kellison
et al. 2003), and brook trout (Lachance & Magnan 1990a). In cases where dietary differences
between stocked and wild fish do not occur, interspecific impacts of stocking on food
resources may be limited, but only where food or prey resources are plentiful (Arnekleiv &
Raddum 2001). Mixed results between studies may be attributed to rearing conditions in
hatcheries (i.e. food ration, predation, competition), or release conditions of fish in
environments (i.e. dietary differences between hatchery and release sites). In artificial
hatchery conditions, stocked fish may be unable to adapt to natural food resources consumed
by wild fish (Smirnov et al. 1994). As a result, stocked fish are often considered inefficient
feeders that are unable to adapt to the consumption of wild prey (Olla et al. 1998). If fish are
raised in fertilised ponds then they may be already feeding on natural food resources prior to
stocking and therefore such an effect may be minimal.
The degree to which stocked fish impact food resources, will to some extent, also depend on
the size and age classes of stocked fish, and the numbers of fish introduced. Stocking fish at a
22
larger size may result in a disproportionate ability of those fish to out compete smaller wild
fish [Large sp A (stocked yearling) > Small sp B (wild yearling), but Small sp A (stocked
fingerling) < Small sp B (wild fingerling)]. Similarly, if an area to be stocked is food
resource limited, the impact of stocked fish on the consumption of food resources is likely to
be large. Although little quantitative data support the different impacts of stocked fish due to
food resource availability, in Canadian lakes, a relationship between fish productivity and
food availability has been detected. Stockner and Macisaac (1996) describe how the addition
of nutrients to Canadian lakes, increased plankton biomass by between 50 and 60%, and
resulted in increased growth (by > 60%) and survivorship of sockeye salmon, Oncorhynchus
nerka. Thus, it is conceivable that fish stocked in high densities have the potential to limit
available food resources, and reduce the growth and survivorship of both stocked and wild
fish via competitive interactions.
Likely effects in Murray-Darling Basin – Likely impacts will largely depend on the
availability of food resources at the stocking location relative to fish biomass, and the
competitive nature of both the stocked and wild fish. A key to understanding how stocking
will impact wild fish is, therefore, to outline the trophic status of stocking sites. Locations
with large food resources, such as wetlands, undercut banks or snags, may support a greater
number of stocked fish before reaching a threshold abundance, above which competition and
negative impacts of stocking will occur. As the availability of food resources varies both
spatially and temporally, as do the resource requirements of wild fish, it is conceivable that
fish stocking may negatively impact several species either collectively or in succession
(Table 3). The initial use of different food resources by stocked fish is likely to lessen pulse
impacts and competition. Stocked and wild fish that require similar food resources will,
however, directly compete with one another. Competition for similar food resources may lead
to press effects of starvation, diet switching, and displacement of stocks. The degree to which
prey switching could occur is relatively unknown, however, Raborn et al. (2002) indicate that
removal of stocked fish may increase prey items consumed by wild fishes, causing an
increase in biomass of between 3 to 12%.
Of particular relevance to the MDB are species that are susceptible to declines in abundance,
possibly due to restricted food and or habitat availability (i.e. IUCN listed species). Little is
known about the food requirements of a majority of MDB fish (but see Table 5 and King
2005). Should competition for food exist between species, a stocked species that is a
23
dominant and generalist feeder, such as Murray cod, may out-compete species with more
restricted food requirements. Such competition may result in a bottleneck effect by reducing
fish abundance, which may affect recruitment success of fish stocks in consecutive cohorts.
Thus, the introduction of large numbers of stocked species to one particular site may affect
abundances of several fish species, with these effects likely to have long lasting press impacts
over several years.
Habitat resources – The impact of fish stocking on habitat resources is likely to depend on
the abundance of stocked and wild fish, and extent of suitable habitat. The addition of
stocked fish can result in competition for space and habitat. Competition will be extreme if
habitat is a limited resource and stocked and wild fish have similar habitat requirements.
Competition for habitats may be either aggressive (interference) or passive (exploitation);
however, this largely depends on the territorial nature of species.
The extent and impact of stocking on habitats is difficult to assess, as few quantitative data
sets and experiments examining habitat use of stocked fish have been done. Of the studies
that have examined habitat resource use by fish, Fjellheim et al. (1995) reported restricted
movements of stocked brown trout, with 30.3% of stocked fish remaining within 30 meters of
the release site, and the greatest movements being 100 meters. Such restricted movements of
stocked fish may result in localized competition for habitat, and localized displacement of
wild populations.
Stocked and wild fish may interact if habitat use overlaps. Several studies have described
habitat overlaps between stocked and wild fish, particularly for salmonids (Lachance &
Magnan 1990a, Kahilainen & Lehtonen 2001, Peery et al. 2004). However, differences in
habitat use have also been reported for stocked and wild brown trout (Hesthagen et al. 1995).
In an experimental investigation, habitat use by stocked chinook salmon, Oncorhynchus
tshawytscha, overlapped that of wild fish within streams, which was consistent regardless of
the density of stocked fish (Peery et al. 2004). Competition was, however, detected between
wild and stocked chinook salmon when water temperatures were high (averaged 8.7 ºC vs 5
ºC), during which time wild salmon held preferential stream positions. The apparent and
conceivable interactions between variables, such as stocking size, density, season, and
available habitat, require further investigations before strong conclusions on the negative
impacts of stocking fish on habitat resources are inferred.
24
Likely effects in Murray-Darling Basin – Likely impacts will depend on several factors,
including habitat availability, number and size of stocked fish, and populations of wild fish.
In general, stocking of fish will have either no effect or detrimental effects on habitat
resources used by wild fish stocks. Detrimental effects may arise from a lack of habitat
resources, where stocked and wild fish have habitat requirements that overlap. The overlap of
habitat resources will be most apparent for species that have specific habitat requirements,
such as large fish requiring snags, most of which have been removed for navigation
requirements (Cadwallader 1978). Competition is likely to be both intra- and inter-specific
and territorial dominance may increase in species that require habitat for protection or
breeding. The impacts of stocking on habitat resources are likely to result in both pulse and
press effects. After initial introductions of stocked fish, a short-term pulse of competition
over habitat resources is likely, with this competition being localized. As stocked fish often
display high initial mortality (Fjellheim et al. 1995, Buckmeier et al. 2005), impacts may not
manifest over long periods of time, however, surviving fish are likely to have a long lasting
press impact on habitat resources (Table 3).
The impact of stocking is likely to depend on stocking methods, such as timing and stocking
abundances, and the extent of any impact will be dependent on the species involved. Stocked
fish, such as golden perch, that move large distances (up to 290 km) (O'Conner et al. 2005)
are unlikely to result in large impacts on wild fish, however, fish displacement may occur.
Wild fish with restricted habitat requirements or movement capabilities will, however, be
susceptible to competition with stocked fish. In this instance, many of the IUCN listed
species, such as Yarra pygmy perch, Nannoperca obscura, may be susceptible to competitive
interactions should large numbers of fish be introduced.
Indirect impacts of competition
Habitat alteration – Habitat alterations caused by stocking native fish may occur if
individuals exceed the carrying capacity for a particular habitat. Few documented cases of
habitat alteration exist, but perhaps the most common is the speculated increase in turbidity
associated with introductions of common carp, Cyprinus carpio (Fletcher et al. 1985).
However, even this impact has proved difficult to attribute to introductions alone due to
natural variation in turbidity. Habitat alterations may arise indirectly via additional grazing on
25
macrophytes, which can alter habitat attributes, such as sediment stability (see also Food and
habitat resources sections).
Likely effects in Murray-Darling Basin – Habitat alteration caused by stocked native fish
populations is likely to be minimal within the MDB. In a review by Arthington (1991), there
was little evidence of habitat alteration occurring in Australia, with the exception of common
carp. Moreover, as common carp are invasive and not native, impacts are likely to be
exacerbated compared to those of native fish introductions. A potential impact of stocked
native species is the increased need for food resources, such as macrophytes, the additional
grazing of which may alter biomass and, therefore, associated habitats. Any impacts of
habitat alteration are likely to result in both pulse and press effects. Short-term pulse effects
may include a decrease in macrophyte abundance, or long term changes to benthic
assemblages. Impacts of habitat alteration within the MDB are, however, likely to be
restricted to sites of fish stocking (Table 3). In general, potential impacts of native fish
stocking on habitat alteration has received little attention, and is an area where additional
research is required.
Behavioural changes
Artificial cultural conditions of hatchery-reared fish are likely to result in behavioural
differences between stocked and wild fish. Hatcheries rear fish at high densities, often with
constant food supplies, which can lead to behavioural responses that are inadequate for river,
lake, and stream environments. Behavioural impacts can manifest by two different
mechanisms, (i) stocked fish can have behaviours that cause competition with wild fish and
(ii) stocked fish can alter the behaviour of wild fish. Regardless of the mechanism responsible
for the impact, fish stocking often results in detrimental behavioural changes to wild fish.
Behavioural changes in response to fish stocking can be summarized under two broad
categories, (i) aggression and (ii) activity. Predator avoidance as a behavioural response will
be dealt with under a separate heading (see below). Weber and Fausch (2003) have reviewed
the behavioural responses of salmonids in streams containing wild populations. Of the 16
papers reviewed on aggressive behaviour between stocked and wild salmonids, 12 papers
outlined hatchery fish as being more aggressive than wild fish. In only two cases were wild
fish more aggressive than hatchery, and only one paper reported aggressive dominance of
wild fish (see Weber & Fausch 2003 and references there in). In general, aggressive
26
behaviour appears to result from fish stocking, however, the nature of this impact (i.e.
stocked or wild fish aggression) appears to differ. Although, it is hard to assess why
differences in aggression were detected in different studies, it is conceivable that
aggressiveness between stocked and wild fish manifested due to size class differences
between fish. Reduced aggression of wild chinook salmon has been detected when stocked
fish were large in size (Peery et al. 2004).
Of particular relevance to competitive interactions, is the density dependent effect on
aggression in Atlantic salmon, Salmo salar, where wild fish were more aggressive at low
densities, yet hatchery fish more aggressive at high densities (Fenderson & Carpenter 1971).
The degree to which fish density influenced aggressiveness of stocked fish towards wild fish
is difficult to determine, given that density is not manipulated in most experiments. Further
investigations on density dependent behavioural responses are needed, both within hatchery
situations and between stocked and wild fish.
Hatchery-reared fish and wild fish are generally considered to have different rates of activity.
Stocked species of salmonids generally have lower activity rates, and raise the activity rates
of wild fish (Scott et al. 2003, Peery et al. 2004). The impact of stocking on wild fish activity
may again be density dependent or related to habitat types. Movements of brown trout have
been influenced by stocking densities, with wild fish having greater movements (5% to 30%
more movements) at high stocking densities, however, this effect was dependent on stream
habitat type (Weiss & Schmutz 1999a). A general consensus is that hatchery fish are less
active, having lower feeding rates and reduced stamina and swimming ability (Olla et al.
1998), yet, these fish still negatively impact wild fish movements.
Likely effects in Murray-Darling Basin
Native fish stocking in the MDB is likely to impact the behaviour of wild fish, however, the
degree to which this could occur is difficult to predict. As discussed earlier, behavioural
responses may be in the form of aggression or affect fish activity, and impacts are likely to be
density and habitat dependent. In addition, behavioural changes as a result of stocking may
indirectly impact wild fish fitness, predation risk, or force the expansion of species ranges.
The degree to which direct and indirect behavioural responses occur, will depend on the
species stocked. Conceivably, stocking of large dominant fish will alter the behaviour of both
con-specifics and other species, whereas stocking small fingerlings will have a lesser impact.
27
The spatial and temporal impacts of fish stocking will, to some degree, be controlled by the
method of fish stocking. Stocking large numbers of fish at one location is likely to contribute
to localized competition and impact on fish behaviour, whereas, stocking fewer fish at many
locations may lessen impacts. Behavioural impacts resulting from stocking may have pulse
and press effects. Initial changes in fish behaviour as a result of stocking are likely to occur.
In areas where resources are lacking, stocking fish can have long lasting impacts by evoking
aggressive behaviour in wild fish (Table 3).
Expansion of species range and displacement of wild stocks
The introduction of stocked fish can have both negative and positive effects on the expansion
of a species range. In addition, stocked fish are likely to displace wild fish where resources
are limited, or where numbers of stocked fish exceed wild recruitment rates. The impact of
stocking on species ranges largely depends on where stocked fish are introduced. For
instance, if fish are stocked within the current or former geographical distributions, then
impacts may be positive (re-establishment of a stock) or negative (local competition with
wild fish). If fish are stocked to naïve environments beyond species distributions, interspecific competition is likely to occur, with resulting impacts similar in nature to the
introduction of invasive fish (Mills et al. 2004).
Possible impacts of stocking fish within former distributions or in naïve (i.e. species stocked
outside their natural range) environments will largely depend on competitive interactions
between fish, and the natural recruitment capacity of environments. It is also important to
note that environments of locally extirpated fish may no longer match ecological
requirements of stocked fish, especially if habitat change has occurred. The degree to which
stocking will have negative impacts when re-establishing former distributions is likely to be
affected by food and habitat resource availability, as well as behavioural differences between
stocked and wild fish (see sections on Food resources, Habitat resources and Behaviour).
Habitat and food resource remediation prior to stocking may reduce competition and increase
survivorship of stocked fish (Stockner & Macisaac 1996), however, detecting these effects
can be difficult given time delays between treatments and detected differences in fish
abundance (Allen et al. 2003).
28
Similarly, impacts of stocking fish on the displacement and replacement of wild fish are
difficult to detect. The replacement of wild populations by hatchery-reared fish can occur
regardless of whether hatchery fish reproduce. If stocking rates exceed natural recruitment
rates within environments (Evans & Willox 1991), then numerical displacement or
replacement can occur. In a modelling situation, Evans and Willox (1991) describe the ability
of hatchery produced lake trout, Salvelinus namaycush, to significantly replace wild stocks
within approximately 80 years, even when stocked fish do not reproduce. If stocked fish do
reproduce, both physical and numerical displacement of wild stocks may occur over
relatively short time periods.
Likely effects in Murray-Darling Basin
The likely impacts of stocking fish in the MDB on species range expansion and fish
displacement may be positive or negative. Impacts of stocking on fish displacement are likely
to go unnoticed. A priority of any stocking program should be to determine natural
recruitment and carrying capacities of environments (food and habitat resources) so that rates
of fish stocking will not exacerbate displacement and competition.
Impacts of fish stocking on expansion of species range and displacement of fish are likely to
have both pulse and press aspects. For instance, pulse impacts will occur immediately after
stocking, with impacts being particularly severe in translocated areas. Press effects of
stocking will occur if stocked fish survival is high, and especially if stocked fish naturally
reproduce. In areas formerly occupied by species, if the cause of initial species decline is not
rectified, long-term press benefits of stocking will not occur. Long-term benefits of
expanding species ranges may occur, however, these will be accompanied by an increased
likelihood of displacement of wild stocks. Impacts of fish stocking on expanding species
ranges and displacement are likely to be both localized and broad in spatial extend, again
largely depending on the survivorship of stocked fish, and their ability to move post stocking
(Table 3).
Predation
Predation occurs when one fish (predator) consumes another (prey), with the consumption
occurring while the prey is still alive (Begon et al. 1996). In a stocking situation, predation
can have both positive and negative effects, such that wild fish may prey upon stocked fish
providing them with additional food supplies, or stocked fish may prey upon wild fish. The
29
extent to which predation occurs between stocked and wild fish will, to some extent, be
driven by the size distribution of both fish categories (and size-related ecological
differences). Consider the example in which a large and small fish are stocked to a system
(species A) containing individuals of a different species (species B), where species A is
dominant over species B. If adult fish are stocked to systems, predation on both juvenile and
adult wild fish may occur (i.e. Large sp A > Large sp B ~ Small sp A > Small sp B) (Mills et
al. 2004). It is, however, difficult to conceive a situation where stocking juvenile fish will
result in an increase in predation on large wild fish (i.e. Small sp A > Large sp A or B)
(Garman & Neilsen 1982). In the latter case, stocking has a positive effect on the wild fish
population, by acting as a supplemental food resource, but this effect ignores the underlying
issue of limited food resources possibly due to habitat degradation or that the wild population
may already be at carrying capacity.
Predation of stocked fish on wild fish has received much attention. However, most
investigations on predation have based interpretations on gut contents and correlative
evidence between abundance of predators (stocked fish) and prey (wild fish) (Moyle 1976,
Oguto-Ohwayo 1991, Bravo et al. 2001, Wysujack et al. 2002). For example, Wysujack et al.
(2002) examined pikeperch, Esox lucius, diet and noted that > 50% by number and > 90% by
biomass of pikeperch diet consisted of roach, Rutilus rutilus, with a positive correlation
detected between pikeperch length and length of roach in gut contents. Although correlative
evidence provides useful information, with studies often reporting large shifts in species
abundance post stocking (Oguto-Ohwayo 1991), examining diet is not strong evidence for
direct predation (see below; Methods for determining predation). Experiments have also
documented predation of stocked fish on wild fish. Experimental caging of stocked brown
trout provided stronger evidence of predation, with gut contents of large brown trout
containing more wild fish compared to gut contents of small brown trout (Garman & Neilsen
1982). The addition of large brown trout to caged stream sections resulted in a significant
reduction of wild fish; no reduction of wild fish was detected with the stocking of small
brown trout. In general, the stocking of aggressive fish appears to result in predation upon
wild fish, with this effect likely to depend on the size of stocked fish. Additional
experimental research is required on individual species before generalizations can be made.
Predation of wild fish on stocked fish is likely to occur, even though evidence is limited,
perhaps due to difficulties in obtaining data. In an experimental tethering study of stocked
30
and wild summer flounder higher predation of stocked fish occurred, however, this effect was
only detected in marsh habitats, with predation being equal in beach habitats (Kellison et al.
2003) (Fig. 5). Thus, potential interactions between competition and predation can occur
(Garvey et al. 1994, Wahl et al. 1995). In addition, tagging data have shown huge predation
rates of stocked fish. Buckmeier et al. (2005) examined predation of stocked fingerling
largemouth bass, Micropterus salmoides, and determined that up to 27.5% of stocked bass
were consumed by predators within 12 hours of stocking. Natural mortality of stocked
largemouth bass in predator-free enclosures was 3.5% in 84 hours, thus, predation of newly
stocked fish was substantial. Predation by wild fish on small stocked fish is likely, which may
have little effect on the success of stocking programs given high natural mortalities
(Fjellheim et al. 1995). Although predation of stocked fish by wild fish can increase food
resources, there is a possibility that wild fish can become dependent on artificial food
resources. Should wild fish increase in abundance, biomass, or health, due to increased prey
availability, then long-term (press) impacts of stocking may be detected due to prey switching
once stocked fish are depleted (see Food resources).
Methods for determining predation
Strictly speaking, predation refers to the direct consumption of fish, therefore, it excludes the
consumption of fish that have died as an indirect result of a stocking procedure. As high
mortality of stocked fish occurs post release (Smirnov et al. 1994), consumption of carcasses
by stocked or wild fish can occur, and as such, evidence of predation by examining gut
contents is not ideal. Assessing predation as direct consumption in natural environments
(rivers, streams, ponds) is difficult to achieve, however, caging experiments are possible (e.g.
Connell 1997). General methods to assess predation include those described earlier for
competition, and additional methods, such as fish tethering.
Tethering fish involves attaching a length of nylon line to a fish’s mouth, dorsal, or pectoral
fin, and this line being weighted at one end, effectively restricting fish movement (Belanger
& Corkum 2003, Adams et al. 2004, Manderson et al. 2004). Tethered fish are then attached
to the substrate, with pegs or weight, and predation determined after a set time period (several
hours) as a proportion of tethers without fish to tethers with fish. Rates of fish ‘escapes’ from
tethers can be assessed using caged and non-caged treatments. Similarly, effects of habitat on
predation can be assessed by manipulating the habitat surrounding tethered fish. The act of
tethering can conceivably increase predation susceptibility, as fish have little to no way of
31
avoiding predation, thus, information gained from tethering experiments should be coupled
with further predatory evidence. Data from tethering experiments may be useful when
addressing the question of interactions between competition driven responses (i.e. habitats
resource use) and predation. Tethering studies may be difficult to conduct because ethical
approval is now required in many places and the end point of such experiments is regarded as
death.
Likely effects in Murray-Darling Basin
High rates of predation are likely to be associated with fish stocking within the MDB due to
the size and ecology (e.g. predatory nature) of the wild and introduced species present.
Impacts on stocked fish are likely to be high for fish stocked as fingerlings and non-existent
to low for fish stocked at larger sizes. Although predation on small stocked fish is detrimental
to a stocking program, it may be deemed more desirable to have stocked fish abundances
decline than have wild fish stocks reduced due to predation by large stocked fish. Impacts of
predation are likely to be localized and also pulse in nature, however, longer term effects may
occur if predation significantly enhances fish growth, development, and survivorship. For
example, large fish in the MDB, such as Murray cod may receive significant benefits that aid
growth and survivorship due to stocking of small fish (i.e. a small species of stocked fish, and
fingerlings of stocked fish or Murray cod). Increased growth and survivorship of Murray cod
may give rise to greater inter- and intra-specific competition and predation. Any associated
impacts of predation, both on wild and stocked fish, may be reduced by considering the
spatial and temporal aspects of stocking (Table 3). In addition, predation effects are likely to
be high in areas where competition is exacerbated. Thus, in areas where food and habitat
resources are scarce, and behavioural changes are likely to occur, competition may interact
with predation and enhance impacts.
Incidental captures
The impact of incidental captures of wild fish by humans has received little attention. In
theory, stocking should increase fish numbers and allow harvesting to increase, or resume
where harvesting had ceased. As fish stocking can alter wild fish distribution, and in some
cases where habitat resources overlap (Lachance & Magnan 1990b), mixed aggregations of
stocked and wild fish may occur, then the possibility of incidentally harvesting wild fish may
increase. Limited data exists to assess to what degree incidental captures of wild fish occur
due to stocking. Detecting incidental captures, particularly in largely closed systems, such as
32
the MDB, can be done if stocked fish are either artificially or naturally tagged (e.g. Campana
2005, Crook et al. 2005). The proportion of stocked fish (tagged) versus wild fish in a
harvested stock could be compared to pre stocking data, to assess if captures of wild fish
increased post stocking. Further research on incidental capture of wild fish due to stocking is
required.
Genetics
The genetic impacts that hatcheries and hatchery fish have on wild populations is one aspect
of stocking and aquaculture that has perhaps received the most attention, as evidenced by the
volume of published literature, including many reviews on the subject (e.g. Allendorf 1991,
Hindar et al. 1991, Waples 1991, Busack & Currens 1995, Campton 1995, Utter 1998, 2003).
However, this literature is primarily theoretical in nature (Keenan 2000). In addition, the
majority of the literature deals with the effect of cultured fish on native populations of
salmonids. Because of the long history of salmonid stocking (Waples 1991) much is known
about the ecology and genetics of salmonids (Hindar et al. 1991) and the role of salmon
hatcheries and their impacts on wild salmonid populations has generated the most
controversy, particularly in the Pacific Northwest of North America (Campton 1995). In
contrast, little is known about the genetic diversity and structure of native freshwater fish in
the Murray-Darling Basin. Many of the stocking programs in Australia are done without
knowledge of genetic impacts or knowledge of the genetic relationship between the
broodstock, their offspring that are being used for stocking, and the wild populations into
which the fish are being stocked (Bearlin & Tikel 2003). According to the environmental
impact statement on freshwater stocking in New South Wales (NSW Fisheries 2003) the
current approach to stocking fish in NSW poses a significant risk to the genetic integrity of
wild populations of native fish.
The effects of stocking on the genetics of wild populations can be divided into three main
categories: 1) direct effects, which include hybridisation (interspecific and intraspecific),
introgression and various genetic processes including outbreeding depression; 2) indirect
effects, which can be brought about through altered selection regimes or reduction in
population size caused by factors such as predation, competition and disease, and 3) changes
in the genetics of the hatchery fish by means of selection, genetic drift, or stock transfers
(Waples 1991). The following draws primarily from the reviews of genetic effects of
hatchery fish on wild populations by Campton (1995), Busack and Currens (1995), Krueger
33
and May (1991), Utter (1998, 2003), and Waples (1991) and we refer you to those reviews
and others for further details of the issues and concepts discussed below.
Direct effects
Direct effects include those in which exogenous genes from a donor population (e.g. stocked
fish) infiltrate the gene pool of the indigenous population. These effects include
hybridisation and introgression (or the incorporation of genes after hybridisation and repeated
back crossing of hybrid descendants) (Fig. 6, Table 6).
Interspecific hybridisation and introgression – The introduction and translocation of species
breaks down the barriers responsible for the reproductive isolation inherent in the definition
of species, resulting in the potential for interspecies hybridisation. There are numerous cases
of hybridisation between native and introduced or translocated species. Some examples
include Atlantic salmon interbreeding with introduced brown trout in a river in Nova Scotia,
Canada (Beland et al. 1981), bull trout, Salvelinus confluentus, hybridising with non-native
brook trout in the upper Columbia River drainage in Montana, USA (Leary et al. 1983), and
the hybridisation, and eventual loss, of endemic tilapiine species in Africa after the
translocation of other tilapiine species (see Oguto-Ohwayo 1991). Often the offspring from
the crossing of different species are sterile, in which case direct genetic effects are not an
issue, although these hybrids may cause other effects (e.g. indirect genetic effects,
competition – see Abundance and behavioural responses to fish stocking). However, some
hybrids may be fertile [e.g., lake trout and brook trout (splake) (Krueger & May 1991)], in
which case the genes from one species can become integrated into another. This flow of
genes into a population (introgression) can occur naturally between species (or subspecies)
and if limited (<0.1%) can be considered beneficial because it introduces variation upon
which natural selection can act (Krueger & May 1991). However, if this gene flow is
excessive the natural co-adapted gene complexes can be broken down thus reducing the
fitness of the population (Hindar et al. 1991). Introgression has played a significant role in
the evolution of salmonids in western North America; however, stocking of rainbow trout
outside of its native distribution throughout the west has resulted in the introgression of
numerous trout species and subspecies including golden trout, Oncorhynchus aguabonita,
and the cutthroat trout subspecies. Introgression has also been attributed to the loss of native
populations of cutthroat trout, including one extinct and two endangered sub-species
(Allendorf & Leary 1988).
34
Intraspecific introgression/outbreeding depression – Hybridisation and introgression between
hatchery and wild fish of the same species can lead to increased genetic diversity within a
given population by the incorporation of new alleles, but can also result in the
homogenisation of the genetic variation among populations of the species (Krueger & May
1991, Busack & Currens 1995). For example, if the same population of hatchery fish are
used to stock genetically divergent (locally adapted) wild populations and if the hatchery
broodstock are sourced from one or more of these populations, some receiving populations
will have new alleles introduced, but the overall effect will be a homogenisation of the gene
pools among the populations. In addition, there is the potential for genetic swamping - the
partial or, in an extreme case, complete replacement of the indigenous gene pool with genetic
material from a large number of stocked fish that are generally offspring of relatively few
parents (Campton 1995). A reduction in genetic variability or alteration of the genetic
composition of a population can lead to the loss of locally adapted populations, limit the
evolutionary potential of the population as a whole, and increase the vulnerability of
populations to environmental changes (Waples 1991, Utter 1998) possibly leading to
outbreeding depression.
Outbreeding depression is defined as the erosion of population fitness through mating of
genetically divergent populations (Waples 1991), or more simply, the reduction in fitness of
the population following intraspecific hybridisation, usually through reduction in fertility or
viability (Keenan 2000). Outbreeding depression occurs when co-adapted gene complexes,
which have a positive effect on fitness, begin to breakdown and are recombined in the F2
(second) generation of the hybridised stock resulting in reduced fitness of the population
(Campton 1995). Typically, the F1 (first) generation of the cross between the wild and
introduced populations will exhibit increased fitness (hybrid vigour or heterosis) because they
retain the co-adapted gene complexes of their parents (Waples 1991). The risk of
outbreeding depression is considered significant. For example, computer simulations
indicated that even a small amount of genetic mixing (5-10%) between a locally adapted wild
population and hatchery fish could result in significant declines in fitness and that the
recovery of fitness after a single hybridisation event would require many generations (Emlen
1991). However, few cases of outbreeding depression have been documented in salmonid
populations or other groups of fish (Krueger & May 1991, Campton 1995).
35
It is often difficult to measure the genetic effects involving populations of the same species
and limited information exists on the second generation contribution of stocked fish (Krueger
& May 1991). There are a number of examples where interbreeding between stocked and
wild populations has occurred leading to the homogenisation of genetics [e.g., rainbow trout
on the Olympic Peninsula, Washington, USA after years of hatchery stocking (Reisenbichler
& Phelps 1989)]. However, there are also studies that have found little to no evidence of
introgression between hatchery-stocked fish and the wild populations. For example, Krueger
and Menzel (1979) concluded that much of the genetic variability in brook trout sampled
from several streams in Wisconsin, USA was natural, despite having long histories of
stocking (6 to 35 years). Similarly a more recent study of the only indigenous populations of
lake trout in the upper Mississippi River basin concluded that repeated stockings (primarily
from the 1950s to present) had contributed minimally to the genetics of these populations
(Piller et al. 2005). The lack of evidence of interbreeding can be because of a true lack of
interbreeding between the stocked and wild fish as a result of poor survival of the stocked
fish or some other reproductive isolating mechanism; however, it could also be because the
effects of interbreeding cannot be differentiated from natural genetic drift or the methods
used are not powerful enough to detect the differences between the hatchery and wild
populations (Steward and Bjornn 1990 in Krueger and May 1991).
Indirect Effects
Indirect effects include those where genetic changes within or among wild populations occur
without the infiltration of exogenous genes from the stocked fish (Utter 1998, 2003). Any
factor that leads to a reduction in population size or alters the selection regimes can have an
indirect effect on the genetics of the indigenous population (Krueger & May 1991, Waples
1991, Utter 2003). Such factors include competition, predation (including overharvest by
humans), and introduction of diseases (see relevant sections in this review) (Fig. 6, Table 6).
Reductions in population size can potentially result in loss of genetic variability, increase in
rates of genetic drift and an increase in the potential for inbreeding (Krueger & May 1991,
Waples 1991). Genes or alleles may also be lost from locally adapted populations, which
may result in a decline in the fitness, evolutionary potential and overall adaptability of the
population (Krueger & May 1991). Reduced population size may also lead to the expression
of deleterious or lethal alleles that otherwise would remain at low frequency. These genetic
changes can lead to increased risk of losing populations and ultimately extinction of the
36
population or species. In addition, populations can become fragmented and isolated thus
interrupting migration and gene flow patterns, further compounding these negative genetic
effects. A few examples that can affect both the size of populations and alter selection
regimes are briefly discussed below.
With the stocking of hatchery fish into wild populations there are typically the social,
political and economic pressures to fully exploit the expanded resource (Waples 1991).
Unless managed properly, it has been suggested that this can lead to excess harvest of wild
fish in these mixed-stock fisheries (see Incidental captures), thus reducing the abundance of
the wild population and potentially restricting the gene pool. Overharvesting has been an
important topic in the supplementation and harvest of Pacific salmon and is considered a
major factor in the decline of wild populations (Nehlsen et al. 1991). Although Evans and
Willox (1991) concluded that increased exploitation of a mixed wild and hatchery fishery
could lead to the loss of native populations of lake trout (i.e. extinction), Campton (1995)
stated that, “virtually all of the data related to this question appear to be conjectural,
theoretical, or circumstantial.” It has also been suggested that predators attracted to the
increase in available prey when there are large releases of hatchery fish can cause declines in
wild populations (Waples 1991, Utter 1998, 2003) (see Abundance and behavioural
responses to fish stocking).
Another two factors that have been suggested as potentially contributing to indirect genetic
effects are broodstock exploitation and wastage of gametes. Broodstock exploitation (or
mining) is when more fish are removed from the wild population than can be replaced by
natural reproduction or through recruitment of adults of hatchery origin (Campton 1995).
Wastage of gametes can occur when there is non-introgressive hybridisation (i.e. the first
generation offspring of a mating are sterile), therefore the gametes of the wild donor are lost
and do not contribute to the population (Utter 1998, 2003). These potential factors are
difficult to measure and to our knowledge there is no empirical evidence to support these
assumptions.
Genetic changes in hatcheries
Artificial propagation of fish invariably alters the genetics of captive-bred populations. Some
of the important processes that can lead to genetic changes in hatchery fish include: random
processes that lead to loss of genetic variation, mixing of stocks and transfers among
37
hatcheries that result in introgression and homogenisation of the gene pools, artificial
selection, and domestication (natural selection due to the hatchery environment) (Waples
1991, Campton 1995) (Table 6).
Random processes/loss of genetic variation – The effective population size, or effective
number of breeders, is a key parameter in population genetic models and if low the genetic
variability of the population is reduced, resulting in reduced fitness and evolutionary potential
that may lead to inbred hatchery populations (Waples 1991). There are numerous cases that
have documented significant changes in the genetics of hatchery fish (see Allendorf &
Ryman 1987, Waples 1991, Campton 1995). For example, Waples and Teel (1990) and
Waples and Smouse (1995) found significant changes in allele frequencies and levels of
gametic disequilibrium in hatchery chinook salmon and concluded that the changes were
likely a result of genetic drift or founder effects because of low effective number of breeders
(Waples 1991, Campton 1995).
Often the effective number of breeders is much smaller than the number of spawners in the
hatchery population. Factors that reduce the effective number of breeders are unequal sex
ratios, unequal family sizes, shipping of entire families to other hatcheries, and mixing the
eggs and sperm from several females and males (Campton 1995). Genetic loss can be
minimized by using appropriate spawning procedures such as using equal numbers of both
sexes and limiting matings to pairs of fish or by using a replicated factorial design (Campton
1995). Also by ensuring that each male and female contributes equal numbers of progeny to
the next generation of spawners effective number of breeders can be increased to nearly twice
the number of available spawners; this has the additional benefit of counteracting the genetic
effects of domestication (Campton 1995).
Stock mixing/introgression – Transfer of fish and eggs between hatcheries and mixing of
gene pools from divergent populations and stocks has been a common practice. These
transfers and mixings have resulted in genetically homogenous stocks across multiple
hatcheries (Waples 1991, Campton 1995). The extent of the genetic effects of these
“hatchery” stocks on genetically distinct wild populations is unknown (Waples 1991).
Although, as noted above, Reisenbichler and Phelps (1989) found evidence of
homogenisation of Olympic Peninsula, Washington, USA rainbow trout populations after
many years of stocking.
38
Artificial selection – Artificial selection, or selective breeding by hatcheries, whether it is
intentional or not, is inevitable; simply the selection of which fish to breed and mate together
is a form of artificial selection (Waples 1991). Selective breeding has been commonly
practiced in salmon hatcheries and has been well documented (Campton 1995). For example,
early-spawning salmon are often selected for, partly because by the time late spawners arrive
the hatcheries have filled their quota of eggs (Waples 1991). In the case of steelhead
(anadromous rainbow trout), early spawners are less successful in the wild because of the
variable environment in the early spring, but in the hatchery environment success is greatly
improved (Waples 1991). Campton (1995) notes that the monitoring of basic genetic
measures, such as heritability, genetic correlations for life history or other quantitative
genetic traits has rarely been done in salmon hatcheries. However, Campton (1995) does
concede that to do so, to the extent that it is done with other animals that are bred artificially,
would be logistically and economically difficult. Even if efforts were made to avoid artificial
selection of broodstock (e.g. random samples from entire spawning season) it would be
impossible to mimic natural selection for reproductive success (i.e. in nature, which
individuals mate and with what success is determined by a number of factors) (Waples 1991).
Domestication – Domestication is different from artificial selection in that it is a result of
natural selection in an artificial environment, such as a hatchery (Campton 1995). In the
hatchery environment some level of natural selection would be expected because some fish
will have genotypes better suited for that artificial environment and therefore have a greater
survival rate than those that do not and that the selection pressures would be different than
those experienced by wild fish (Waples 1991, Busack & Currens 1995). Artificial
propagation circumvents the initial high mortality encountered by fish in natural
environments and therefore the patterns of mortality are very different. In salmonids the egg
to smolt survival in hatcheries is typically 50% (cf < 10% in the wild), but after release the
mortality of hatchery fish is much higher (> 99%) than in wild fish (Waples 1991) (Fig. 7).
Fish in the hatcheries are reared in greater than natural densities, different food sources and
availability, and rearing habitats (e.g. concrete raceways). Because of these differences,
behavioural and physiological traits are most often affected (Campton 1995). In the hatchery,
domestic trout are often more active than wild fish, as they swim near the surface in open
water, have reduced stamina, and increased growth; however, in natural conditions they
39
exhibit decreased growth, increased swimming activity, increased aggressiveness, and
decreased survival compared to wild fish (Campton 1995).
Likely effects in the Murray-Darling Basin
It is generally accepted that any type of stocking or translocation program will result in
alteration of the genetic composition of the hatchery reared fish, the receiving population, or
both. Within the MDB any of the direct, indirect, or hatchery effects described above are
likely. However, the extent of these impacts depends primarily on hatchery practices
(including stocking) and an understanding of the taxonomy, phylogeography and population
genetic structure of the native species. An important framework for assessing population
structure from a genetic point of view and the scale at which they can be monitored and
managed are the concepts of Evolutionary Significant Units (ESUs) and Management Units
MUs) (see Moritz 1994b, Moritz 1994a). ESUs are recognised as monophyletic for mtDNA
alleles and significantly divergent at nuclear loci . MUs are recognised as populations that
are significantly divergent in nuclear or mitochonrial alleles, but not necessarily
phylogentically distinct (Moritz 1994b). In other words, MUs represent populations in which
gene flow between them is so low that they are essentially independent (Moritz 1994b).
ESUs therefore, emphasise the evolutionary heritage and historic population structure as well
as long-term conservation needs whereas MUs address the current population structure and
short-term management needs (Moritz 1994b).
Hatchery production of native MDB fish has until recent times been limited to species that
have commercial and recreational importance, such as golden perch, Murray cod, trout cod,
silver perch, and freshwater catfish. However, there have been concerns over the production
practices of the hatcheries supplying fish for stocking in the MDB, particularly with regard to
management of genetics (Rowland & Tully 2004). For example, many hatcheries use a
limited number of broodstock to produce many fish and will often source broodstock from
other hatcheries or from farm dams, which may have been stocked with fish from only a few
parental crossings (Moore & Beaverstock 2003, Rowland & Tully 2004). Therefore, most
hatchery-produced fish that have been stocked or are currently being stocked into the MDB
probably have much reduced genetic diversity and reduced fitness. There are also indications
that inbreeding is common in the hatcheries (e.g. Bearlin & Tikel 2003, Moore &
Beaverstock 2003). As discussed previously, the stocking of fish with reduced genetic
variability can lead to reduction in the heterogeneity of the receiving population if the stocked
40
and wild fish interbreed, and will likely lead to reduced fitness, adaptive potential and ability
of the indigenous population to cope with environmental change. In order to minimize the
genetic impacts from hatchery practices strict hatchery protocols and proper genetic
management of the broodstock and their progeny is required. The New South Wales
Department of Primary Industries has developed a Hatchery Quality Assurance Program in an
attempt to address these issues (see Rowland & Tully 2004, and below).
Measuring genetic variation
There are a number of different molecular techniques available to measure genetic variation
in fish, the most commonly employed methods being allozyme electrophoresis, mitochondrial
DNA (mtDNA) and nuclear DNA (nDNA). These techniques have been reviewed in detail in
the papers included in Baker (2000b), Hallerman (2003) and Avise (2004). While each
technique has its advantages and disadvantages, and may be more suited to the study of
genetic variation at one level versus another (e.g. phylogenetic, population, individual), most
publications do caution using these techniques without understanding some of the basic
assumptions, such as the mode of inheritance. They also advise not to rely upon one
technique, but instead use a combination of genetic methods in order to more fully
understand the genetic variation.
Allozyme electrophoresis - Allozyme electrophoresis was developed in the mid-1960s (May
2003) and until the mid-1970s was the only molecular method available to population
geneticists (Cross 2000). The method explores variation at nuclear loci. At a given locus
different molecular (allelic) forms of an enzyme can be encoded; these are allozymes,
however, the same enzyme may also be encoded at multiple loci, again with different
molecular forms (isozymes) (Richardson et al. 1986, Baker 2000a). The allozymes identified
are presumed to represent putative gene loci and the variation among enzymes is the basis for
allozyme electrophoresis analysis. Allozyme electrophoresis has been used extensively in
fisheries research and management, particularly for understanding species boundaries and for
the planning and monitoring of stocking programs (Cross 2000). Although allozyme markers
are good population markers, they have several distinct disadvantages. Because the enzymes
that are being analysed are very temperature labile, the samples need to be of high quality,
which means using either fresh samples or samples stored at very low temperatures (-40 to 80°C) (Cross 2000, Bearlin & Tikel 2003, May 2003). Also, because some enzymes are
encoded at multiple loci and expressed in different tissues, samples from multiple organs (e.g.
41
muscle, blood, heart, live, brain and eye) should be collected in order to fully understand the
genetic control of the isozymes (Baker 2000a). Thus, the animal needs to be sacrificed which
limits allozyme research on threatened and endangered species or valuable broodstock (Cross
2000, Bearlin & Tikel 2003). Finally, allozyme electrophoresis only examines structural
DNA, so less than 1% of the nuclear genome is examined (May 2003); therefore, this
technique is only able to detect a limited amount of the genetic variation present at the
nucleotide level (Baker 2000a). Despite these disadvantages, allozyme electrophoresis will
likely continue to be used because it is a relatively inexpensive and easy technique, large
quantities of data can quickly be produced, and there are large baseline datasets for many
species (May 2003). In addition, allozyme electrophoresis is generally precise enough for
certain fisheries and aquaculture applications (Cross 2000).
Mitochondrial DNA – With the development of direct DNA techniques in the mid-1970s, the
DNA found in the mitochondria of cells has been used extensively in the study of genetic
variation in organisms at both the phylogenetic and population level. Mitochondrial DNA is
a small haploid molecule, which is clonally and, for the most part, maternally inherited
(Cross 2000, Randi 2000, Avise 2004). Some of the properties of mtDNA that make it useful
for studying genetic variation include the fact that it is maternally inherited, not subject to
recombination, and that it generally evolves faster than nDNA. Because mtDNA is
maternally inherited it is more responsive to reductions in population size (bottlenecks) and
certain gene sequences are, therefore, useful for comparing levels of genetic variability
between populations, such as stocked hatchery populations and the receiving wild
populations (Cross 2000). Mitochondrial DNA does not undergo recombination so it
provides a complete set of homologous markers that are linked to maternal lineage, which
makes mtDNA a good discriminator between common ancestry and convergence (Billington
2003). In higher vertebrates, mtDNA generally evolves much faster than the nuclear genome
(5 to 10 times greater rate of nucleotide substitution) (Billington 2003). This faster rate of
evolution means that the genetic variation in the mitochondrial genome accrues within and
between populations in short evolutionary time spans, thus increasing the ability to
discriminate among populations (Randi 2000). A further advantage of mtDNA is that it is
homoplasmic (i.e. identical throughout the organism), meaning samples can be taken from
any tissue including muscle, fin clips, barbel clips and even scales (although liver and
gonadal tissues tend to have high mitochondrial densities) (Billington 2003). Preservation is
less of a concern for mtDNA analysis than it is for allozymes. Useable mtDNA can be
42
extracted from samples that have been stored frozen, in ethanol, or even formalin (short
segments), but fresh samples provide the highest quality mtDNA (Billington 2003).
The development of polymerase chain reaction (PCR) amplification has been a major asset to
mtDNA analysis. PCR is a technique that is used to amplify the numbers of copies of
specific DNA fragments. This means that only small quantities of tissue are needed and does
not require sacrificing the organism, which is helpful when working with threatened and
endangered species or populations (Billington 2003). In addition, PCR has contributed to
making mtDNA analysis a relatively inexpensive and versatile technique, especially when
combined with restriction fragment length polymorphism (RFLP) analysis (Randi 2000).
Of the literature reviewed, there was little in the way of specific disadvantages of mtDNA.
However, because the mode of inheritance is maternal and many sections lack variability, one
needs to carefully select regions of the mitochondrial genome (Cross 2000). In addition, a
combination of information from mtDNA and nDNA analysis should be used because
mtDNA is effectively one linked gene (Randi 2000). A further disadvantage is that mtDNA
will not detect hybridisation (which requires nuclear markers). However, mtDNA analysis
can provide additional information on the direction of hybridisation and occurrence of
introgression (Billington 2003).
Microsatellites – Nuclear DNA contains segments of repetitive and non-repetitive DNA
sequences. Repetitive DNA sequences are further divided into several different types, which
include VNTR sequences (variable number of tandem repeats). Microsatellites are a subclass
of VNTRs and are simple sequences of nucleic acids, 2 to 8 base pairs long that are tandemly
repeated where the number of repeats define a particular allele (Cross 2000, Brown &
Epifanio 2003). Microsatellites are considered superior population markers compared to
allozymes and mtDNA (Bearlin & Tikel 2003), although this may depend on the research
question, spatial scale and variability present within a species. They have rapidly become
widely used in population genetic studies of fish (Cross 2000, Scribner & Pearce 2000,
Brown & Epifanio 2003). Some of the properties of microsatellite loci that make them
particularly useful for studying genetic variation at the population level are that they are
common throughout the genome, highly variable (i.e. many are polymorphic and have
multiple alleles), lack physical linkage with each other so are statistically independent and
exhibit codominant inheritance (Scribner & Pearce 2000). In addition, they are non43
functional coding sequences and thus considered selectively neutral (Brown & Epifanio
2003). Microsatellites, like allozymes, are very sensitive to population demographic changes,
such as bottlenecks and population size fluctuations (Scribner & Pearce 2000). PCR
techniques are used to detect and amplify microsatellites, as with mtDNA; therefore DNA
can be extracted from a variety of tissues that do not necessarily require destructive sampling
and can even be extracted from small quantities of preserved and archived tissue (including
scales and otoliths) (Cross 2000, Scribner & Pearce 2000).
As with mtDNA, there was little in the reviewed literature on direct disadvantages of using
microsatellites. One of the drawbacks with using microsatellites are that in general the
number of heterozygotes found is lower than expected, which may be caused by the presence
of null alleles (sequences that are not amplified in PCR) (Cross 2000). The presence of null
alleles are likely to be more common when using primers that were developed for species
other than those being analysed (Scribner & Pearce 2000). Also, the number of loci surveyed
is important for correct determination of relationships among populations (Scribner & Pearce
2000). Although microsatellites are considered superior population markers, they are more
time consuming and less economic to develop than for mtDNA (Bearlin & Tikel 2003).
Summaries of genetic structuring in MDB native fish species
Very little is known about the genetic structure of native fish populations in the MDB
(Bearlin & Tikel 2003). However, from the work that has been done it is clear that the
genetic structuring of native fish found in the MDB is commonplace and complex, including
the presence of cryptic species and subspecies within the MDB and adjacent basins, as well
as population structuring within catchments. The summaries below (and Table 7) illustrate
the complexities in population structuring present in MDB fish species studied thus far, as
well as the paucity of information that is available and the need to collect more
comprehensive genetic data for these species and other native MDB species for which there is
no genetic information. These summaries in no way suggest the full extent of true genetic
structuring is known or that none exists if no structuring has been detected. In many cases
sampling may not have been at a large enough spatial scale or extensive enough to capture
the full extent of genetic structuring.
Golden perch – A common and widely distributed species within the MDB and adjacent
basins, golden perch is an important species for recreational fishing and aquaculture. Musyl
44
and Keenan (1992) surveyed golden perch populations in the Lake Eyre, Bulloo River,
Murray-Darling, and Fitzroy River basin and concluded that golden perch do have genetically
distinct populations at a broad level. Genetic variation among populations was determined
using allozyme electrophoresis of liver, muscle and eye samples. They concluded that Lake
Eyre golden perch (Barcoo and Diamantina rivers) should be considered a distinct species
and that Bulloo River Basin golden perch be considered a subspecies more closely related
Lake Eyre golden perch than MDB golden perch. East of the Great Dividing Range, the
Fitzroy River Basin population (Dawson and Nogoa rivers) were found to have allelic
distribution indicative that there has not been genetic exchange between it and the MDB
population (Murray River, Lake Keepit and Condamine River), although lack of fixed genetic
differences suggest that mechanisms that result in reproductive isolation probably have not
developed (Musyl & Keenan 1992). More recently, in a study on the population genetics of
fish in the Murray-Darling Basin (36 sites in 16 rivers and 2 impoundments) evidence of
significant population structuring among golden perch was found for samples from central
Murray-Darling, lower Murray River, Paroo River, Ambathalla Creek, Lachlan River, and
two impoundments (Keenan et al. 1996). As with the previous study genetic variation among
populations was determined using allozyme electrophoresis of liver, muscle and eye samples.
Murray cod – An important recreational and aquaculture species, the abundance and
distribution of Murray cod has been severely reduced by overfishing and other anthropogenic
factors (Allen et al. 2002). Very little is known about the genetic structure of this species, but
mtDNA analyses of samples from seven catchments (one river per catchment) located in the
northern and southern regions of the MBD suggested that there are potential regional
differences in frequency and type of haplotypes in Murray cod (Bearlin & Tikel 2003).
Although there is differentiation among the populations examined, particularly in the
northern drainages (Gwydir, Macintyre and Namoi), the structure is unclear without
evaluating the influence of past stocking and translocation events (Bearlin & Tikel 2003).
Hatchery broodstock from 9 hatcheries (sampled as part of the same study) were found to be
genetically similar to wild fish; however, the Murray cod that were being stocked into
Victorian water in 2001 and 2002 had only 6 of the 11 haplotypes found in wild populations,
plus an additional haplotype not found in samples of wild Murray cod (Bearlin & Tikel
2003).
45
Trout cod – Trout cod were once abundant throughout the southern MDB, but are now
severely restricted both in abundance and range and are listed as endangered or threatened
under various state, territory and commonwealth conservation acts. Only a few natural and
translocated populations of trout cod still exist, with the population in the Murray River
below Yarrawonga Weir being the only natural population. There are two translocated
populations – one within the MDB in Seven Creeks (tributary to the Goldburn River) and one
in Cataract Dam, NSW, which is outside its native range. There are also two stocked
populations within the MDB (Murrumbidgee and Ovens rivers). Bearlin and Tikel (2003)
surveyed the Murray River, Seven Creeks, and Ovens River populations and found a total of
11 mtDNA haplotypes, of which all were present in the Murray River population. However,
the trout cod from both Seven Creeks and Ovens River had fewer mtDNA haplotypes and
were significantly different to Murray River fish. In addition, it was found that the
broodstock of the two government hatcheries that supply trout cod were also missing a
number of the haplotypes present in the Murray River population, the source of most
broodstock (Bearlin & Tikel 2003). Furthermore, natural hybrids between trout cod and
Murray cod have been reported in both the Murray River population and the translocated
population in Cataract Dam (Wajon 1983, Douglas et al. 1995). Potential for hybridisation
has implications with regards to the translocation or stocking of trout cod or Murray cod to
areas where the two species historically did not overlap and barriers for reproductive isolation
are not established. In addition, in the hatcheries, if hybrid broodstock are used to produce
offspring for stocking the effects of introgressive hybridization may be passed through
hatcheries into the wild.
Silver perch – Silver perch were once abundant in the MDB and are now listed as threatened.
Silver perch have been extensively stocked and translocated for over 25 years and are
considered to be the most genetically altered of the native fish species in the MDB and in the
greatest danger of inbreeding (Bearlin & Tikel 2003). Keenan et al. (1996) were unable to
detect genetic differences among the two wild populations they surveyed (Warrego River,
QLD and Murray River near Torrumbarry Weir – the only large populations not stocked)
using allozyme electrophoresis of liver, muscle and eye samples, in which 36 loci were
examined, but only 3 could consistently be scored. However, Bearlin and Tikel (2003) found
that there was significant differentiation among wild silver perch populations sampled from
10 catchments in four different regions of the MDB (particularly in the Macintyre,
Condamine, Mid-Murray, and Lachlan rivers). The observed genetic differences in these
46
populations were mainly due to variations in frequency of occurring mtDNA haplotypes and
not region specific haplotypes. Comparisons of stocked and wild populations indicated that
the stocked populations have much less genetic diversity (Keenan et al. 1996, Bearlin & Tikel
2003). A survey of 46 hatcheries found that each of the silver perch broodstocks were
characterized by a few dominant haplotypes; however, taken as a whole it appears that the
hatcheries have managed to capture a large diversity of haplotypes (Bearlin & Tikel 2003).
In addition, of the hatcheries surveyed only one non-government hatchery maintained
pedigree records.
Freshwater catfish – Freshwater catfish were once an important commercial and recreational
fish species and are one of the five native Murray-Darling species that have been successfully
cultured in hatcheries. Historically freshwater catfish were commonly distributed throughout
the MDB and the coastal rivers east of the Great Dividing Range; however, in recent years
their abundance has declined substantially and they are now considered restricted or rare
(Musyl & Keenan 1996). The limited studies that have surveyed freshwater catfish suggested
that complex population structures and even cryptic species do exist. Musyl and Keenan
(1996) were the first to discover the possibility of two undescribed species of Tandanus in the
coastal rivers of NSW using allozyme electrophoresis. Tandanus from the Bellinger and
Nymboida rivers were genetically distinct from the other populations sampled in the eastern
coastal drainages and the MDB (six impoundments in eastern QLD and NSW plus the
Lachlan River). Further work by Jerry and Woodland (1997) using allozyme electrophoresis
found the ‘Bellinger’ catfish (Tandanus sp.) were present in three other mid-northern NSW
coastal rivers. In addition, Tandanus from three other northern coastal rivers were
genetically similar to samples from the Namoi River (MDB), but exhibited genetic variability
that suggested a degree of population structuring. Within the MDB, riverine populations
(Macintyre, Lower Murray, Lachlan, Macquarie, Bogan, and Warrego rivers) were found to
be genetically similar, but populations in the eight impoundments located throughout the
MDB exhibited some genetic differentiation (Keenan et al. 1996). Genetic variation was
determined using allozyme electrophoresis of liver, muscle and eye samples. It should also
be noted that the purpose of Musyl and Keenan (1996) and Jerry and Woodland (1997) were
to identify the presence of cryptic speciation and not population structuring of freshwater
catfish, while Keenan et al. (1996) aimed to examine population genetics within the MDB.
47
Two-spined blackfish – Two spined blackfish, Gadopsis bispinosus, is the less common of the
two Gadopsis species found in the MDB. The two-spined blackfish has a limited distribution
with populations identified in the central and north-eastern tributaries of the Murray River in
Victoria and in a tributary of the Murrumbidgee River in the ACT (Waters et al. 1994).
Comparisons of these populations, using mtDNA, indicate that there are differences among
them, with the King Parrot Creek (Murray River tributary) population being distinct, but the
populations from King River and Stony Creek (Murray River) and Cotter River
(Murrumbidgee) were not significantly divergent (Ovenden et al. 1988, Waters et al. 1994).
Natural historic river capture (drainage rearrangement) possibly explains the genetic pattern
observed in these populations (Waters et al. 1994).
Southern purple-spotted gudgeon – Once widespread throughout the MDB, the southern
purple-spotted gudgeon is now only common in the northern Basin. Although, to our
knowledge, there are no published studies of the phylogeography of purple-spotted gudgeon
in the MDB, studies in the Atherton Tablelands in north-eastern Queensland suggest that
there is genetic structuring in populations of this species. Little genetic differentiation was
found among many of the catchments sampled (Herbert, Barron, and North Johnstone rivers),
but there were distinct populations in the Tully River Basin (Hurwood & Hughes 1998). The
widespread presence of certain mtDNA haplotypes among some catchments indicates that
gene flow is possible among rivers for this species, which would be expected given its
historically wide distribution and broad habitat preferences. However, the genetic structuring
in the Tully River Basin does indicate that there is limited genetic exchange among
populations. This limited genetic flow is possibly a result of natural barriers and drainage
rearrangement (i.e. river divergence or capture due to geologic processes) (Hurwood &
Hughes 1998). Similar genetic structuring of southern purple-spotted gudgeon populations is
likely within the MDB. Given the threatened status of this species and the fact that efforts to
re-introduce populations have already been attempted (see Gilligan 2005), it is essential that
the genetic structuring within the MDB be determined so as not to adversely impact the
remaining populations.
Southern pygmy perch – Southern pygmy perch, Nannoperca australis, is a widely
distributed and common fish found in coastal drainages from eastern Victoria to the Inman
river and extending inland into the Murray and Murrumbidgee rivers, with populations also in
northern Tasmania and the King and Flinders islands (Bass Strait) (Allen et al. 2002).
48
Although common throughout most of its range, southern-pygmy perch appear to have
become regionally extinct in some locations within the MDB and some populations are
considered ‘locally-endangered’ (Hammer 2001). A genetic survey of populations across
south-eastern Australia indicated that ‘southern pygmy perch’ likely comprises two species
including a morphologically cryptic species (N. sp. nov.) in the south-eastern extent of the
range. Within the western species, there was evidence for deep regional differences between
populations in the MDB and those populations occurring in coastal drainages of south-east
South Australia, western coastal Victoria, and northern Tasmania (Hammer 2001).
Furthermore, within the MDB populations, substantial genetic variation suggested that there
was limited dispersal among populations, possibly due to due to naturally poor dispersal
ability and more recent habitat alteration/fragmentation (Hammer 2001).
Minimising genetic impacts/knowledge gaps
The major concern with stocking is the alteration of the natural genetic composition of wild
populations (Bearlin and Tikel 2003). Continued poor hatchery practices and stocking or
translocating fish with different genetic composition to the receiving population will
eventually erode the genetic diversity of the population and result in homogenisation of the
gene pool and loss of population structure, locally adapted populations and potentially
extinction.
Good hatchery protocols are an essential component of any supplemental or conservation
stocking program. However, indications are that current practices are inadequate (Bearlin &
Tikel 2003, Rowland & Tully 2004). Genetic guidelines and protocols have been developed
for hatcheries producing fish for supplemental and conservation stocking and NSW has
developed the Hatchery Quality Assurance Program to address these issues (see Miller &
Kapuscinski 2003, Rowland & Tully 2004 for examples of such guidelines). One of the
caveats that Miller and Kapuscinski (2003) stress with regards to genetic guidelines for
hatchery supplementation programs is that hatchery supplementation is an unproven
technology and that there is still much that needs to be learned; therefore it should be
approached with the concept of adaptive management in mind. Adaptive management
requires continual monitoring, systematic evaluation of actions and implementation of
necessary changes in order to meet the goals and objectives of the program.
49
Considerations for the production of fish for supplementation or conservation stocking
include the source of broodstock, spawning of the fish (i.e. numbers, mating), and rearing. It
is recommended that only broodstock from the wild be used and that they be sourced from
the population or genetic strain that is going to be stocked (or connected populations)
(Aprahamian et al. 2003, Sanger & Talbot 2003, Rowland & Tully 2004). Broodstock should
be collected from multiple locations and throughout the spawning season (Aprahamian et al.
2003). With rare or endangered species it is important that removal of broodstock does not
threaten the persistence of the source population (Aprahamian et al. 2003). Bearlin and Tikel
(2003) suggest that where a local population is not available, broodstock should be collected
from as many unrelated populations as possible and note that it is essential to then monitor
the genetic character of progeny. However, this approach is contentious and could lead to
some of the direct genetic effects outlined above when mixing divergent populations (e.g.
breakdown of co-adapted gene complexes and outbreeding depression). Other hatchery
recommendations include maintaining an effective population size of at least 50 individuals
to minimize inbreeding, using only single cross matings, and keeping the eggs from each
pairing separate (Ingram et al. 1990, Miller & Kapuscinski 2003, Sanger & Talbot 2003)
Furthermore, it is recommended that the time spent in the hatchery environment is minimized
so as to limit domestication (Aprahamian et al. 2003). It has also been recommended that a
central genetic register for broodstock be established and that detailed reporting, such as
broodstock source, matings, stocking locations be mandatory for hatcheries (NSW Fisheries
2003).
There are a number of recommendations for stocking of fish into the wild. These include
stocking equal numbers of offspring from multiple pairings at any given site (Sanger &
Talbot 2003, Rowland & Tully 2004). Fish should only be stocked into populations that are
genetically similar in order to reduce the possibility of outbreeding depression and maintain
biodiversity (Bearlin & Tikel 2003, Rowland & Tully 2004). More detailed protocols and
recommendations for stocking fish can be found in Rowland and Tully (2004) and Miller and
Kapuscinski (2003).
The lack of information on the genetic diversity of native fish populations is a critical
knowledge gap, and the cause of uncertainty as to the impacts of stocking and translocating
fish. Developing a comprehensive understanding of the genetic composition of wild
populations is essential for defining significant evolutionary and management units (see
50
Moritz 1994b), which in turn direct where broodstock are collected from and their progeny
are stocked (Bearlin & Tikel 2003, NSW Fisheries 2003). Similar requirements also apply to
translocating fish from one location to another. A priority should be to determine baseline
genetic data and units of conservation management, and to establish a genetic library to map
the distribution of native species and identify subpopulations (Bearlin & Tikel 2003, NSW
Fisheries 2003). This information can then be used to guide broodstock collections, locations
for the release of hatchery fish, and aid in the ability to recognise when genetic diversity is
being compromised. In addition, risk assessments of proposed stocking or translocation
programs should be done prior to their commencement (Bearlin & Tikel 2003).
Evaluating the genetic impact of stocking programs is very difficult. Monitoring and
evaluation programs are generally not powerful enough to detect genetic effects until many
generations later when it is too late to rectify (Waples 1991). Manipulative experiments in
natural settings are required to better understand the risk of intraspecific hybridisation,
introgression and inbreeding/outbreeding depression (Krueger & May 1991). However, the
limited experimental data currently available is an indication of the difficulty in doing such
research, but also the reluctance to experiment with natural populations. In the meantime, it
has been recommended that prior to initiating a stocking or translocation program it should be
determined whether or not it is appropriate, or if there is another approach (e.g. habitat
rehabilitation) that would remedy the situation (Bearlin & Tikel 2003). Models developed to
investigate the benefits of stocking fish have suggested that the greatest benefit to the wild
populations is habitat restoration and rehabilitation (Oosterhout et al. 2005). Stocking too
many fish may lead to redistribution of wild fish from good habitat and result in lower
survival of both wild and stocked fish. Continual stocking will eventually have negative
genetic impacts on the wild population. Waples (1991) suggested that the primary rule in
supplementation stocking programs should be “do no harm” to existing populations.
Disease, parasites, exotic organisms
Pathogenic organisms, such as bacteria, viruses and fungi, are an integral part of any natural
system. However, the risks associated with the introduction or translocation of organisms
that are potentially harmful to native fish populations and their environment as a result of
stocking activities are clearly recognised in Australia (see Department of Primary Industries
2003, NSW Fisheries 2003, Commonwealth of Australia 2005) and internationally (Hnath
1993, FAO 1996, ICES 2005). Despite potential risks, the impacts of introducing diseases,
51
parasites and exotic organisms unintentionally when stocking fish have historically received
little attention. Diseases in wild populations usually only receive attention when there are
mass mortalities that have affected population sizes (Stewart 1991). Thus, little is known
about the historic distribution of specific diseases – whether they are endemic or not (Waples
1991) and what impacts may have already occurred from past stocking events.
The threat of introducing pathogens through stocking of cultured fish is a major concern
because the nature of aquaculture practices makes aquaculture facilities prone to the
proliferation of disease (Taylor et al. 2005). In hatcheries, fish are typically reared at
unnaturally high densities, which can lead to increased nutrient and organic loads in the water
and thus proliferation of pathogens. Also, because of the unnatural rearing environment (e.g.
concrete raceways), high fish densities and handling stress, hatchery fish can be more
vulnerable to disease and parasites. Hatchery fish may also come in contact with disease via
their food, other species housed at the hatchery, and equipment used in transporting and
transferring the fish.
The introduction of an exotic pathogen or translocation of an endemic pathogen can alter the
‘pathogen status’ of the ecosystem and result in increased occurrence and severity of
infections, thus, reducing the ability of the affected population to compete for resources
(Department of Primary Industries 2003). Populations of fish exposed to a new pathogen can
be extremely susceptible leading to increased mortality, potentially to the point beyond
natural recovery and result in the loss of locally adapted populations. If present, individuals
resistant to the pathogen may help in the natural recovery of the population, but the original
genetic diversity of the wild population will have been reduced (see Genetics), potentially
leaving the population more susceptible to other impacts (e.g. competition) and stochastic
events (e.g. droughts). There is often a lag between the introduction of the pathogen or the
exotic organism and the expression of the clinical disease, or when the impacts of the
introduction are clearly recognised; therefore, the connection between stocking and the
occurrence of the disease is difficult to infer (Stewart 1991). For example, Myxobolus
cerebralis, the causative agent of whirling disease, was not known to be present in the
Madison River, Montana, USA until a 90% decline in the abundance of juvenile rainbow
trout alerted Montana Fish, Wildlife and Parks that something was wrong (Vincent 1996).
However, it is not known when M. cerebralis (native to Europe) was introduced into the
Madison River or by what means. In addition, some species may be carriers of pathogens
52
and not necessarily exhibit any symptoms, yet they can pass the disease to other more
susceptible species, thus, deducing the exact time and point of introductions becomes near
impossible.
There is a general lack of historical data and strong evidence linking specific stocking events
with establishment of exotic organisms in aquatic systems. It is, therefore, difficult to focus
specifically on diseases introduced through the stocking and translocation of native species,
especially if organisms are pathogenic (i.e. producing physical disease or relating to the
production of physical disease) across multiple species and families. However, there are
several examples that illustrate the potential impacts of pathogens introduced through
stocking and translocation. In 1975, the monogenean Gyrodactylus salaris was introduced
into Norwegian populations of Atlantic salmon through resistant Baltic smolts from Sweden
(Johnsen & Jensen 1986, Bakke et al. 1990). In less than a decade the parasite had been
detected in over 20 rivers in Norway, the effects of which resulted in significant declines in
both the production of parr and catches of adults (Johnsen & Jensen 1986).
In North America, whirling disease has become a major concern both in hatcheries and wild
populations. As such, it is one of two pathogens listed in USA federal legislation that limit
importation of salmonids into the USA (Hoffman 1990). Myxobolus cerebralis is native to
Europe and was first detected in cultured rainbow trout in Germany (Hedrick et al. 1998). It
is believed that M. cerebralis was initially introduced into North America in a shipment of
trout to a Pennsylvania hatchery in 1958. Since then, M. cerebralis has spread across North
America through transfer and stocking of infected hatchery fish, as well as unintentionally via
anglers waders, boats, etc. that are contaminated with spores and has been linked to the
severe declines (up to 90%) in wild trout populations, especially in the western USA
(Nehring & Walker 1996, Vincent 1996).
Additional examples of the unintentional spread of pathogens through the stocking and
transfer of fish include the stocking of ayu, Plecoglossus altivelis, in Japan. These fish were
infected with bacterial coldwater disease, Flavobacterium psychrophilum, which resulted in
the spread of the disease to wild populations (Amita et al. 2000, Iguchi et al. 2003).
Similarly, the transfer of starry sturgeons, Acipenser stellatus, from the Caspian Sea in 1934
lead to the introduction of the monogenean flatworm, Nizschia sturionis, into the Aral Sea,
53
which severely reduced populations of the native Aral or fringebarbel sturgeon, A.
nudiventris (Bauer et al. 2002).
In Australia, the discovery of a nodavirus, which causes viral encephalopathy and retinopathy
(VER) in Australian bass, Macquaria novemaculeata, (see Anonymous 2005) in a hatchery in
NSW that produced fish that have been translocated to a variety of watersheds in at least two
states is cause for concern. Mortality rate in Australian bass with VER is high and the
hatchery was de-stocked and decontaminated, costing about $200,000. The Department of
Agriculture Fisheries and Forestry (2004) noted that a rapid and vigorous response by
regulatory authorities is justified if an outbreak occurs in freshwater zones outside the normal
distribution of these viruses. Although no outbreaks have occurred, it is likely that the
nodavirus responsible for the NSW outbreaks that occurred during 2004 has been
translocated outside its normal range (Deveney, Primary Industries and Resources South
Australia pers. comm.).
Likely effects in the Murray-Darling Basin
The unintentional introduction of a pathogen with the stocking of native species will most
likely have a negative impact on wild populations. The severity of the impact will, however,
be species-specific depending on a number of factors including host specificity of the
particular pathogen, susceptibility of native species and how stressed wild populations are.
Within the Murray-Darling Basin there are examples of the spread of pathogens through
stocking and transfer of cultured fish. Epizootic haematopoietic necrosis virus (EHNV) is a
pathogen of considerable concern because some native species are susceptible to this virus.
Although its spread has primarily been attributed to the non-native redfin perch, Perca
fluviatilis, and has been documented in trout hatcheries in New South Wales (Langdon et al.
1988, Cadwallader 1996). The origin of EHNV is unknown, although it is believed that the
virus was not introduced with redfin perch or salmonids (Langdon 1989a). Native fish
species in the MDB that are highly susceptible to EHNV include Macquarie perch, silver
perch, and mountain galaxias, Galaxias olidus (Langdon 1989b, Cadwallader 1996). Murray
cod are potential carriers, whereas golden perch are not likely to be naturally susceptible.
Langdon (1989) notes that this example illustrates the hazards of transmission of diseases
between families and species of fish and the risk of translocating fish (Cadwallader 1996).
54
Another example of a pathogen able to infect multiple species including native freshwater
fish is Chilodonella cyprini. This protozoan has spread within Victoria through the stocking
of infected trout. But like EHVN, C. cyprini can infect many fish species native to the
Murray-Darling Basin including galaxiids, Australian smelt, Retropinna semoni, Macquarie
perch, Murray cod, trout cod, river blackfish, Gadopsis marmoratus, striped gudgeon,
Gobiomorphus australis, and southern pygmy perch (Cadwallader 1996).
In addition to disease causing organisms, non-pathogenic organisms can unintentionally be
introduced into a system through the stocking and translocation of fish. These organisms
may be present in or on the fish being stocked or in the water used to transport the fish from
the hatchery to the stocking site (Department of Primary Industries 2003). Examples from
Australia of fish species that have established populations after being accidentally released
during stocking include common carp, redfin perch, Murray cod, trout cod, Macquarie perch,
and eastern mosquito fish, Gambusia holbrooki (Department of Primary Industries 2003).
Although, the exact nature of these accidental releases are not elaborated upon, one could
envision any number of scenarios in which unwanted individuals of another species are
included in a lot of stocked fish either mixed within the rearing ponds/raceways at the
hatchery, or inadvertently left in the water used to transport the fish to the stocking site, or in
the case of translocated fish inadvertently included in the sample of fish taken from the
source location.
How are impacts of introduced pathogens evaluated (or controlled for/mitigated)?
Understanding and evaluating the impact of introduced pathogens on wild populations is
extremely difficult. Quantification of the disease status in wild populations is hindered
because in the wild sick fish often die or are preyed on and are therefore not available to
sample. In addition, wild fish can move and migrate over time, thus, dispersing pathogenic
organisms (Moffitt et al. 2004). Long-term monitoring of the health of wild populations is
often lacking, and standard sampling methods and good sample designs are absent (Williams
& Moffitt 2001, 2003 address these issues).
The occurrence and spread of disease in cultured fish exceeds that of wild populations
because of the high densities and artificial rearing conditions. Thus, pathogens are easier to
control and prevent at the hatchery before they are transferred into natural systems. Good
hatchery practices are the primary means of preventing the unwanted spread of pathogens,
55
invertebrates, and fish (Moffitt et al. 2004). Covered rearing facilities limit interaction
between fish and vectors, such as birds, that may transmit pathogens or act as intermediate
hosts. In the case of M. cerebralis it was found that rearing salmonids in concrete raceways
as opposed to earthen ponds limited the occurrence of whirling disease in hatcheries (Hedrick
et al. 1998). Subsequently, it was found that the oligochaete worm, Tubifex tubifex, which
was naturally abundant in earthen ponds, acted as a secondary host allowing M. cerebralis to
complete its lifecycle. The risk of introducing pathogens from hatcheries to the surrounding
environment can be minimised by properly treating the effluent, waste products, and transport
water. Quarantine practices and routine disease testing for both broodstock entering the
hatcheries and progeny leaving the facilities are critical to reduce the introduction of new
pathogens into the hatchery and to other locations.
To ensure that good hatchery and stocking practices are established and maintained, it is
essential that the appropriate regulatory agencies have inspection and certification programs
for hatcheries (Stewart 1991), including certification of hatcheries, targeted monitoring and
surveillance programs, and disease zoning policies (Department of Primary Industries 2003).
In conjunction, research should focus on determining processes that lead to the introduction
of pathogens and their impacts on natural populations. Methods of minimizing impacts
should be investigated via monitoring of healthy wild populations (Stewart 1991), so that
distribution of pathogens and acceptable infection levels are known (Blankenship & Leber
1995). The number of existing control and monitoring programs by various agencies and
organizations indicates the worldwide awareness of the threat of introduced pathogens into
aquatic systems. Agencies with an interest in pathogen control include the Food and
Agriculture Organization of the United Nations, the Great Lakes Fishery Commission, and
the Pacific Northwest Fish Health Protection Committee (Krueger & May 1991). In
Australia, initiatives at both the state and national level have attempted to address the risk of
pathogen spread through fish stocking. These include: Aquaplan –Australia’s national
strategic plans for aquatic animal health, which is being developed by the Australian
Government Department of Agriculture, Fisheries and Forestry (Commonwealth of Australia
2005) and the Hatchery Quality Assurance Program developed by the New South Wales
Department of Primary Industries (NSW Fisheries 2003, Rowland & Tully 2004).
It must be realized that despite establishing and enforcing good hatchery practices,
monitoring of broodstock, their progeny, and stocked systems the risk of spreading pathogens
56
and other organisms will never be eliminated. Prevention and vigilance is, however, likely to
minimise impacts of pathogens on fish populations because once introduced it is near
impossible to eliminate a pathogen.
Ecosystem level effects
Ecosystem alteration from stocking of fish is extremely difficult to demonstrate and has
mostly been attributed to introduced species rather than native species. Ecosystem effects
may include reduction in clarity of water from fish stirring up the bottom while feeding and
alteration of zooplankton communities that reduce growth and survival of planktonic-feeding
species, thus, manifesting in indirect effects on native species.
Exceeding the carrying capacity of an ecosystem
It is often viewed that a maximum number of fish can be supported in a given area (often
referred to as carrying capacity). Stocking fish into a system would then help abundances of
fish to achieve their carrying capacity, but would only be applicable where the number of fish
are below the carrying capacity indicating that the habitat and food resource is under-utilised
(Aprahamian et al. 2003). If, however, attempts are made to increase populations beyond the
carrying capacity then higher mortality and/or emigration may make stocking appear
ineffective (Aprahamian et al. 2003). Carrying capacity may exhibit spatiotemporal
variability depending on flow regime, food availability and temperature, as well as
ontogenetic changes in habitat use or feeding of the stocked species (Aprahamian et al. 2003).
It will also depend greatly on exploitation of the stocked population, such that at high
exploitation rates, higher stocking densities may be justifiable (FAO 1999). Most stocking
programs assume that the system is below the carrying capacity, but there are few empirical
studies that have assessed carrying capacities, although it is acknowledged that this is
problematic and often not feasible. If the carrying capacity is assessed then this is often done
at one time and place, yet considerable year-to-year and site-to-site variability may exist
(Aprahamian et al. 2003). Other approaches to assessing carrying capacity include habitat
models (e.g. HABSCORE for salmon and trout – Milner et al. 1998) and analysis of time
series data.
Potential exists for the carrying capacity of the system to be exceeded given repeated
stocking (Welcomme & Bartley 1998). In such circumstances, other measures to increase the
57
sustainability of the water mass may be required (e.g. fertilisation of the water, modification
of habitat), although most of these methods have been undertaken in reservoirs, dams or
lakes.
Likely effects in the Murray-Darling Basin
We are not aware of any studies that have assessed the carrying capacity of a section of the
MDB prior to stocking, although the stocking density of impoundments and dams is often
based on surface area. For example, Queensland stocking rates are based on a maximum of
100-200 fish per hectare of stocking area. It should however be noted that most stocking
groups do not stock anywhere near the maximum number permitted. Stocking generally
occurs in areas where numbers of fish have been reduced or from public pressure when
anglers are not catching fish. The combination of carrying capacity and resource limitations
may exacerbate impacts. If resources are reduced in areas then there is potential to exceed
the carrying capacity of the system, even by stocking only a few fish.
Trophic cascades/ecosystem shifts
An understanding of trophic interactions is essential to predict the potential effects of
stocking fish species. Increasing abundance of fish at different trophic levels will have
consequences for lower trophic levels. Increased piscivore abundance may lead to reduced
abundances of planktivores, increased abundance of herbivores, and reduced phytoplankton
biomass through a cascade of interactions (Carpenter et al. 1985) (Fig. 8). Likewise, if the
abundance of zooplanktivores was increased, a similar cascade of effects (e.g. reduction in
abundance of herbivorous zooplankton, an increase in phytoplankton, decrease in water
clarity) could be initiated. Specific growth rates at each trophic level may show the opposite
(Carpenter et al. 1985). An intermediate biomass of predators will lead to maximum
productivity at any given tropic level (Carpenter et al. 1985). Research on trophic cascades
in Australia has largely focused on lakes or impoundments and very little work has focused
on streams. Most of the research has focused on only one trophic group being fish.
Likely effects in the Murray-Darling Basin
Many of the species being stocked are piscivores (e.g. golden perch, Murray cod).
Theoretically, trophic cascades may be found if stocked species reduce abundance of prey
species. Indirect effects on other species would then be likely. Whether such cascades affect
multiple trophic levels of fish will depend on whether piscivores, planktivores and herbivores
58
are present. A number of MDB fish consume small fish, while others feed on zooplankton
(see Table 5) and therefore trophic cascades would seem plausible within this system.
Extinctions
An increase in the abundance of released fish and ecosystem shifts may result in local
extirpation of a species. Such extinctions are most likely to be localised. Several studies
have reported declines in native fishes after stocking of fish, but in the majority of cases the
stocked species were not native to the area (e.g. trout) (see details in McDowall 1987). We
are not aware of any extinctions resulting from stocking of native species, although the
distribution and abundance of potential prey species in areas where predatory species are
released is often poorly known. If fitness of populations declines or if stocked fish are
genetically different to the receiving population then there is also potential for extinctions
(see Genetics).
Conclusions
Stocked fish will have overarching negative impacts through competition with and predation
on wild fish. Furthermore, any possible short-term benefits from fish stocking are likely to
eventually lead to a negative impact: stocked fish may act as food for wild fish, but this will
eventually lead to starvation or prey switching in wild populations. Conceivably, impacts
may be reduced if fish are stocked in low numbers at multiple locations, however, to what
degree changing stocking procedures reduces impacts is largely unknown.
Behavioural impacts of fish stocking are difficult to predict and detect. The information
available suggests that fish reared in hatcheries have behaviours that differ from wild fish.
Differences in aggression and activity between stocked and wild fish are likely to impede
both stocking efforts and exacerbate direct competition between fish. Hatchery-reared fish
may also exhibit poor post-stocking survival and reproduction because of morphological,
physiological or behavioural problems (White et al. 1995). Within the MDB, there have been
no investigations of behavioural differences between stocked and wild fish; thus, the extent of
impact is relatively unknown. Similarly, despite possible competition and predation of
stocked fish on wild fish, there remains little conclusive experimental evidence of impacts for
any species within the MDB. Experiments using species from the MDB are recommended to
provide conclusive evidence of impacts. These experiments should be conducted in river and
floodplain habitat rather than just in reservoirs or dams.
59
The genetic impacts of hatcheries and stocking of hatchery fish have received much attention;
however, much of the literature is theoretical or deals mainly with salmonids. Information on
potential genetic impacts of stocked native fish on wild fish within the MDB can only be
gleaned from this body of literature and from limited studies that have been done within the
MDB. Genetic changes in wild populations as a result of stocking are likely and sometimes
desirable (e.g. conservation stocking of a small inbred population). However, genetic
changes are most often undesirable and negative. Genetic changes in wild populations can be
both direct (e.g. hybridisation, introgression, inbreeding and outbreeding depression) and
indirect (e.g. excess harvest of wild stock, introduction of disease, predation). Within the
hatcheries, changes can also occur and include loss of genetic variation, stock mixing,
artificial selection, and domestication. Good hatchery and stocking protocols are an essential
part of any supplementation or conservation stocking programs. However, indications are
that current practices in hatcheries supplying fish for MDB stocking are inadequate. The
limited data available on the genetic population structure of native fish species in the MDB
indicates that complex genetic structuring exists, but the extent is likely to vary with how
widespread the species is (or was) and specific life history traits (e.g. dispersal ability, habitat
preference, spawning migrations). A better understanding of the genetic composition of
populations of native fish species in the MDB is critical in order to minimise the potential
genetic impacts of stocking and to define significant evolutionary and management units,
which in turn will direct where broodstock are collected from and where their progeny can be
stocked. In addition, genetic information is essential for monitoring the impact of stocking
programs on not only the species being stocked, but also other species in the community to
protect unique and significant evolutionary units within species.
Pathogens and parasites are an integral part of any natural system, but hatcheries and stocking
practices can promote the occurrence and spread of disease and parasites, and pose a potential
risk to wild populations receiving hatchery-reared fish. Little is known about the historic
distribution and spread of these organisms and the impacts that they have had on wild
populations as a result of past stocking events, especially in the MDB. There are a number of
documented cases of the introduction and spread of diseases and parasites from stocked to
wild fish, which have resulted in subsequent declines in wild populations (e.g. Gyrodactylus
salaris in Norwegian Atlantic salmon populations and whirling disease in wild trout
populations in Western USA). The risk of introducing and spreading of pathogens through
60
stocking of native fish within the MDB is high, but can be prevented or minimized by
ensuring that good hatchery and stocking practices are followed by all hatcheries and groups
participating in the stocking of fish. To ensure that good hatchery and stocking practices are
established and maintained, the appropriate State and Commonwealth agencies need to
institute effective control and monitoring programs for both the hatcheries and the river
systems that are receiving fish. This may include the establishment of hatchery certification
and fish health monitoring programs. However, it is difficult to understand and evaluate the
impact of introduced pathogens in wild populations of fish without long term monitoring
programs and good sample designs with standard sampling methods.
Stocking needs to be considered within an ecosystem context, if the effects of stocking are to
be predicted (Vollestad & Hesthagen 2001). Releasing large numbers of hatchery-reared
juveniles may initially increase abundance of the stocked species, but does not necessarily
lead to long term population level increases (Heppell & Crowder 1998). The life history of
the fish to be stocked also needs considering, if stocking is to be an effective management
tool. Stocking may affect the genetics of wild populations and this should be considered
early on so that genetic information can be obtained prior to stocking; practices can then be
constantly reviewed/monitored. Likewise, release of diseased fish into the wild may
contaminate wild stocks and also lead to low survival of stocked fish. Ideally, fish should be
certified as being free from disease prior to release and hatchery practices should aim to
minimise diseases.
Because a number of risks and problems can be associated with stocking programs, it is
essential that these risks are reduced as much as possible. A level of risk can be associated
with each of the broad potential impacts from stocking and occur at both the stocking site and
broader spatial scales (see Table 8). Obviously, the level of risk depends on which species,
population or community (e.g. RAMSAR sites) are being investigated. We suggest that a
responsible approach to stocking would embody a framework such as that proposed by
Blankenship and Leber (1995) and outlined below (Table 9, see also Taylor et al. 2005). A
variety of species are already used widely in stocking programs. If additional species are to
be stocked, selection criteria need to be identified, and species prioritised. This could be
done via workshops, community surveys, interviews with local experts (see Blankenship &
Leber 1995). It is essential that management plans are developed, and if necessary these
incorporate other aspects such as habitat restoration that may assist stocking efforts (see
61
Taylor et al. 2005). Quantitative measures of success need to be defined, which will require a
method to mark hatchery-reared fish that will allow them to be tracked (see below).
Hatchery fish need to be identified from wild fish if the effectiveness of stocking is to be
determined. At present a variety of methods are available (Crook et al. 2005), but many of
these methods are not feasible for large numbers of fish or non-government hatcheries.
Several projects are currently underway to investigate quick and efficient methods for
marking fish (). Optimum release strategies need to be determined, even though it can be
logistically difficult to quantify survival of fish especially in river systems. Each stocking
could be used as a pilot-scale experiment where survival could be monitored and release
strategies (e.g. size at release, time of release) that maximise survival determined. Costbenefit analyses are also required to predict the value of stocking. After each stocking event
the process should be continually assessed and improvements made (i.e. an adaptive
management framework used).
According to Walters & Martell (2004) key monitoring and experimental requirements
should be to mark all stocked fish (or at least a high and known proportion), mark wild fish
that are of similar size to the stocked fish at all of the stocking locations, experimentally vary
hatchery releases over years and areas, monitor changes in recruitment, production and
fishing effort in the fishery rather than just percentage contribution of hatchery fish to
production (in the case of stocking recreationally fished species), monitor changes in fishing
mortality rates of both wild and hatchery fish, and monitor reproductive performance of
hatchery-origin fish and hatchery-wild hybrid crosses in the wild. While some of these
monitoring and experimental requirements have been done in some cases, we are not aware
of any studies that have addressed all requirements.
Given the continued increase in stocking of hatchery-reared fish and the potential for
interactions with wild fish, it is essential to take a responsible approach and to monitor and
experimentally evaluate any stocking program. Only with such an approach will the success
of stocking programs be evaluated and the risks mitigated.
62
Table 1. Reasons for stocking (adapted from Aprahamian et al. 2003).
Reason
Mitigation
Restoration
Enhancement
Creation of new fisheries
Research and development
Conservation
Description
Stocking is conducted to mitigate lost
production due to a scheme or activity
(e.g. hydro-development) which can not
be prevented or removed
Stocking that is carried out after the
removal or reduction of a factor (e.g.
water quality, habitat improvements) that
has been limiting or preventing natural
production
Stocking which is carried out to
supplement an existing stock where the
production is less than the water body
could potentially sustain (or harvest is
high)
Stocking which aims to transfer fish into
new water bodies or when new species
are introduced into existing fisheries
Stocking which aims to address particular
fisheries management issues
Stocking which aims to conserve the
stock of fish
63
Table 2. Numbers of four key native species stocked into different catchments of the MDB.
The table includes stocking data up to and including 2001 for Queensland, 2002 for New
South Wales, and 2004 for both the ACT and Victoria. Note no native stocking is legally
undertaken in the MDB section of South Australia (excluding farm dams).
State
Catchment
Australian Capital Territory (ACT)
Upper Murrumbidgee
New South Wales (NSW)
Darling
Gwydir
Lachlan
Macintyre
Macquarie
Moredun
Murray
Murrumbidgee
Namoi
Upper Murray
Upper Murrumbidgee
Sub-total for NSW
Queensland
Balonne
Border Rivers
Condamine
Macintyre
Maranoa
Severn
Warrego
Sub-total for Queensland
Victoria
Avoca
Broken
Campaspe
Goulburn
Kiewa
Loddon
Mallee
Ovens
Upper Murray
Wimmera
Sub-total for Victoria
Total for Murray-Darling Basin
Golden
perch
Murray
cod
1,393,015
546,316
18,900
140,879
2,804,685
3,200,531
2,214,505
2,093,175
3,642,971
1,906,230
1,524,500
160,000
17,706,376
17,520
10,212
484,775
669,360
480,735
3,086
284,888
511,183
372,813
87,600
36,000
2,958,172
774,277
540,405
2,314,944
1,436,111
80,921
856,905
323,652
6,327,215
40,661
7,250
85,895
54,350
9,400
108,633
8,500
314,689
397,000
633,000
998,800
1,000,420
927,494
135,000
258,785
177,200
896,285
5,423,984
30,850,590
Silver
perch
Trout cod
561,138
93,240
1,788,600
1,179,513
1,102,150
17,200
237,600
1,542,245
1,150,760
193,400
199,675
7,393,943
63,800
340,940
63,600
485,540
166,647
29,750
491,632
179,789
16,667
95,399
979,884
30,000
278,125
291,625
813,300
80,000
411,125
209,122
30,000
55,900
360,250
192,365
36,200
2,721,812
66,200
6,540,989 9,001,165
0
15,900
10,900
58,550
243,331
56,403
385,084
963,864
64
Table 3. Extent of impacts associated with abundance and behavioural responses to stocking
fish, including both spatial and temporal aspects.
Potential impact
Extent of impacts in MDB
Spatial
Predation
(a) on stocked fish
(b) by stocked fish on
natives and also on
exotics
Competition for food
survival.
Localised impacts to area of
stocking, unless fish move,
which will cause broad
competition.
Competition for habitats
Localised impacts to area of
stocking, unless fish move,
which will cause broad
competition.
Displacement of wild stock
(a) physical displacement
(b) numerical displacement
Localised (within site) and
broad scale (within basin)
displacement can occur. Spatial
extent dependent on degree of
competition.
Localised and broad impacts
likely, depending on extent of
stockings and subsequent fish
movement.
Expansion of species range
(a) positive impacts of recolonisation
(b) negative impacts of
competition
Habitat alteration
Behavioural changes
Temporal
Localised to area of stocking, Pulse impacts after stocking. Press
impacts if predation increases fish
broad effects detected if
growth, development, and therefore
species move.
Minimal evidence. Impacts
likely to be localised.
Localised impacts to area of
stocking, unless fish move,
which will cause broad scale
behavioural effect.
Change in abundance of stocked
species
Broad impacts likely if stocked
species expand species range.
Incidental captures
Minimal evidence. Impact
likely to be both localized and
broad depending on wild fish
movements in relation to
stocking (i.e. mixing with
stocked fish)
Pulse impacts lessened if stocked fish
have different diets or use different
food resources to wild fish. Press
impacts likely if stocked fish persist
to compete with wild fish for
resources, which may cause
starvation, diet switching and
displacement.
Pulse impacts lessened if stocked fish
have different habitat requirements or
are ill adapted to habitat. Press
impacts likely if stocked fish persist
to compete with wild fish for habitat.
Physical displacement likely to have
both pulse and press impacts.
Numerical displacement will have
press impacts.
Press impact of re-colonisation if fish
survival occurs. Pulse and press
negative impacts of competition.
Minimal evidence. Impacts likely to
be both pulse and press.
Immediate pulse impacts likely, press
impacts may develop over
consecutive cohorts of fish.
Aggressive behaviour may be
adaptive due to competition for
resources, causing press impacts.
Immediate pulse impacts. Press
impacts will occur if stocked species
abundance remains high.
Minimal evidence. Pulse impacts
directly after stocking. Press impacts
if fish harvesting continues after
stocked fish have been removed.
65
Table 4. Appropriate experimental designs manipulating both intra- and inter-specific
competition between two fish species. Each treatment outlines appropriate densities of fish
species, and the comparisons of treatments that lead to conclusions about intra- and interspecific competition. Designs are for (A) two manipulated densities, and (B) three
manipulated densities. * For a stocking procedure, species A can represent wild fish, and B
represents stocked fish. In this case, the designs test three levels of intra-specific competition;
e.g. for (A) two densities manipulated: 1 vs 2 (wild on wild), (4 vs 5) stocked on stocked, and
(1 vs 3, and 3 vs 4) stocked on wild (and vice versa). Adapted from Underwood (1986).
(A) Two densities manipulated
Treatments
1
2
Density:
Species A
10
20
(2 × 10A)
Species B*
-
3
4
5
10
-
-
10
10
20
(2 × 10B)
Comparisons: Intra-specific Competition
1 vs 2 (A on A)
4 vs 5 (B on B)
Inter-specific Competition
A’s in 1 vs 3 (B on A)
B’s in 3 vs 4 (A on B)
(B) Three densities manipulated
Treatments
1
2
3
Density:
Species A
10
20
30
4
5
6
7
8
9
10
10
20
-
-
-
Species B*
10
20
10
10
20
30
-
-
-
Comparisons: Intra-specific Competition
1 vs 2 vs 3 (A on A)
7 vs 8 vs 9 (B on B)
Inter-specific Competition
A’s in 1 vs 4 vs 5 (B on A)
B’s in 4 vs 6 vs 7 (A on
B)
66
9
9
9
9
Worms
Insects
Fish
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
Other vertebrates
Molluscs
9
Crustaceans
Zoobenthos
Zooplankton
Detritus
Phytoplankton
Species
Ambassis agassizii
Anguilla australis
Anguilla reinhardtii
Craterocephalus amniculus
Craterocephalus fluviatilis
Craterocephalus stercusmuscarum fulvus
Pseudaphritis urvilli
Nematalosa erebi
Hypseleotris sp.
Mogurnda adspersa
Philypnodon grandiceps
Gadopsis bispinosus
Gadopsis marmoratus
Galaxias brevipinnis
Galaxias fuscus
Galaxias maculatus
Galaxias olidus
Galaxias rostratus
Galaxias truttaceous
Plant material & algae
Table 5. Summary of known dietary information for fish from the Murray-Darling Basin (adapted from Froese & Pauly 2005). Prey items are
listed in each column. Insects includes aquatic and terrestrial insects and larvae, other vertebrates includes birds, mammals and reptiles. Note
also that adults of Mordacia mordax are parasitic.
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
67
Geotria australis
Melanotaenia fluviatilis
Mordacia mordax
Nannoperca australis
Nannoperca obscura
Maccullochella macquariensis
Maccullochella peelii peelii
Macquaria ambigua
Macquaria australiasica
Macquaria colonorum
Neosilurus hyrtlii
Tandanus tandanus
Retropinna semoni
Bidyanus bidyanus
Leiopotherapon unicolour
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
68
Table 6. Potential genetic effects of stocking hatchery fish on wild populations, including
causes of these effects and whether they are positive (+) or negative (-). References provide
examples where empirical evidence is available for the cause or effect, but may not indicate
that a cause and effect relation has been documented. Adapted from Campton (1995).
Genetic effect
Cause of effect
References
+ or –
Straying
No definitive evidence
–
(Reisenbichler &
Phelps 1989)
–
No definitive evidence
–
(Bearlin & Tikel 2003)
–
Introgressive
hybridisation and
outbreeding
depression
(Gharrett et al. 1999,
Gilk et al. 2004)
–
Genetic changes in
hatchery stock
See hatchery effects
–
Introgressive
hybridisation
No definitive evidence
+
Predation
(Garman & Neilsen
1982, Beamish et al.
1992, Kellison et al.
2003)
–
Competition
(Nickelson et al. 1986,
Lachance & Magnan
1990b, Fjellheim et al.
1995)
–
Overharvest in mixed
fishery
(Evans & Willox 1991)
–
Direct
Decrease in between
population variation
Time and location of
release/stock transfer
Decrease in within
population variation
Genetic swamping
Low effective
population size in
hatchery
Decrease in fitness
Increase in
between/within
variation and fitness
Indirect
Decrease in
abundance
No definitive evidence
Disease transfer
–
69
No definitive evidence
–
Broodstock extraction
or mining
Increase in
abundance
Fisheries targeted on
stocked fish
(Mezzera & Largiader
2001)
Successful rebuilding
of population through
hatchery
supplementation
No definitive evidence fo +
long-term self-sustaining
population
Loss of genetic
variation (random
processes)
Genetic drift and
small effective
number of breeders
(Allendorf & Phelps
1980, Waples & Teel
1990, Bouza et al.
1997, Iguchi et al.
1999, Calcagnotto &
Toledo 2000)
–
Introgression of
exogenous genes
Stock mixing
(Reisenbichler &
Phelps 1989, Nielsen et
al. 1994)
– ( could be
+ if adding
to inbred
population)
Interspecific
hybridization
(Gross et al. 2004)
–
Domestication
(Reisenbichler &
McIntyre 1977,
Fleming & Einum
1997, Iguchi et al.
1999, Glover et al.
2004)
–
Artificial selection
(Danzmann et al. 1994, –
Yamamoto & Reinhardt
2003)
+
Hatchery
Phenotypic change
in life history or
other quantitative
characteristics
70
Table 7. Summary of studies of genetic structuring in native fish species of the MurrayDarling Basin. Adapted from NSW Fisheries (2003).
Species
Structure
Marker
References
Study location
Silver perch
Bidyanus bidyanus
insufficient
data
allozymes
Murray-Darling
Basin
yes
mtDNA
(Keenan et al. 1996)
(Bearlin & Tikel
2003)
Two-spined
blackfish
Gadopsis bispinosus
yes
mtDNA
Murray-Darling
Basin
Trout cod
Maccullochella
macquariensis
yes
mtDNA
(Ovenden et al.
1988, Waters et al.
1994)
(Bearlin & Tikel
2003)
Murray cod
Maccullochella
peelii peelii
insufficient
data
mtDNA
(Bearlin & Tikel
2003)
Murray-Darling
Basin
Golden perch
Macquaria ambigua
yes
microsatellites
allozymes
Murray-Darling
Basin
Tikel and Nock,
unpublished data
(Musyl & Keenan
1992, Keenan et al.
1996)
Eastern
drainage:
Fitzroy Basin
Western
drainages:
Bulloo River
Lake Eyre Basin
Murray-Darling
Basin
Purple-spotted
gudgeon
Mogurnda adspersa
yes
mtDNA
(Hurwood & Hughes
1998)
Eastern
drainages:
Southern pygmy
perch
Nannoperca
australis
yes
allozymes
(Hammer 2001)
South-eastern
Australia
including
southern MDB
Australian smelt
Retropinna semoni
yes
allozymes
Crook, Victorian
Department of
Sustainability &
Environment, pers.
comm.
Murray-Darling
Basin
Hammer, University
of Adelaide, pers.
comm.
South-eastern
Australia
including
5 catchments in
NE QLD
mtDNA
microsatellites
71
southern MDB
Freshwater catfish
Tandanus tandanus
yes
allozymes
(Keenan et al. 1996,
Musyl & Keenan
1996, Jerry &
Woodland 1997)
Murray-Darling
Basin
Eastern
Drainages
72
Table 8. Summary of potential impacts of commonly stocked fish (which are predatory as
adults) on different types of fish, populations and communities within the MDB.
The level of risk (negligible, low, medium or high) is given for each section of the table at the
stocking site and adjacent habitats (in brackets; only reported for species and populations).
See sections of report and Tables 3, 6 and 7 for further details of potential impacts.
Recreationally important predatory species includes Murray cod, trout cod and golden perch;
small fish (which are widespread and abundant in the Basin) includes un-specked
hardyheads, smelt, Murray rainbow fish, bony herring, flathead gudgeon, carp gudgeons;
endangered and vulnerable species includes Murray hardyhead, trout cod, silver perch, Yarra
pygmy perch, Macquarie perch; endangered populations includes southern pygmy perch,
southern purple spotted gudgeon, olive perchlet; endangered ecological communities includes
the lower Murray river and lower Darling river, significant ecological assets includes
Barmah-Millewa forest, Gunbower and Koondrook-Perricoota forest, Hattah Lakes, Chowilla
Floodplain and Lindsay-Wallpolla Islands system, Murray Mouth, Coorong and Lower
Lakes, River Murray Channel; and Ramsar sites includes Currawinya Lakes National Park
(Qld), Macquarie Marshes Nature Reserve (NSW), Barmah Forest (Vic), Gunbower Forest
(Vic), Hattah-Kulkyne Lakes (Vic), Kerang Lakes (Vic), Lake Albacutya (Vic), Coorong and
Lakes Alexandrina and Albert (SA), Riverland, including Chowilla Floodplain System (SA),
Ginini Flats, Namadgi National Park (ACT).
Abundance
&
behavioural
responses
High (low to
medium)
Genetics
Disease/parasites Ecosystem
level effects
High (high)
High (medium)
Medium (low
but could be
higher
depending on
movement of
stocked fish)
Small fish
(e.g. potential
prey species)
(not likely to
be part of a
stocking
program)
Low
(negligible)
Low
(negligible)
Medium to low
(low to
negligible)
depending on
whether disease is
infectious across
multiple species
Low (e.g.
extinction) to
medium (e.g.
trophic
cascade)
(low)
Endangered &
vulnerable
species (likely
to be stocked
for
conservation)
High (medium
to high as
most rare
species have
restricted
distributions)
High +/depending on
whether
conservation
or recreational
stocking (low)
High (high)
Medium to
high (low)
Endangered
populations
High (medium High +/to low)
depending on
whether
conservation
High (medium)
Medium to
high (low)
Recreationally
important
predatory
species (likely
to be stocked
for recreation)
73
or recreational
stocking
(medium if
conservation
stocking, but
low if
recreational
stocking)
Endangered
ecological
communities
Potentially
high (+/depending on
whether
conservation
or recreational
stocking)
Difficult to
predict
because of
limited
information,
but direct
genetic impact
likely to be
low to
negligible, but
may be
potential
indirect
impacts which
will depend
on level of
risk of other
effects
Potentially
medium to high
depending on
whether disease is
infectious across
multiple species
Difficult to
predict, but
potentially
high if lose
critical
species
Significant
ecological
assets
Low to
medium
(potentially
high + if
conservation
stocking)
Same as
above
Same as above
Same as
above
Ramsar sites
Same as
above
Same as
above
Same as above
Difficult to
predict, but
potentially
high if lose
critical
species or
birds feed on
stocked fish
74
Table 9. Considerations for stock enhancement programs proposed by Blankenship and
Leber (1995).
Principles
(1) Prioritize and select target species for enhancement
Workshop to identify and rank selection criteria
Community survey to solicit opinion on selection criteria and generate list of possible
species for stock enhancement
Interviews with local experts to rank candidate species with regard to selection criteria
Workshop where results are discussed and consensus sought
(2) Develop a species management plan
Clearly identify goals and objectives of stocking program in terms of testable
hypotheses
Identify genetic structure of wild stocks targeted for enhancement
Evaluate performance and operation of stocking plan
(3) Define quantitative measures of success
Indicate an explicit indicator of success
Indicate what marking and assessment system will be used for tracking hatchery fish
(4) Use genetic resource management
Identify genetic risks and consequences of enhancement
Define an enhancement strategy
Implement genetic controls in the hatchery (e.g. sufficiently large and representative
broodstock population) and a monitoring and evaluation program for wild stocks
(prior to, during and after enhancement)
Outline research needs and objectives
Develop a feedback mechanism
(5) Use disease and health management
Certify that fish are free from bacterial and viral infections and parasites prior to
release
(6) Form enhancement objectives and tactics
Consider ecological factors that may contribute to success or failure of hatchery
releases
Consider physiological and behavioural factors that may affect hatchery fish once
released
(7) Identify released hatchery fish and assess stocking effects
Indicate method for marking hatchery fish
Determine impacts of hatchery fish on wild populations
(8) Use an empirical process to define optimum release strategies
Quantify and control effects of release variables through pilot releases
(9) Identify economic and policy objectives
Use cost-benefit analyses to determine value of enhancement
75
(10) Use adaptive management
Assess process and allow changes over time
76
Figure 1. Map of Murray-Darling basin showing major river systems.
.
77
5
Numbers (millions)
4
Total numbers
3
Golden perch
2
1
Murray cod
Silver perch
Trout cod
0
60
19
1
5
96
70
19
75
19
80
19
85
19
1
0
99
95
19
00
20
05
20
Financial year
Figure 2. Numbers of native fish stocked into the Murray-Darling Basin. Small numbers of
freshwater catfish, Tandanus tandanus, and Macquarie perch, Macquaria australiasica, have
also been stocked (see also Table 2).
78
180
8
160
6
140
Fish weight
(g)
% Recovery
120
4
100
80
Fish genetic origin
Domestic
Hybrid (F1 x Dom) 60
Wild (F1)
2
40
0
Inter- Inter+ Intra-
Inter- Inter+ Intra-
Figure 3. Inter- and intra-specific competition assessed as changes in mean fish weight and
% recovery (mean ± SEM) of three different breeds of stocked brook trout, Salvelinus
fontinalis, two years post introduction. Fish breeds reared from domestic and wild trout were:
domestic trout, hybrid trout (male wild × female domestic), and wild (F1 wild trout reared in
captivity). Differences between trout breeds provides a test of inter-specific competition, in
the absence of control lakes (trout stocked to lakes void of fish competition). Data adapted
from Lachance and Magnan (1990b).
79
Dietary components (% volume)
Stocked
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
Wild
June
September
0
1
2
3
0
1
2
3
Years in Lake
Figure 4. Dietary differences between stocked and wild brown trout, Salmo trutta, in a
subarctic lake. Data represent gut contents in June and September of fish that have spent 0, 1,
2, and 3 years within the lake. Hatched bars = whitefish (Coregonus sp.); shaded bars =
insects (order Trichoptera with aquatic larvae); clear bars = surface insects; black bars =
other. Data adapted from Kahilainen and Lehtonnen (2001).
80
Proportion of fish eaten
1.0
Wild
0.8
Hatchery reared
0.6
0.4
0.2
0.0
Beach
Marsh
Habitat Type
Figure 5. Predation of tethered wild and hatchery-reared summer flounder, Paralichthys
dentatus, as indicated by the proportion of fish eaten over three hours. Tethering was done in
beach and marsh habitats to elucidate habitat effects of predation. Data adapted from Kellison
et al. (2003).
81
INDIRECT GENETIC
EFFECTS
EXOGENOUS
POPULATION
DIRECT GENETIC
EFFECTS
PRE-INTRODUCTION
NATURAL
POPULATION
overharvest through
mixed-stock fisheries
resulting from
cultured releases
Effects on
natural
populations
by means of
EXOGENOUS
POPULATION
introgressive
hybridisation
disease introduction
from resistant carriers
outbreeding
depression
habitat reduction and
fragmentation through
naturalization
modified growth,
survival,
reproduction,
and behaviour
wasted reproduction
from non-introgressive
hybridisation
Reduced population size and
fragmentation with resulting
loss of genetic variability
Greatly increased likelihood
of displacement and extinction
Persisting
capabilities
for migration,
geneflow, and
adaptation
Hybrid swarms
replace original
populations and
promote spreading
gametic degradation
Modified selection regimes
or remanent subgroups
Figure 6. Some direct and indirect genetic effects of releases of exogenous, hatchery-reared
fish on native population. Figure from (Utter 2003).
82
3.0
80
hatchery
wild
2.5
60
1.5
40
% survival
% survival
2.0
1.0
20
0.5
0
0.0
egg-smolt
smolt-adult
life history stage
Figure 7. Egg-smolt and smolt-adult survival of hatchery and wild spring chinook salmon,
Oncorhynchus tshawytscha, from the Deschutes River, Oregon, USA. Wild data are for
brood years 1975-81; hatchery data are for brood years 1977-1983 at Warm Springs and
Round Butte Hatcheries (Lindsay et al. 1989). Mortality in nature is difficult to measure, so
values for the wild population should be regarded as approximate. Although survival rates
can be expected to vary somewhat among populations and species, the shift in pattern of
mortality to a later-history stage is typical for cultured populations of anadromous salmonids.
Figure from Waples (1991).
83
Herbivore
Phytoplankton
Biomass
Production
Planktivore
Piscivore biomass
Figure 8. Piscivore biomass in relation to biomass (solid line) and production (dashed line)
of vertebrate zooplanktivores, large herbivores and phytoplankton. Figure from Carpenter et
al. (1985).
84
References
Adams AJ, Locascio JV, Robbins BD (2004) Microhabitat use by a post-settlement stage
estuarine fish: evidence from relative abundance and predation among habitats.
Journal of Experimental Marine Biology and Ecology 299:17-33
Adams SB, Frissell CA, Rieman BE (2001) Geography of invasion in mountain streams:
Consequences of headwater lake fish introductions. Ecosystems 4:296-307
Allen GR, Midgley SH, Allen M (2002) Field guide to the freshwater fishes of Australia,
Vol. Western Australian Museum, Perth, WA
Allen MS, Tugend KI, Mann MJ (2003) Largemouth bass abundance and angler catch rates
following a habitat enhancement project at Lake Kissimmee, Florida. North American
Journal of Fisheries Management 23:845-855
Allendorf FW (1991) Ecological and genetic effects of fish introductions: synthesis and
recommendations. Canadian Journal of Fisheries and Aquatic Sciences 48:171-181
Allendorf FW, Leary RF (1988) Conservation and distribution of genetic variation in
apolytypic species, the cutthroat trout. Conservation Biology 2:170-184
Allendorf FW, Phelps SR (1980) Loss of genetic variation in a hatchery stock of cutthroat
trout. Transactions of the American Fisheries Society 109:537-543
Allendorf FW, Ryman N (1987) Genetic management of hatchery stocks. In: Ryman CN,
Utter F (eds) Population genetics and fishery management. University of Washington
Press, Seattle, Washington, p 141-159
Amita K, Hoshino M, Honma T, Wakabayashi H (2000) An investigation on the distribution
of Flavobacterium psychrophilum in the Umikawa River. Fish Pathology 35:193-197
Andrew NL, Mapstone BD (1987) Sampling and the description of spatial pattern in marine
ecology. Oceanography and Marine Biology: an Annual Review 25:39-90
Anonymous (2005) NSW quarterly disease report of the OIE (archived at the Network of
Aquaculture Centres in Asia-Pacific). http://library.enaca.org/Health/QAAD/QAAD2004-3.pdf
Aprahamian MW, Martin Smith K, McGinnity P, McKelvey S, Taylor J (2003) Restocking
of salmonids - opportunities and limitations. Fisheries Research 62:211-227
Arnekleiv JV, Raddum GG (2001) Stocking Atlantic Salmon (Salmo salar L.) and brown
trout (Salmo trutta L.) in rivers: diet selectivity and the effects on the
macroinvertebrate community. Nordic Journal of Freshwater Research 75:109-126
Arthington AH (1991) Ecological and genetic impacts of introduced and translocated
freshwater fishes in Australia. Canadian Journal of Fisheries and Aquatic Sciences 48
Suppl. 1:33-43
Avise JC (2004) Molecular markers, natural history, and evolution, Vol. Sinauer Associates,
Inc. Publishers, Sunderland, Massachusetts
Baker AJ (2000a) Protein electrophoresis. In: Baker AJ (ed) Molecular methods in ecology.
Blackwell Science Ltd, Oxford, UK, p 65-88
Baker AJ, editor (2000b) Molecular methods in ecology, Vol. Blackwell Science Ltd,
Oxford, UK
Bakke TA, Jansen PA, Hansen LP (1990) Differences in host resistance of Atlantic salmon,
Salmo salar L., stocks to the monogenean Gyrodactylus salaris Malmberg, 1957.
Journal of Fish Biology 37:577-587
Bauer ON, Pugachev ON, Voronin VN (2002) Study of parasites and diseases of sturgeons in
Russia: a review. Journal of Applied Ichthyology 18:420-429
85
Beamish RJ, Thomson BL, Mcfarlane GA (1992) Spiny dogfish predation on chinook and
coho salmon and the potential effects on hatchery-produced salmon. Transactions of
the American Fisheries Society 121:444-455
Bearlin AR, Tikel D (2003) Conservation genetics of Murray-Darling Basin fish; silver perch
(Bidyanus bidyanus), Murray cod (Maccullochella peelii), and trout cod (M.
macquariensis). In: Phillips B (ed) Managing fish translocation and stocking in the
Murray-Darling Basin workshop held in Canberra, 25-26 September 2002: Statement,
recommendations and supporting papers. WWF Australia, p 59-83
Beaudou D, Baril D, Roche B, LeBaron M, CattaneoBerrebi G, Berrebi P (1995)
Recolonization in a devastated Corsican river: Respective contribution of wild and
domestic brown trout. Bulletin Francais de la Peche et de la Pisciculture 337-9:259266
Begon M, Harper JL, Townsend CR (1996) Ecology: individuals, populations and
communities, Vol. Blackwell Science, Boston
Beland KF, Roberts RL, Saunders RL (1981) Evidence of Salmo salar x Salmo trutta
hybridization in a North American river. Canadian Journal of Fisheries and Aquatic
Sciences 38:552-554
Belanger RM, Corkum LD (2003) Susceptibility of tethered round gobies (Neogobius
melanostomus) to predation in habitats with and without shelter. Journal of Great
Lakes Research 29:588-593
Bender EA, Case TJ, Gilpin ME (1984) Perturbation experiments in community ecology:
theory and practice. Ecology 65:1-13
Billington N (2003) Mitochondrial DNA. In: Hallerman EM (ed) Population genetics:
principles and applications for fisheries scientists. American Fisheries Society,
Bethesda, Maryland, p 59-100
Blankenship HL, Leber KM (1995) A responsible approach to marine stock enhancement. In:
Schramm HL, Jr.,, Piper RG (eds) Uses and effects of cultured fish in aquatic
ecosystems, Vol 15. American Fisheries Society, Bethesda, Maryland, p 167-175
Blaxter JHS (2000) The enhancement of marine fish stocks. Advances in Marine Biology
38:1-56
Bouza C, Sanchez L, Martinez P (1997) Gene diversity analysis in natural populations and
cultured stocks of turbot (Scophthalmus maximus L). Animal Genetics 28:28-36
Bravo R, Soriguer MC, Villar N, Hernando JA (2001) The dynamics of fish populations in
the Palancar stream, a small tributary of the river Guadalquivir, Spain. Acta
Oecologica-International Journal of Ecology 22:9-20
Brown B, Epifanio J (2003) Nuclear DNA. In: Hallerman EM (ed) Population genetics:
principles and applications for fisheries scientists. American Fisheries Society,
Bethesda, Maryland, p 101-123
Brumley AR (1987) Past and present distributions of golden perch Macquaria ambigua
(Pisces: percichthyidae) in Victoria, with reference to releases of hatchery-produced
fry. Proceedings of the Royal Society of Victoria 99:111-116
Buckmeier DL, Betsill RK, Schlechte JW (2005) Initial predation of stocked fingerling
largemouth bass in a Texas reservoir and implications for improving stocking
efficiency. North American Journal of Fisheries Management 25:652-659
Busack CA, Currens KP (1995) Genetic risks and hazards in hatchery operations:
fundamental concepts and issues. In: Schramm HL, Jr.,, Piper RG (eds) Uses and
effects of cultured fish in aquatic ecosystems, Vol 15. American Fisheries Society,
Bethesda, Maryland, p 71-80
86
Cadwallader PL (1978) Some causes of the decline in range and abundance of native fish in
the Murray-Darling river system. Proceedings of the Royal Society of Victoria
90:211-224
Cadwallader PL (1981) Past and present distributions and translocations of Macquarie perch
Macquaria australasica (Pisces: perchichthyidae), with particular reference to
Victoria. Proceedings of the Royal Society of Victoria 93:23-30
Cadwallader PL (1996) Overview of the impacts of introduced salmonids on Australian
native fauna. Australian Nature Conservation Agency, Canberra
Cadwallader PL, Gooley GJ (1984) Past and present distributions and translocations of
Murray cod Maccullochella peeli and trout cod M. macquariensis (Pisces:
perchichthydae) in Victoria. Proceedings of the Royal Society of Victoria 96:33-43
Calcagnotto D, Toledo SD (2000) Loss of genetic variability at the transferrin locus in five
hatchery stocks of tambaqui (Colossoma macropomum). Genetics and Molecular
Biology 23:127-130
Campana SE (2005) Otolith elemental composition as a natural marker of fish stocks. In:
Cadrin SX, Friedland KD, Waldman JR (eds) Stock identification methods. Academic
Press, New York, p 719
Campton DE (1995) Genetic effects of hatchery fish on wild populations of Pacific salmon
and steelhead: What do we really know? In: Schramm HL, Jr., Piper RG (eds) Uses
and effects of cultured fish in aquatic ecosystems, Vol 15. American Fisheries
Society, Bethesda, Maryland, p 167-175
Carpenter SR, Kitchell JF, Hodgson JR (1985) Cascading trophic interactions and lake
productivity. Bioscience 35:634-639
Collares-Pereira MJ, Cowx IG (2004) The role of catchment scale environmental
management in freshwater fish conservation. Fisheries Management and Ecology
11:303-312
Commonwealth of Australia (2005) Aquaplan 2005-2010, Australian Government
Department of Agriculture, Fisheries and Forestry, Canberra
Connell SD (1997) Exclusion of predatory fish on a coral reef: the anticipation, pre-emption
and evaluation of some caging artifacts. Journal of Experimental Marine Biology and
Ecology 213:181-198
Cowx IG, van Zyll de Jong MC (2004) Rehabilitation of freshwater fisheries: tales of the
unexpected. Fisheries Management and Ecology 11:243-249
Crook D, Munro A, Gillanders BM, Sanger AC, Thurstan S, Macdonald J (2005) Review of
existing and proposed methodologies for discriminating hatchery and wild-bred fish,
Murray-Darling Basin Commission native fish strategy project R5003
Cross TF (2000) Genetic implications of translocation and stocking of fish species, with
particular reference to Western Australia. Aquaculture Research 31:83-94
Danzmann RG, Ferguson MM, Heculuck DM (1994) Heterogeneity in the distribution of
mitochondrial-DNA haplotypes in female rainbow trout spawning in different
seasons. Canadian Journal of Fisheries and Aquatic Sciences 51:284-289
Department of Agriculture Fisheries and Forestry (2004) Disease strategy: Viral
encephalopathy and retinopathy (Version 1.0). In: Australian Aquatic Veterinary
Emergency
Department of Primary Industries (2003) Guidlines for the translocation of live aquatic
organisms in Victoria, Victorian Aquatic Organisms Translocation Guidelines
Steering Committee
Douglas JW, Gooley GJ, Ingram BA, Murray ND, Brown LD (1995) Natural hybridization
between Murray cod, Maccullochella peelii peelii (Mitchell), and Trout Cod,
87
Maccullochella macquariensis (Cuvier) (Percichthyidae), in the Murray River,
Australia. Marine and Freshwater Research 46:729-734
Einum S, Fleming IA (2001) Implications of stocking: ecological implications between wild
and released Salmonids. Nordic Journal of Freshwater Research 75:56-70
Emlen JM (1991) Heterosis and outbreeding depression: a multi-locus model and an
application to salmon production. Fisheries Research 12:187-212
Evans DO, Willox CC (1991) Loss of exploited, indigenous populations of lake trout,
Salvelinus namaycush, by stocking of non-native stocks. Canadian Journal of
Fisheries and Aquatic Sciences 48:134-137
FAO (1996) Precautionary approach to capture fisheries and species introductions. FAO
technical guidelines for responsible fisheries, volume 2, FAO, Rome
FAO (1999) Global characterization of inland fishery enhancements and associated
environmental impacts, FAO Fisheries Circular No. 945, Rome
Fenderson OC, Carpenter MR (1971) Effects of crowding on the behaviour of juvenile
hatchery and wild landlocked Atlantic salmon (Salmo salar L.). Animal Behaviour
19:439-447
Fjellheim A, Raddum GG, Barlaup BT (1995) Dispersal growth and mortality of brown trout
(Salmo trutta L.) stocked in a regulated West Norwegian river. Regulated Rivers
Research and Management 10:137-145
Fleming IA, Einum S (1997) Experimental tests of genetic divergence of farmed from wild
Atlantic salmon due to domestication. Ices Journal of Marine Science 54:1051-1063
Fletcher AR, Morison AK, Hume DJ (1985) Effects of carp, Cyprinus carpio L., on
communities of aquatic vegetation and turbidity of waterbodies in the Lower
Goulbourn River Basin. Australian Journal of Marine and Freshwater Research
36:311-327
Garman GC, Neilsen LA (1982) Piscivority by stocked brown trout (Salmo trutta) and its
impacts on the nongame fish community of Bottom Creek, Virginia. Canadian Journal
of Fisheries and Aquatic Sciences 39:862-869
Garvey JE, Stein RA, Thomas HM (1994) Assessing how fish predation and interspecific
prey competition influence a crayfish assemblage. Ecology 75:532-547
Gharrett AJ, Smoker WW, Reisenbichler RR, Taylor SG (1999) Outbreeding depression in
hybrids between odd- and even-broodyear pink salmon. Aquaculture 173:117-129
Gilk SE, Wang IA, Hoover CL, Smoker WW, Taylor SG, Gray AK, Gharrett AJ (2004)
Outbreeding depression in hybrids between spatially separated pink salmon,
Oncorhynchus gorbuscha, populations: marine survival, homing ability, and
variability in family size. Environmental Biology of Fishes 69:287-297
Gilligan DM (2005) Fish communities in the Murrumbidgee catchment: status and trends,
NSW Department of Primary Industries, Narranderra, NSW, Australia
Glasby TM, Underwood AJ (1996) Sampling to differentiate between pulse and press
perturbations. Environmental Monitoring and Assessment 42:241-252
Glover KA, Taggart JB, Skaala O, Teale AJ (2004) A study of inadvertent domestication
selection during start-feeding of brown trout families. Journal of Fish Biology
64:1168-1178
Gross R, Gum B, Reiter R, Kuhn R (2004) Genetic introgression between Arctic charr
(Salvelinus alpinus) and brook trout (Salvelinus fontinalis) in Bavarian hatchery
stocks inferred from nuclear and mitochondrial DNA markers. Aquaculture
International 12:19-32
Hallerman EM, editor (2003) Population genetics: principles and applications for fisheries
scientists, Vol. American Fisheries Society, Bethesda, Maryland
88
Hammer M (2001) Molecular systematics and conservation biology of the southern pygmy
perch Nannoperca australis (Günther, 1861) (Teleostei: Percichthyidae) in southeastern Australia. B.Sc. (hons), Univeristy of Adelaide
Harris J (2003) Fish stocking and translocation in the Murray-Darling Basin: issues, benefits
and problems. In: Phillips B (ed) Managing fish translocation and stocking in the
Murray-Darling Basin workshop. WWF Australia, Canberra
Hedrick RP, El-Matbouli M, Adkinson MA, MacConnell E (1998) Whirling disease: reemergence among wild trout. Immunological Reviews 166:365-376
Henry G (1997) Further stocking successes by NSW Fisheries. Fisheries NSW 1:31-33
Heppell SS, Crowder LB (1998) Prognostic evaluation of enhancement programs using
population models and life history analysis. Bulletin of Marine Science 62:495-507
Hesthagen T, Hegge O, Skurdal J, Dervo BK (1995) Differences in habitat utilisation among
native, native stocked, and non-native stocked brown trout (Salmo trutta) in a
hydroelectric reservoir. Canadian Journal of Fisheries and Aquatic Sciences 52:21592167
Hindar K, Ryman N, Utter F (1991) Genetic effects of aquaculture on natural fish
populations. Aquaculture 98:259-261
Hnath JG (1993) Great Lakes fish disease control policy and model program (supersedes
September 1985 edition). Great Lakes Fishery Commission Special Publication 93:138
Hoffman GL (1990) Myxobolus cerebralis, a worldwide cause of salmonid whirling disease.
Journal of Aquatic Animal Health 2:30-37
Hurwood DA, Hughes JM (1998) Phylogeography of the freshwater fish, Mogurnda
adspersa, in streams of northeastern Queensland, Australia: evidence for altered
drainage patterns. Molecular Ecology 7:1507-1517
ICES (2005) ICES code of practice on the introductions and transfers of marine organisms,
Copenhagen
Iguchi K, Ogawa K, Nagae M, Ito F (2003) The influence of rearing density on stress
response and disease susceptibility of ayu (Plecoglossus altivelis). Aquaculture
220:515-523
Iguchi K, Watanabe K, Nishida M (1999) Reduced mitochondrial DNA variation in hatchery
populations of ayu (Plecoglossus altivelis) cultured for multiple generations.
Aquaculture 178:235-243
Imre I, Grant JWA, Cunjak RA (2005) Density-dependent growth of young-of-the-year
Atlantic salmon Salmo salar in Catamaran Brook, New Brunswick. Journal of Animal
Ecology 74:508-516
Ingram BA, Barlow CG, Burchmore JJ, Cooley GJ, Rowland SJ, Sanger AC (1990)
Threatened native freshwater fish in Australia - some case histories. Journal of Fish
Biology 37:175-182
Jerry DR, Woodland DJ (1997) Electrophoretic evidence for the presence of the undescribed
‘Bellinger’ catfish (Tandanus sp.) (Teleostei :Plotosidae) in four New South Wales
mid-northern coastal rivers. Marine and Freshwater Research 48:235-240
Johnsen BO, Jensen AJ (1986) Infestations of Atlantic salmon, Salmo salar, by Gyrodactylus
salaris in Norwegian rivers. Journal of Fish Biology 29:233-241
Kahilainen K, Lehtonen H (2001) Resource use of native and stocked brown trout Salmo
trutta L., in a subarctic lake. Fisheries Management and Ecology 8:83-94
Keenan CP (2000) Should we allow human-induced migration of the Indo-West Pacific fish,
barramundi Lates calcarifer (Bloch) within Australia? Aquaculture Research 31:121131
89
Keenan CP, Watts RJ, Serafini LG (1996) Population genetics of golden perch, silver perch
and eel-tailed catfish within the Murray-Darling Basin. In: Banens RJ, Lahane R (eds)
1995 Riverine environment research forum. Murray-Darling Basin Commission, p 1726
Kellison GT, Eggleston DB, Taylor JC, Burke JS, Osborne JA (2003) Pilot evaluation of
summer flounder stock enhancement potential using experimental ecology. Marine
Ecology Progress Series 250:263-278
King AJ (2005) Ontogenetic dietary shifts of fishes in an Australian floodplain river. Marine
and Freshwater Research 56:215-225
Krebs CJ (1978) Ecology: the experiemental analysis of distribution and abundance, Vol.
Harper and Row, New York
Krueger CC, May B (1991) Ecological and genetic-effects of salmonid introductions in North
America. Canadian Journal of Fisheries and Aquatic Sciences 48:66-77
Krueger CC, Menzel BW (1979) Effects of stocking on genetics of wild brook trout
populations. Transactions of the American Fisheries Society 108:277-287
Lachance S, Magnan P (1990a) Comparative ecology and behaviour of domestic, hybrid, and
wild strains of brook trout, Salvelinus fontinalis, after stocking. Canadian Journal of
Fisheries and Aquatic Sciences 47:2285-2292
Lachance S, Magnan P (1990b) Performance of domestic, hybrid, and wild strains of brook
trout, Salvelinus fontinalis, after stocking: the impact of intraspecific and interspecific
competition. Canadian Journal of Fisheries and Aquatic Sciences 47:2278-2284
Langdon JS (1989a) Disease risk of fish introductions and translocations. In: Pollard DA (ed)
Introduced and translocated fishes and their ecological effects. Proceedings of the 8th
Australian Society for Fish Biology Workshop, Magnetic Island, 24-25 August 1989.
Langdon JS (1989b) Experimental transmission and pathogenicity of epizootic
haematopoietic necrosis virus (EHNV) in redfin perch, Perca fluviatilis L. and 11
other teleosts. Journal of Fish Diseases 12:295-310
Langdon JS, Humphrey JD, Williams LM (1988) Outbreak of an EHNV-like iridovirus in
cultured rainbow trout, Salmo gairdneri Richardson, in Australia. Journal of Fish
Diseases 11:93-96
Le Cren ED (1973) Some examples of the mechanisms that control the population dynamics
of salmonid fish. In: Bartlett MS, Hiorns RW (eds) The mathematical theory of the
dynamics of biological populations. Academic Press, London, p 125-135
Leary RF, Allendorf FW, Knudsen KL (1983) Consistently high meristic counts in natural
hybrids between brook trout and bull trout. Systematic Zoology 32:369-376
Levin PS, Zabel RW, Williams JG (2001) The road to extinction is paved with good
intentions: negative association of fish hatcheries with threatened salmon.
Proceedings of the Royal Society of London, Series B 268:1153-1158
Levings CD, McAllister CD, Change BD (1986) Differential use of the Campbell River
estuary, British Columbia, by wild and hatchery-reared juvenile chinook salmon
(Oncorhynchus tshawytscha). Canadian Journal of Fisheries and Aquatic Sciences
43:1386-1397
Maheshwari BL, Walker KF, McMahon TA (1995) Effects of regulation on the flow regime
of the River Murray, Australia. Regulated Rivers: Research and Management 10:1538
Manderson JP, Pessutti J, Hilbert JG (2004) Shallow water predation risk for a juvenile
flatfish (winter flounder; Pseudopleuronectes americanus, Walbaum) in a northwest
Atlantic estuary. Journal of Experimental Marine Biology and Ecology 304:137-157
Matthews WJ (1998) Patterns in freshwater fish ecology, Vol. Chapman and Hall, New York
90
May B (2003) Allozyme variation. In: Hallerman EM (ed) Population genetics: principles
and applications for fisheries scientists. American Fisheries Society, Bethesda,
Maryland, p 23-36
McDowall RM (1987) Fish stocks for cool fresh waters, FAO, Rome
McMichael GA, Pearsons TN (2001) Upstream movement of residual hatchery steelhead into
areas containing bull trout and cutthroat trout. North American Journal of Fisheries
Management 21:943-946
Mezzera M, Largiader CR (2001) Evidence for selective angling of introduced trout and their
hybrids in a stocked brown trout population. Journal of Fish Biology 59:287-301
Miller LM, Kapuscinski AR (2003) Genetic guidelines for hatchery supplementation
programs. In: Hallerman EM (ed) Population genetics: principles and applications for
fisheries scientists. American Fisheries Society, Bethesda, Maryland, p 329-355
Mills MD, Rader RB, Belk MC (2004) Complex interactions between native and invasive
fish: the simultaneous effects of multiple negative interactions. Oecologia 141:713721
Moffitt CM, Haukenes AH, Williams CJ (2004) Evaluating and understanding fish health
risks and their consequences in propagated and free-ranging fish populations.
American Fisheries Society Symposium 44:529-537
Molony BW, Lenanton R, Jackson G, Norriss J (2003) Stock enhancement as a fisheries
management tool. Reviews in Fish Biology and Fisheries 13:409-432
Moore AS, Beaverstock PR (2003) Trends in fish hatchery practices within New South
Wales: what do they tell us about the maintenance of genetic diversity in fish stocking
programs? In: Phillips B (ed) Managing fish translocation and stocking in the MurrayDarling Basin workshop held in Canberra, 25-26 September 2002: Statement,
recommendations and supporting papers. WWF Australia, Sydney, p 104
Moritz C (1994a) Applications of mitochondrial DNA analysis in conservation: a critical
review. Molecular Ecology 3:401-411
Moritz C (1994b) Defining 'evolutionarily significant units' for conservation. Trends in
Ecology & Evolution 9:373-375
Morris SA, Pollard DA, Gehrke PC, Pogonoski JJ (2001) Threatened and potentially
threatened freshwater fishes of coastal New South Wales and hte Murray-Darling
Basin. Report No. 33, NSW Fisheries, Sydney
Moyle PB (1976) Fish introductions in California: History and impact on native fishes.
Biological Conservation 9:101-118
Murray-Darling Basin Commission (2005) Native fish strategy annual implementation report
2003-2004, Murray-Darling Basin Commission, Canberra
Musyl MK, Keenan CP (1992) Population genetics and zoogeography of Australian
freshwater golden perch, Macquaria ambigua (Richardson 1845) (Teleostei,
Percichthyidae), and electrophoretic identification of a new species from the Lake
Eyre Basin. Australian Journal of Marine and Freshwater Research 43:1585-1601
Musyl MK, Keenan CP (1996) Evidence for cryptic speciation in Australian freshwater eeltailed catfish, Tandanus tandanus (Teleostei: Plotosidae). Copeia:526-534
Nehlsen W, Williams JE, Lichatowich JA (1991) Pacific salmon at the crossroads: stocks at
risk from California, Oregon, Idaho, and Washington. Fisheries 16:4-21
Nehring RB, Walker PG (1996) Whirling disease in the wild: the new reality in the
Intermountain West. Fisheries 21:28-30
Nickelson TE, Solazzi MF, Johnson SL (1986) Use of hatchery coho salmon (Oncorhynchus
kisutch) presmolts to rebuild wild populations in Oregon Coastal streams. Canadian
Journal of Fisheries and Aquatic Sciences 43:2443-2449
91
Nickum MJ, Mazik PM, Nickum JG, MacKinlay DD (2004) Propagated Fish in Resource
Management, Vol. American Fisheries Society, Berthesda, Maryland
Nielsen JL, Gan C, Thomas WK (1994) Differences in genetic diversity for mitochondrialDNA between hatchery and wild populations of Oncorhynchus. Canadian Journal of
Fisheries and Aquatic Sciences 51:290-297
NSW Fisheries (2003) Freshwater fish stocking in NSW. Environmental Impact Statement.
Public Consultation Document, NSW Fisheries
O'Conner JP, O'Mahony DJ, O'Mahony JM (2005) Movements of Macquaria ambigua, in the
Murray River, south-eastern Australia. Journal of Fish Biology 66:392-403
Oguto-Ohwayo R, Hecky, R.E. (1991) Fish introductions in Africa and some of their
implications. Canadian Journal of Fisheries and Aquatic Sciences 48:8-12
Olla BL, Davis MW, Ryer CH (1998) Understanding how the hatchery environment
represses or promotes the development of behavioral survival skills. Bulletin of
Marine Science 62:531-550
Oosterhout GR, Huntington CW, Nickelson TE, Lawson PW (2005) Potential benefits of a
conservation hatchery program for supplementing Oregon coast coho slamon
(Oncorhynchus kisutch) populations: a stochastic model investigation. Canadian
Journal of Fisheries and Aquatic Sciences 62:1920-1935
Ovenden JR, White RWG, Sanger AC (1988) Evolutionary relationships of Gadopsis spp.
inferred from restriction enzyme analysis of their mitochondrial DNA. Journal of Fish
Biology 32:137-148
Peery CA, Bjornn TC, Bjornn C (2004) Interactions between natural and hatchery chinook
salmon parr in a laboratory stream channel. Fisheries Research 66:311-324
Piller KR, Wilson CC, Lee CE, Lyons J (2005) Conservation genetics of inland lake trout in
the upper Mississippi River basin: Stocked or native ancestry? Transactions of the
American Fisheries Society 134:789-802
Raborn SW, Miranda LE, Driscoll MT (2002) Effects of simulated removal of striped bass
from a southeastern reservoir. North American Journal of Fisheries Management
22:406-417
Randi E (2000) Mitochondrial DNA. In: Baker AJ (ed) Molecular methods in ecology.
Blackwell Science Ltd, Oxford, UK, p 136-167
Reisenbichler RR, McIntyre JD (1977) Genetic differences in growth and survival of juvenile
hatchery and wild steelhead trout. Journal of the Fisheries Research Board of Canada
34:123-128
Reisenbichler RR, Phelps SR (1989) Genetic variation in steelhead (Salmo gairdneri) from
the north coast of Washington. Canadian Journal of Fisheries and Aquatic Sciences
46:66-73
Richardson BJ, Baverstock PR, Adams M (1986) Allozyme electrophoresis: a handbook for
animal systematics and population studies, Vol. Academic Press, Sydney
Rowland SJ, Tully P (2004) Hatchery quality assurance program for Murray cod
(Maccullochella peelii peelii), golden perch (Macquaria ambigua) and silver perch
(Bidyanus bidyanus), New South Wales Department of Primary Industry
Sanger AC, Talbot B (2003) Management of fish stocking in New South Wales. In: Phillips
B (ed) Managing fish translocation and stocking in the Murray-Darling Basin
workshop held in Canberra, 25-26 September 2002: Statement, recommendations and
supporting papers. WWF Australia, Sydney, p 88-93
Scott RJ, Noakes DLG, Beamish FWH, Carl LM (2003) Chinook salmon impede Atlantic
salmon conservation in Lake Ontario. Ecology of Freshwater Fish 12:66-73
Scribner KT, Pearce JM (2000) Microsatellites: evolutionary and methodological background
and empirical applications at individual, population and phylogenetic levels. In: Baker
92
AJ (ed) Molecular methods in ecology. Blackwell Science Ltd, Oxford, UK, p 235273
Smirnov BP, Chebanova VV, Vvedenskaya TV (1994) Adaption of hatchery-rased chum
salmon, Oncohynchus keta, and chinook salmon, O. tshawytscha, to natural feeding
and effects of starvation. Journal of Ichthyology 34:96-106
Stewart JE (1991) Introductions as factors in diseases of fish and aquatic invertebrates.
Canadian Journal of Fisheries and Aquatic Sciences 48 (Supplement 1):110-117
Stockner JG, Macisaac EA (1996) British Columbia lake enrichment programme: two
decades of habitat enhancement for sockeye salmon. Regulated Rivers Research and
Management 12:547-561
Taylor MD, Palmer PJ, Fielder DS, Suthers IM (2005) Responsible estuarine finfish stock
enhancement: an Australian perspective. Journal of Fish Biology 67:299-331
Underwood AJ (1986) The analysis of competition by field experiments. In: Kikkawa J,
Anderson DJ (eds) Community ecology: pattern and process. Blackwell Scientific,
Melbourne, p 240-268
Underwood AJ (1990) Experiments in ecology and management: their logics, functions and
interpretations. Australian Journal of Ecology 15:365-389
Underwood AJ (1997) Experiments in ecology. Their logical design and interpretation using
analysis of variance, Vol. Cambridge University Press, Cambridge
Utter F (1998) Genetic problems of hatchery-reared progeny released into the wild, and how
to deal with them. Bulletin of Marine Science 62:623-640
Utter F (2003) Genetic impacts of fish introductions. In: Hallerman EM (ed) Population
genetics: principles and applications for fisheries scientists. American Fisheries
Society, Bethesda, Maryland, p 357-378
van Zyll de Jong MC, Gibson RJ, Cowx IG (2004) Impacts of stocking and introductions on
freshwater fisheries of Newfoundland and Labrador, Canada. Fisheries Management
and Ecology 11:183-193
Vincent ER (1996) Whirling disease and wild trout: the Montana experience. Fisheries
21(6):32-33
Vollestad LA, Hesthagen T (2001) Stocking of freshwater fish in Norway: management goals
and effects. Nordic Journal of Freshwater Research 75:143-152
Wahl DH, Stein RA, DeVries DR (1995) An ecological framework for evaluating the success
and effects of stocked fishes. In: Schramm HL, Jr., Piper RG (eds) Uses and effects of
cultured fish in aquatic ecosystems, Vol 15. American Fisheries Society, Bethesda,
Maryland, p 176-189
Wajon S (1983) Hybridisation between Murray cod and trout cod in Cataract Dam, NSW.
B.Sc. (Hons), University of New South Wales, Sydney (cited in Douglas et al. 1995)
Walker KF (1992) The River Murray, Australia: a semiarid lowland river. In: Calow P, Petts
GE (eds) The rivers handbook: hydrological and ecological principles, Vol 1.
Blackwell Scientific Publications, Oxford, p 472-492
Walters CJ, Martell SJD (2004) Fisheries ecology and management, Vol. Princeton
University Press, Princeton
Waples RS (1991) Genetic interactions between hatchery and wild salmonids: Lessons from
the Pacific North West. Canadian Journal of Fisheries and Aquatic Sciences 48:124133
Waples RS, Teel DJ (1990) Conservation genetics of Pacific salmon .1. Temporal Changes in
Allele Frequency. Conservation Biology 4:144-156
Waters JM, Lintermans M, White RWG (1994) Mitochondrial DNA variation suggests river
capture as a source of vicariance in Gadopsis bispinosus (Pisces, Gadopsidae).
Journal of Fish Biology 44:549-551
93
Weber ED, Fausch KD (2003) Interactions between hatchery and wild salmonids in streams:
differences in biology and evidence for competition. Canadian Journal of Fisheries
and Aquatic Sciences 60:1018-1036
Weiss S, Schmutz S (1999a) Performance of hatchery-reared brown trout and their effects on
wild fish in two small Austrian streams. Transactions of the American Fisheries
Society 128:302-316
Weiss S, Schmutz S (1999b) Response of resident brown trout, Salmo trutta L., and rainbow
trout, Oncorhynchus mykiss (Walbaum), to the stocking of hatchery-reared brown
trout. Fisheries Management and Ecology 6:365-375
Welcomme RL, Bartley DM (1998) Current approaches to the enhancement of fisheries.
Fisheries Management and Ecology 5:351-382
White RJ, Karr JR, Nehlsen W (1995) Better roles for fish stocking in aquatic resource
management. In: Schramm HL, Jr., Piper RG (eds) Uses and effects of cultured fish in
aquatic ecosystems, Vol 15. American Fisheries Society, Bethesda, Maryland, p 527547
Williams CJ, Moffitt CM (2001) A critique of methods of sampling and reporting pathogens
in populations of fish. Journal of Aquatic Animal Health 13:300-309
Williams CJ, Moffitt CM (2003) Bayesian estimation of fish disease prevalence from pooled
samples incorporating sensitivity and specificity. In: Williams CJ (ed) Bayesian
inference and maximum entropy methods in science and engineering. American
Institute of Physics, College Park, Maryland, p 39-51
Wysujack K, Kasprzak P, Laude U, Mehner T (2002) Management of a pikeperch stock in a
long-term biomanipulated stratified lake: efficient predation vs. low recruitment.
Hydrobiologia 479:169-180
Yamamoto T, Reinhardt UG (2003) Dominance and predator avoidance in domesticated and
wild masu salmon Oncorhynchus masou. Fisheries Science 69:88-94
94