Download Introduction of fish species in freshwaters : a major threat to aquatic

Towards a New Partnership
Edited b y
Francesco di Castri
Director of Research, C N R S
Centre d'EcoZogie Fonctionelle et Evolutiue
1919 Route de Mende
B.P. 5051
34033 Montpellier Cedex
France
and
Talal Younès
Executive Director
Internafional Unioiz of Biological Sciences
51, Boulevard de Moiztmorency
75016 Paris
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CAB INTERNATIONAL
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in nssociation with the
International Union of Biological Sciences
Freshwater ecosystems occupying less than 1%of the surface of the Earth, are
inhabited by more than 25% of the vertebrates. Fish which consitute the
bulk of this fauna, are currently exposed to the consequences of major impacts
resulting from anthropogenic activities: management of water systems for
hydropower or agriculture purposes, pollutions, overexploitation of natural
resources including living resources, introduction of alien species. The result
is that a great number of endemic fish species are presently endangered.
Among major threats, the question of the introduction of alien fish
species became a matter of debate among freshwater ecologists and managers
during the last decades. The main goals of voluntary introductions, most
usually decided by fishery officers, were initially to improve sport fisheries,
artisanal fisheries and aquaculture, or to develop biological control of aquatic
diseases, insects and plants. More recently, transfers of ornamental fish
for the aquarium trade have also sharply increased, as well as 'accidental'
introductions of these species into freshwater ecosystems.
The future promises a continuing spread of exotic species, and managers
of aquatic ecosystems will be confronted increasingly with a shifting mix of
native and non-native species. However, although the introduction of species
has been encouraged all around the world both by managers and scientists
for centuries, many ecologists consider it today to be like Pandora's box.
The consequence is that any planned introduction is viewed as a potential
ecological catastrophe, and that both its ecological and economical potential
values are not objectively considered.
Are introductions of fish speciesta way to encourage in order to improve
fisheries, or a major threat to freshwater biodiversity, or a game of chance
as suggested by number of ecologists? Whereas many scientists, more or less
O 1996 CAB INTERNATIONAL. Biodiversity, Science and
Development: Towards a New Partnership (eds F. di Castri and T. Younès)
Introduction of Fish Species in Freshwafers
447
intuitively, claim that the introduction of exotic species is risky, there are also
examples of assumed success when the goal was to improve artisanal fisheries.
We need therefore a very careful assessment of past experiences in order to
develop general guidelines and policies about fish introductions which are
scientifically based.
Purposes of Introductions
The first attempts to introduce true exotic species are not exactly known, but
the modern era of introductions started during the second half of the nineteenth century. In Europe, these introductions of exotics involved mainly
North American species: salmonids (the brook trout was introduced to the
United Kingdom in 1869), catfishes (Icfalurus spp.) and centrarchids.
There are different motives for performing introductions (Welcomme,
1988; Moreau efal., 1988). During the niheteenth century and until about
World War II, fish introductions were largely an outgrowth of colonialism and
nostalgia by settlers in their newly adopted homelands for the familiar species
and surroundings left behind (Welcomme, 1984). Acclimatization societies
proliferated for instance in New Zealand in the late 1800s to a point that of
the 46 fish species now found, 20 are exotic (McDowell, 1990). Several species,
usually salmonids o r top predators, have been introduced in different parts
of the world as sport fish. The most spectacular example is that of the brown
trout (Salmo f r u f f a ) ,native from Europe and now occurring worldwide
(Baglinière and Maisse, 1991).
More than a hundred fish species have been introduced for fish culture,
but less than ten now have a worldwide distribution. The common carp
(Cyprinus Carpio) and the herbivorous carp (Cfenopharyngodon i d d a ) , now
occur in many temperate countries. In the intertropical area, several species
of tilapiines (family Cichlidae) native to Africa, such as Oreochromis nilotirus
and to a lesser extent Oreochromis mossambicus, make up the bulk of the fish
culture production in Southeast Asia. They have also been introduced in
South America, as well as Australia.
One of the purposes of fish introductions has been the control of the aquatic
vegetation. The grass carp, Cfenopharyngodon idella, has been successfully
used in many countries for the control of aquatic macrophytes. Tilapia rendalli
as well has been introduced into Sudanese irrigation channels and some artificial
lakes of Shaba to control aquatic vegetation (Philippart and Ruwet, 1982).
Fish introduction has also been suggested for disease control. That is the
case for instance for Asfatoreochromis alluaudi, a 'mollusc crushing' cichlid
under threat of extinction in East Africa, whose introduction to Cameroon has
been proposed for control of snails involved in schistosomiasis transmission
(Slootweg, 1989). The mosquitofish (Gambusia affinis) native from southern
United States, and guppy (Poecilia reticulafa)native from north-eastern South
America, were introduced worldwide after the World War II to control larvae
of mosquito vectors of malaria.
A current major cause of introduction is the trade of ornamental fish. For
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C. Lévêque
the past few decades, several thousands of small, usually tropical species, have
been disseminated around the world by the aquarium fish trade. A famous
and ancient example is the popular gold fish, Carassius aurafus aurutus, which
has achieved a wide distribution for rearing in ornamental ponds. Most of
these fish are rarely recorded as introduced, but can accidentally escape or be
introduced in natural water bodies. It is generally assumed that they cannot
survive in temperate climates, although populations may survive in artificial
warm waters.
Many introductions of alien fish into natural environments appear to have
been more or less accidental, and occurred during the deliberate introduction
of another species, because of confusions in identification. That is the case,
for instance, with batches of stock-fish contaminated with unnoticed juveniles
of another species.
What Have We Learned about the Consequences of Fish
Introductions on Biodiversity and the Aquatic Environment?
From the different experiences in fish introduction, some general reflections
should be derived.
The introduction of large predators can be almost catastrophic in complex
fish communities which evolved in tropical lakes with considerable specialization in feeding and reproduction. A well-documented example was the
accidental introduction of the cichlid piscivore, Cichla ocellaris, to Gatun
lake, Panama, around 1967 (Zaret and Paine, 1973). In less than five years,
the introduced species which has voracious predatory habits, led to the
elimination of six of the eight common fish and decimated a seventh.
The large predator Lates niloficus was introduced into Lake Victoria
during the 1950s and early 1960s to boost fisheries. During the first 25 year
after its introduction, catches of the Nile perch were insignificant. However,
in the early 1980s an explosive increase in the catches took place and
simultaneously stocks of endemic cichlids have been decimated (OgutuOhwayo, 1990).It can be assumed that the establishment of the Nile perch
was initially supported by the large quantities of native haplochromines which
were their main prey. As a result, about two-thirds of the flock of 300
haplochromine cichlid species have disappeared or are threatened with extinction (Witte ef al., 1992).
One of the major problems in fish species introductions, is their irreversibility, at least on the scale of a human's lifetime, Once introduced and established, it is impossible, given current technology, to eradicate a fish species
from a large natural water body.
Most information about the effects of introduced species on native fish
is often anecdotal and fragmentary, not only because so little is known about
native fish, but because the effects of the introduced fish have been overshadowed by the effects of environmental changes which have occurred
simultaneous1.y. For instance, although it has been claimed that the introduced
Introducfion of Fish Species in Freshwaters
Nile perch was responsible for the decline of the native cichlids, the impact
of human activities in the vicinity of Lake Victoria could also be involved.
The lake has experienced profound changes over the last 30 years (Witte et a2, ,
1992). Eutrophication, as well as the introduction of new fishing techniques
(trawling) certainly contributed significantly to the decline of the Haplochromis
fauna. It has been shown that the haplochromine stock was already affected by
fisheries before the establishment of Lafes (Ogutu-Ohwayo, 1990).
In newly created habitats (geologically young lakes, reservoirs) only a few
species of riverine fish are adapted to take advantage of the deep o r open
waters and to occupy adequately the newly created niches. The creation of
reservoirs provides the habitat but not the preadapted inhabitants (Fernando,
1991). From the vast number of tropical freshwater fish only Cichlidae and
Clupeidae, and to a limited extent Cyprinidae, have proved to be preadapted
to colonize lacustrine habitats and to increase the total fish yield substantially.
However, it seems reasonable to limit their introduction into reservoirs or
natural lakes of recent origin, while introduction into ancient lakes must be
very carefully considered (Fernando and Holcik, 1982).
In aquatic ecosystems containing a very depauperate indigenous freshwater fauna as a result of historical and biogeographical events, the introduction of appropriate fish species that would occupy presently vacant or
underutilized niches, could significantly improve the fishery. For example,
O. mossambicus introduced in the Sepik River (Papua New Guinea) makes
up about 50% of the present catch in inland fisheries (Coates, 1987). It should
also be recalled that Ireland's current freshwater fish fauna consists of
20 species, whereas after the last glaciation, the fish fauna probably consisted
of only about eight anadromous species. At least seven species were introduced into Ireland within the last 400 years or so (Fitzmaurice, 1984).
Introductions for the purposes of fish conservation is a new option, for
conservationists. A captive breeding programme may be considered as a way
to save taxa threatened with extinction in the wild and to provide propagules
for the repopulation of natural habitats. However, the restocking of captive
bred fish to their native habitats is not always realistic and cannot necessarily
be considered as the ultimate goal of breeding programmes. Indeed, reintroduction should take place only in localities where the original causes of extinction have been removed, and where habitat requirements of the species are
satisfied. In the case of Lake Victoria for instance, reintroduction would be
futile as long as the Lates threat persists, and the eutrophication of the lake
is not controlled. To save endangered fish species, conservationists, therefore
suggested introducing them into other ecologically suitable habitats, similar
or identical to the original one, in order to initiate new safety net populations
in appropriate waters. In other words, fish introductions may receive particuIar attention in that case, as a way of saving endemic species from extinction,
providing similar habitats may be found for these species.
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C. Lkvêqtie
Conclusions
Are introductions a game of chance? Most scientists more or less agree that
the introduction of exotic species is risky, and many examples have been given
that provide evidence of the potential threats. But there are also examples of
apparent success which demonstrate the need for very careful impact assessments prior to any introduction. The assessment of the impact of fish introduced in Asia (De Silva, 1989), is that this continent has suffered few losses
of native species to introductions.
The need for better communication between the scientific community and
managers or administrators has been raised many times (Balon and Bruton,
1986) as well as the need for establishing formal protocols for the evaluation
of risks, both for indigenous species and for ecological balance, prior to any
introduction. Balon and Bruton (1986) also clearly pointed out the difficulties
of such procedures: are decision-makers willing to accept advice which does
not conform to their schemes? Do scientists have sufficient knowledge to give
clear and unequivocal advice? In fact, biologists have a limited ability to
predict the outcome of introductions because of an inability to answer some
fundamental biological questions.
Applied research programmes should be devised urgently to improve the
documentation of the consequences of fish introductions, starting with the
more common species. Both field and controlled experiments should be conducted using the already available information to assess direct and indirect
effects of introductions. But decision-makers should also realize that the lack
of regulation and control makes it difficult nowadays to avoid possible major
ecological threats in the near future.
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