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8. GENERAL DISCUSSION
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8.1. SUMMARY
The previous chapters have shown that the Clyde Sea area supports a diverse
epibenthic fauna, members of which are frequently caught and discarded by Nephrops
trawlers (Chapter 2). Trawling and exposure to air caused considerable damage to
fragile organisms such as brittlestars, swimming crabs and squat lobsters (Chapter 3)
and ephemeral physiological stress to decapod crustaceans (Chapter 4). Brittlestars
suffered 100% mortality in the long-term while most hermit crabs and whelks
survived (Chapters 5 and 6). Injury significantly increased mortality of starfish,
swimming crabs and squat lobsters.
Discarded invertebrates, crustaceans in
particular, attracted a number of facultative megafaunal scavengers and amphipods
and were consumed within 24-48 h (Chapter 7).
8.2 METHODOLOGY
As mentioned in Chapter 1, not all organisms on the sea bed are caught in trawls,
some pass through the mesh of the net whilst others inhabit burrows that are out of the
reach of trawls. In this study, no assessment has been made of the catch efficiency of
Nephrops otter trawls for epibenthic invertebrates, so it is difficult to determine the
extent of disturbance caused by discarding at a population level.
While catch
efficiency data are available for Nephrops and undersized commercial fish (Sardà et
al., 1993; Mytilineou et al., 1998; Sangster & Breen, 1998; Briggs et al., 1999;
Madsen et al., 1999), only limited information is available for non-target
invertebrates.
Craeymeersch et al. (1998) estimated the catch efficiency of
commercial beam trawls for epibenthic invertebrates in the North Sea at <10%,
although 10%-70% of the animals belonging to larger size classes were caught. In the
same study, catch efficiencies of Nephrops trawls were assessed for a limited range of
epibenthic invertebrates: at 2%-12% for Crangon allmanni, Dendronotus frondosus,
Dichelopandalus bonnieri and Pasiphaea sivado. However, tow durations were 1020 times shorter compared with commercial trawls and the animals under
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consideration are quite small in size. Longer tow durations generally result in larger
catches as the accumulating mass of the catch helps to retain later animals and can
clog the meshes of the net (Craeymeersch et al., 1998, Wileman et al., 1999). The
catch efficiency for Nephrops can be assumed to be lower compared with epibenthic
species of similar size as a large proportion of Nephrops will be out of the reach of
trawls, i.e. retreated in their burrows. Tuck et al. (1997a) have shown that Nephrops
densities estimates based on burrow counts (using a towed underwater camera)
exceeded those determined by trawl catches by two orders of magnitude. Demersal
fish behave differently to invertebrates in response to trawls and escape of undersized
fish from the cod-end has been facilitated by recent regulations enforcing the
introduction of a square mesh panel into the cod end (Madsen et al., 1999). One
approach to determine the catch efficiency of Nephrops trawls would be to estimate
the density of epifauna by the use of a towed epibenthic video sledge analogous to its
current application in Nephrops stock assessments (Tuck et al., 1997a,b; Marrs et al.,
2000) followed by trawling along the same track.
The data from by-catch analysis should be regarded as a baseline study only.
A comprehensive study employing more than three trawlers is required in order to
cover a wider area and overcome the effects of local preferences by fishermen as they
often tend to trawl in the same areas. For logistical reasons it was not possible to
make use of a greater number of boats in the present study.
8.3 SYNTHESIS AND EVALUATION OF RESULTS
In this section the findings for each discard species from different chapters are
synthesised and an attempt is made to evaluate potential impacts of trawling and
discarding in the light of each species' biology.
Asterias rubens
This starfish is frequently discarded in high numbers from Nephrops trawlers and so
some 30% of A. rubens sustained damage such as arm loss and puncture wounds.
Although short-term mortality was low, physical trauma such as punctures and arm
loss rendered A. rubens more susceptible to infection, disintegration and subsequent
death in the longer term (22%-32%). So, it can be assumed that a minimum of some
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10% of the individuals discarded died in the longer term. Longer righting times
indicated that starfish experienced post-trawling stress. As stated above, it is not
known what proportion of A. rubens populations are caught and discarded by
Nephrops trawlers. Therefore it is difficult to estimate the extent of disturbance
caused to starfish populations. However, Marrs et al. (2000) have shown that the
fishing intensity in the Clyde Sea is high, so trawling could have an impact at the
population level. Vevers (1949) and Anger et al. (1977) reported that A. rubens
populations on mud consisted of very small starfish, probably as a result of low food
availability in this habitat. Growth rates of A. rubens are related to environmental
factors such as food availability and temperature, rather than age (Vevers, 1949;
Barnes & Powell, 1951; Nauen, 1978; Guillou, 1979). Starfish that are starving have
the ability to autolyse their body wall and shrink (Jangoux & van Impe, 1977). The
food supply of soft sediments may be further decreased in frequently trawled areas
due to increased benthos mortality and a decrease in benthic biomass although
abundance of individuals may increase (Ball et al., 2000). Jangoux & van Impe
(1977) showed that A. rubens build up glycogen reserves in the pyloric caeca during
summer and autolyse these reserves in the winter for gametogenesis. Hence, foodlimited populations may be less fecund (Vevers, 1949; Barker & Nichols, 1983). The
investment of energy reserves into regeneration of lost arms could further reduce
fecundity. Wieczorek et al. (1999) and Dare (1982) reported surface predation of
herring gulls (Larus argentatus) which could additionally decrease A. rubens
population densities or increase damage to individuals picked but not eaten. A lower
A. rubens standing crop will produce fewer larvae for recruitment, particularly in
food-limited areas. Veale et al. (2000) found a decrease in A. rubens population
densities in areas subject to intense scallop-dredging.
Underwater TV and SCUBA observations along with baited-trap deployments
showed that A. rubens were attracted to discarded invertebrates, crustaceans in
particular.
Starfish in originally food-limited populations may capitalize upon
discards and damaged benthos (Ramsay et al., 1997a; 1998; Ramsay & Kaiser, 1998;
Wieczorek et al., 1999) facilitated by their well-developed chemical olfactory sense
(Castilla & Crisp, 1970) in a similar swarming manner as observed on mussel and
oyster beds. Dare (1982) reported A. rubens swarms forming dense bands (300-400
individuals m-2), 1600 m long and 15 m wide, travelling 300 m in two months,
clearing some 50 ha of mussel beds in the Irish Sea. Similar aggregations have been
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observed on Arctica islandica and mussel beds off Norway and Lancashire (Brun,
1968; Sloan & Aldridge, 1981). Falk-Petersen (1982) reported that egg production in
A. rubens was "enormous", indicating high fecundity when food is not limiting.
Ramsay et al. (2000a) found that starfish numbers increased with increasing
fishing effort but declined after a threshold level had been reached in the Irish and
North Seas. The authors suggest that at low fishing effort such artificially increased
food availability and removal of predators could lead to population growth until these
benefits were outweighed by deleterious effects such as fishing mortality of starfish
and their prey organisms. However, their analysis was based on fishing effort data
recorded per ICES rectangle and Rijnsdorp et al. (1998) and Marrs et al. (2000) have
shown that fishing effort is very patchy within these large units which could explain
the variance not accounted for in these models. Anger et al. (1977) concluded that A.
rubens plays an important role as a consumer and competitor for food to demersal fish
on soft bottoms of the western Baltic. Hence, both increases or decreases in starfish
populations could seriously affect benthic community structure. Anecdotal reports
from local fishermen suggest that discarding practices could have led to an
introduction of starfish to areas where their densities used to be low, e.g. in the Fairlie
Channel.
There has been much debate as to whether discards or damaged benthos could
subsidize scavenger populations (Kaiser & Spencer, 1994, 1996b; Evans et al., 1996;
Ramsay et al., 1996, 1997a,b, 1998, 2000a; Kaiser et al., 1998a,b; Castro et al., 1999;
Wieczorek et al., 1999; Demestre et al., 2000; Groenewold & Fonds, 2000; Mensink
et al., 2000; Veale et al., 2000).
An emerging consensus is that while fishing
temporarily augments the diet of scavengers it is unlikely to constitute its major part.
In food-limited systems, however, such as some soft-sediment systems it could
provide starfish with the additional nutrients required for gonad growth and
gametogenesis. The reproductive benefit would need to surpass the minimum 10%
discard mortality though.
Ophiura ophiura
This brittlestar constituted 0%–66% (mean of 8%) of the discarded catch, with highest
numbers captured when a clean net was employed. In several studies it has been
suggested that O. ophiura is resilient to bottom fishing (Hill et al., 1996; Ramsay et
al., 1998; Bergman & van Santbrink, 2000) owing to its high capacity for
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regeneration. Yet, in the present study, almost all individuals sustained damage and
died within three weeks in several trials. Consequently, one could expect O. ophiura
populations to suffer increased fishing mortality and population decline. Collie et al.
(2000) found that, together with holothurians, ophiuroids were the echinoderm group
most negatively affected by bottom fishing. It was predicted that chronic fishing
disturbance could lead to a 93% reduction of ophiuroid densities. Recent ecological
modelling by Frid et al., (1999) suggests that shifts in demersal fish assemblages and
size distribution may have led to a change in benthos predation with an increase in
ophiuroid consumption. However, the sheer abundance of O. ophiura caught in
trawls and recorded in underwater camera observations (Wieczorek et al., 1999) give
the impression that populations have not been decimated by the last 40 years of Clyde
Sea Nephrops trawling. Tuck et al. (1998) found a significant increase in O. ophiura
densities after experimental trawling (without a net). As discussed in more detail in
Chapter 6, O. ophiura could be considered omnivorous and can substantially affect
benthic community structure (Thorson, 1966; Feder, 1981). Overwintering oocytes
and the presence of pelagic ophioplutei from March to October, suggests that
spawning can potentially take place throughout most of the year (Tyler, 1977).
Hence, O. ophiura populations that had been exposed to trawling could be restocked
with larvae from adjacent populations at the margins of fishing grounds.
Although brittlestars have been reported to aggregate on damaged benthos left
behind in trawl tracks (Ramsay et al., 1998; Hall-Spencer & Moore, 2000b), they
were only rarely caught in baited traps (Chapter 7) but were attracted in high numbers
to baited cameras (Wieczorek et al., 1999). However, in the latter study it could not
be discerned reliably if O. ophiura were actually feeding on discards.
In summary, it would appear that increased brittlestar mortality from trawling
might be outweighed at the population level by the species' reproductive resilience
and advantages attributable to fishing such as a decrease in predator densities
although further investigation is needed to substantiate this hypothesis. There has
been evidence from continuous plankton recorder (CPR) survey data in the North Sea
of an increase of pluteus larvae, from the 1950s to the early 1990s and a simultaneous
increase in several benthic echinoderm populations (Lindley et al., 1995).
The
removal of predators of echinoderms due to increased fishing effort, along with
increased food supply owing to eutrophication, were suggested as possible causes for
this increase.
However, more information on larval and juvenile mortality and
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settlement is required to prove such links. Brun (1976) suggested that discarding
activities between scallop grounds around the Isle of Man may have led to an
expanded distribution of the ophiuroid Ophiothrix fragilis.
Liocarcinus depurator
This swimming crab accounted for <54% of the discarded catch (mean of 13%). High
concentrations of ammonia, lactate and glucose in haemolymph samples from trawled
and air-exposed swimming crabs indicated physiological stress.
Damage was
sustained by 47% of the individuals and decreased post-trawling mortality to 28%–
49% suggesting vulnerability to bottom fishing.
Veale et al. (2000) found
significantly lower numbers of Liocarcinus spp. in an area with a history of high
scallop-dredging effort and concluded that the deleterious effects of dredging
outweighed potential benefits.
In the Clyde Sea, the nocturnal L. depurator (Abelló et al., 1991; Freire et al.,
1991) may be protected to a certain degree from capture by trawls as Nephrops
trawling usually takes place from dawn till dusk during which swimming crabs have
been observed to recess into the sediment surface (Mori & Zunino, 1987;
M.
Bergmann, personal observation).
Figure 8.1. Trisopterus minutus capelanus and Liocarcinus depurator feeding on
discards in the central Adriatic (from Wieczorek et al., 1999).
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High abundances of Liocarcinus depurator and Liocarcinus spp. feeding on damaged
benthos in trawl tracks and discards have been reported in the North Sea, Irish Sea
and Mediterranean Sea (Ramsay et al., 1997a; Wieczorek et al., 1999; Demestre et
al., 2000; Fonds & Groenewold, 2000). Figure 8.1 shows a dense aggregation of L.
depurator at discard bait on fishing grounds in the central Adriatic Sea (from
Wieczorek et al., 1999). Analogously, high densities of this species have been found
under mussel raft cultures in Galicia (e.g. González-Gurriarán et al., 1989; Fernández
et al., 1991; Freire et al., 1991; Freire, 1996). Swimming crabs exploit a wide range
of dietary items from algae to sponges, annelids, molluscs, crustaceans, echinoderms,
fish etc. (Abelló & Cartes, 1987; González-Gurriarán et al., 1989; Freire et al., 1991,
Hall et al., 1990; Freire, 1996 ) and are thus omnivorous. Freire et al. (1996)
concluded that the high diversity of food items in the diet of L. depurator was due to
the functional structure of the chelipeds being more versatile than in other crab
species.
Females attain sexual maturity within their first year of life and are capable of
2-3 broods per year owing to short incubation times of eggs (Wear, 1974; Mori &
Zunino, 1987; Abelló, 1989; Fernández et al., 1991). In the Clyde Sea, spawning
takes place from January to June and females produce two broods per year (Allen,
1967). Liocarcinus depurator is omnivorous, has a high fecundity and hence is able
to rapidly re-colonise frequently fished areas and capitalize upon any food items
available.
Fariña et al. (1997) reported a significant increase in Liocarcinus
depurator population densities between 1980 and 1987 in Galician waters.
Enclosure experiments in a sea lough in Ireland have shown that high densities
of this decapod led to a significant depletion of infaunal organisms (Thrush, 1986), so
L. depurator can have profound effects on benthic community structure.
Munida rugosa
These squat lobsters accounted for large proportions of the discarded catch (<63%,
mean of 12%), especially on rougher grounds. Almost 60% sustained damage and, as
with crabs, loss of appendages reduced post-trawling survival to 16%-32% in the
longer-term. Population density estimates of Munida intermedia, a smaller galatheid
similar in its lifestyle and morphology, based on underwater camera observations in
the Adriatic Sea were greater by an order of magnitude than those deducted from
trawl catches (Gramitto & Froglia, 1998), indicating a catch efficiency of ~10% for
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smaller squat lobsters of small mesh trawls (12 mm). Hartnoll et al. (1992) and
Gramitto & Froglia (1998) hypothesized that female Munida spp. are protected from
trawl capture as a result of cryptic behaviour. If this also holds true for M. rugosa,
populations would to a certain degree be naturally protected from the impacts of
trawling.
Munida rugosa inhabits a variety of substrata ranging from gravelly muddy
sand, muddy sand, sandy mud, mud and bedrock, often seeking shelter in rock
crevices, beneath overhangs, hydrozoans, bryozoans, beneath pebbles and in
abandoned burrows from Nephrops and red band fish (Cepola rubescens) (Zainal,
1990; Howard, 1981; M. Bergmann, personal observation).
Hence, it could be
assumed to be relatively tolerant to dislocation during discarding, which may also be
true for A. rubens.
Zainal (1990) found ovigerous females from November to May. The smallest
ovigerous M. rugosa found had a carapace length of 21 mm. Gramitto & Froglia
(1998) found that most egg-bearing M. intermedia had a carapace length of 11 mm
and concluded that females attain sexual maturity in the first year of life. The closely
related species Munida sarsi, which overlaps with M. rugosa in its geographic
distribution at the lower end of its bathyal range, reached sexual maturity at a
carapace length of 10 mm (Hartnoll et al., 1992). The smallest egg-bearing female
found in the present study measured 13 mm (although the fishing gear might have
failed to retain small individuals) indicating that M. rugosa, like its congeners, may
start breeding within the first or second year of life (R.J.A. Atkinson, UMBS Millport,
personal communication). Hartnoll et al. (1992) stated that Munida rugosa produce
larger numbers of eggs than other Munida spp. The longevity of the smaller-sized
Galathea intermedia has been estimated at one year (Samuelsen, 1970) whereas
Gramitto & Froglia (1998) estimated life span of Munida intermedia at less than four
years.
Zainal (1990) deemed M. rugosa a generalist feeder capable of herbivory,
carnivory and deposit-feeding owing to its versatile mouthparts (Garm & Høeg,
2000), but with a preference for animal food acquired both by scavenging and active
predation.
Based on anecdotal evidence from fishermen, Burd & Jamieson (1986)
reported that Munida quadrispina increased as prawn stocks declined in British
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Columbia. Fariña et al. (1997) found an increase in Munida sarsi and Munida
intermedia populations in Galician waters between 1980 and 1987.
Pagurus bernhardus
These hermit crabs represented a comparatively small fraction of the discarded
material in the Clyde Sea (<4%) with low incidence of damage and high long-term
survival, as a result of its protective host shell. Some hermit crabs were observed to
leave their shell, possibly in response to hypoxia (Côté et al., 1998).
However, this species was one of the most common scavengers recorded both
in baited trap experiments and underwater camera observations in the Clyde Sea
(Wieczorek et al., 1999) though not in Loch Sween. Pagurus bernhardus has also
been frequently observed feeding on damaged animals in trawl tracks (Ramsay et al.,
1996, 1997a,b, 1998 Kaiser et al., 1998a,b) and it has been suggested that this species
might benefit from the input of carrion from fisheries.
Lancaster (1990) reported that female P. bernhardus breed within their first
year of life at a size substantially below that that could be attained in the shells usually
available and are capable of two broods per year. Dawirs (1984) stated that P.
bernhardus larvae represent an evolutionary advanced developmental mode that
completely protects them against starvation, as megalopae can metamorphose without
food supply. Larval settlement is encouraged in the littoral zone where gastropod
species are diverse and abundant and where the youngest will find suitable shells. As
they grow larger, however, hermit crabs become limited by the supply of larger
gastropod shells, limiting further growth and fecundity (Lancaster, 1990). Pagurus
bernhardus are considered omnivorous, being able to adopt filter-feeding and depositfeeding in times of food shortage (Gerlach et al., 1976).
Overall it would appear that P. bernhardus population dynamics are governed
by resource shortage of shells rather than food so that trawling is unlikely to be
beneficial although dislocation might deliver hermit crabs to areas of higher shell
abundance.
Buccinum undatum and Neptunea antiqua
Whelks represented only a small fraction of the catch (<3%) and, being protected by
hard shells, they survived trawling well in the longer term. Baited creels deployed on
Nephrops grounds often yielded large catches of whelks (Chapter 7, Wieczorek et al.,
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1999), indicating that Nephrops otter trawls may not be efficient for collecting
whelks, e.g. that have been seen recessed into the sediment (M. Bergmann, personal
observation).
The use of clean nets seemed to yield higher whelk catches as
demonstrated in Figure 2.1 (data from this trawl was not included in Chapter 2). High
survival rates and the benefits from discarded and damaged carrion has been
suggested to benefit B. undatum (Evans et al., 1996; Kaiser & Spencer, 1996b).
However, Cadée et al. (1995) and Mensink et al. (2000) reported high mortalities of
B. undatum caught in beam trawls and suggested that bottom fishing was one of the
main causes for the decline of Buccinum undatum in the Wadden Sea. Indeed, B.
undatum only attains maturity at 4-7 years (Gendron, 1992). The fact that egg masses
are laid on the sea bed and hatch in situ may render whelk populations vulnerable to
disturbances as recolonization from adjacent areas can be expected to be low due to
low powers of larval dispersal.
Furthermore, aerial exposure and subsequent
dislocation of egg clusters (Mensink et al., 2000), and the resuspension and burial of
benthic juveniles, by bottom fishing could reduce reproductive success. Ramsay &
Kaiser (1998) found an increased risk of predation of trawled B. undatum. Similarly,
B. undatum densities decreased significantly following experimental otter-trawling
(without a net) in a Clyde Sea loch (Tuck et al. 1998) and the species was found to be
almost absent from areas subject to high scallop-dredging intensity (Veale et al.,
2000). However, in the same study whelks were common at a medium-effort site
implying that B. undatum populations may tolerate or even benefit from moderate
fishing until a threshold of fishing intensity is reached that provokes a population
decline. Otter trawls can be assumed to have less deleterious effects than scallop
dredges or beam trawls (Lindeboom & de Groot, 1998; Collie et al., 2000b) so that
this threshold may not have been reached in the Clyde Sea as yet.
8.4 WIDER ECOLOGICAL ISSUES
Although stable soft-sediment (muddy sand) habitats are generally very susceptible to
trawling few studies have addressed fishing impact in such habitats (Collie et al.,
2000b). A meta-analysis has shown recently that recovery rates of muddy-sand and
muddy habitats from various impact studies worldwide are very low (500 and ~200
days respectively) (Collie et al., 2000). The constant removal of Nephrops could
reduce the construction of burrows thus reducing bioturbation and oxygenation of the
sediment. This is particularly significant as such sediments often have a high organic
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carbon content (Ball et al., 2000). Physical disturbance leads to community changes.
Where fishing effort is high this could lead to impoverished communities in an
alternative stable state adapted to regular fishing disturbance.
Disturbed benthic
communities are generally characterized by small, short-lived r-strategists (e.g.
Pearson & Rosenberg, 1978; Feder & Pearson, 1988) as has been reported from a
variety of fished habitats (Jennings & Kaiser, 1998; Lindeboom & de Groot, 1998;
Hall, 1999; Collie et al., 2000). Thrush et al. (1998) found changes in population
structure involving a decrease in juvenile stages living nearer to the sediment surface
while the deeper living adult stages were less affected. In the long run this can be
expected to have severe ramifications for community structure.
Opportunistic
infaunal organisms such as certain polychaetes can form important prey for large
opportunistic megafauna as these species appear to be quite plastic in their food
requirements (see section 8.2). This altered community in turn may support changed
demersal fish assemblages (Rogers & Ellis, 2000) that are more reliant on benthos and
ophiuroid predation in particular (Frid et al., 1999). Frid & Hall (1999) reported an
increased prevalence of scavengers and decreased occurrence of sedentary
polychaetes in the diet of dab Limanda limanda sampled in the 1950s and 1996/7.
More research into food web changes in relation to fishing effort or natural
disturbance is needed to prove such links.
Recruitment patterns of benthic organisms are generally poorly understood.
Currently, knowledge on stock-recruitment relationships is lacking even for wellstudied species of high commercial value such as American lobsters (Homarus
americanus). Elner & Campbell (1991), for example, found a marked increase in
lobster populations during the 1980s in spite of a simultaneous increase in fishing
intensity and suggested climatic changes as a possible factor governing recruitment.
Chesson (1998) suggested that both recruitment limitation and density-dependent
interaction jointly determine the densities of benthic populations. Juvenile mortality
of benthic invertebrates is typically high, often in excess of 90% (Gosselin & Qian,
1997). These authors suggested that a trait that enhances early juvenile survivorship
even slightly would have a stronger effect on population and community structure
than any trait enhancing survival at a later stage of life (Gosselin & Qian, 1997). This
suggests that even if larval supply was increased by increased adult population
densities, factors governing juvenile mortality can be expected to have a larger impact
on overall recruitment rates, unless other factors favour larval and juvenile survival
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such as a decrease in predator or density dependence. If fishing decreases the density
of predators and effectively decouples juvenile survivorship from predation this could
have serious 'knock-on' effects on benthic community structure (Thrush et al., 1998).
Habitat complexity is of primary importance for settlement of red king crab
(Paralithodes camtschaticus) populations in Alaska (Loher & Armstrong, 2000).
Although larval supply was highest at a muddy site, most settlement occurred at a site
characterized by rocky cobble, providing shelter compatible with cryptic behaviour.
Consequently, one could expect increased postlarval/juvenile mortality due to
decreased habitat complexity in trawled areas. It is known, however, for at least two
of the species regularly discarded, Asterias rubens and Pagurus bernhardus, that
larval settlement primarily occurs in shallow waters owing to positive larval
phototaxis (Anger et al., 1977; Lancaster, 1990) which could contribute to recruitment
resilience in regions such as the Clyde Sea which has large areas of untrawled shallow
water habitat.
There is a conspicuous lack of knowledge of the population dynamics and
habitat architectural function of epifauna of soft-sediment habitats and their ecological
rôle in relation to infaunal communities. Therefore it is difficult to evaluate the
overall ecological effects of discarding.
The Clyde Sea area has been subject to Nephrops trawling for some 40 years.
Marrs et al. (1999) have shown that the fishing effort in the Clyde Sea is intense and
has been so for the past decade judging by landing statistics (ICES, 1999). Overall it
would appear that the predominant species discarded from Clyde Sea Nephrops
trawlers are characterized by their resilience to the impacts of fishing and discarding
as a result of morphological protection, dietary plasticity and high fecundity (see
section 8.2). It could be hypothesized that while the species studied here are adapted
to frequent trawling disturbance, those that are more sensitive to bottom fishing may
have become rare or locally extinct as a result of a long history of demersal fishing.
Studies on the impact of trawling on soft sediment communities in the Irish
Sea and a Clyde Sea loch have shown that otter-trawling affects benthic communities
in the medium- and long-term with recovery periods exceeding one year (Ball et al.,
2000). Tuck et al. (1998) concluded that even fishing during a restricted period of the
year may maintain soft-bottom communities in an altered state. Changes in the
epifauna post-fishing, described by these authors, included a decrease in densities of
plumose anemones Metridium senile, Buccinum undatum and long rough dab
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Hippoglossoides platessoides and an increase in Ophiura ophiura densities although
recovery was achieved within six months. However, experimental trawling did not
involve capture of epibenthic organisms since no net was attached to the groundrope,
so that epifaunal changes and recovery periods of commercial fishing activities may
well have been underestimated. Thrush et al. (1998) stated that recovery rates of
benthic organisms are extremely dependent upon locality and proximity to recruit
source areas and that isolated and small experimental disturbances therefore cannot
mimic fishing disturbances, which take much longer to recover than experimental
plots.
Ball et al. (2000) noted that although present at fished and unfished sites
certain species (e.g. Amphiura chiajei, Bryssopsis lyrifera and several bivalve species)
were larger in size at unfished sites. This is in accord with findings from an ongoing
study by Dr R. Coggan in the Clyde Sea area. Average size of epibenthic species
could therefore potentially be used as an indicator of non-target fishing mortality
analogous to its current application in stock assessments of target species.
8.5 MANAGEMENT & REGULATION FISHERIES
8.5.1 An outlook
The Nephrops fishery is regulated within the framework of the EU Common Fisheries
Policy (CFP). There is abundant evidence to the effect that EU fisheries management
under the CFP, with Total Allowable Catch (TAC) and quota regulation as its main
measures, has failed lamentably with catch per unit efforts (CPUE) of many target
species having declined to unprecedentedly low levels since the introduction of the
regime in 1983 (Hillis, 1998). This could in part be due to the EU Commission
regularly exceeding the 'Recommended TAC' advised by ICES biologists in their final
'Agreed TAC' (Hagler, 1995). Along with shortcomings in the protection of target
species, the CFP has failed to implement the precautionary principle in spite of its
inclusion in the Treaty of Maastricht (1992, §130(2), Symes, 2000). It has also not
taken onboard ecosystem issues despite the outcome of numerous studies,
commissioned by the very same body, that clearly indicate adverse ecosystem effects
of fishing. The review of the CFP in 2002 gives a chance to address such criticism
although it has been argued that preservation of the status quo is the most likely
scenario as a compromise between EC member states (Symes, 2000).
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The Nephrops fishery is currently regulated by a combination of output and input
control measures.
In the next section a brief evaluation of the most important
instruments is attempted.
8.5.2 Output control
Individual transferable quota (ITQ) systems entail limiting the number of fishing units
(e.g. vessels), by allocating a quota (or share of the TAC) in respect of each and
allowing the sale or lease of the right to quotas (FAO, 1998). Although ITQs may
help to a certain extent to protect target stocks, they also encourage fishers to highgrade their landings (Pascoe, 2000) as larger Nephrops command higher prices.
High-grading often involves increased trawling and discarding (Fairlie, 1995). It also
encourages illegal landings and misreporting rendering informed management
decisions based on official landing statistics impossible. Anecdotal reports suggest
that 'black' landings of Nephrops in the Clyde Sea may well be in excess of 30%–50%
of the legal quota. Overseas this may even be higher.
Legislated minimum landing size (MLS) aims to protect target species from
overfishing.
It has been shown, however, that the majority of both discarded
Nephrops and fish show very poor longer-term survival (Symonds & Simpson, 1971;
Edward & Bennett, 1980; Kaiser & Spencer, 1995; Wileman et al., 1999) so that this
tool can be regarded as ineffective. This measure will inevitably increase trawling
activity (in order to reach the quota) and discarding (FAO, 1998), whilst imposing
additional operational costs on the fisher.
In conclusion, the output control measures applied to the Nephrops fishery are
unlikely to reduce discarding as they do not decrease fishing effort per se. Symes
(2000) suggested that the introduction of transferable effort quotas to replace ITQs
would help to resolve this problem. Such an instrument would help to alleviate
discarding and environmental impacts while decreasing operational costs for fishers.
8.5.3 Input control/ Technical measures
Input control measures aim to reduce fishing capacity thereby preserving target stocks
and decreasing discarding. Measures include licensing systems, temporal and spatial
closures and restrictions to fishing vessels and gear (Chapter 1). By restricting fishing
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effort per se, input control measures have the potential to restrict environmental
impacts.
Technical measures involving the improvement of gear selectivity are often
favoured by the fishing industry as more selective gear reduces the undesirable bycatch and time spent sorting the catch. Although larger mesh sizes may lead to a loss
in landings and profits in the short-term, since operational costs remain stable, the
resulting catch will be of a higher grade (larger Nephrops) and thus more valuable and
discarding costs in terms of sorting time are reduced. Owing to overall decreased
landings, the value of the commodity will increase in the long-term, so that
operational costs of fishing may be outweighed and the resource, including non-target
by-catch, allowed to recover. Pascoe (2000) stated that if the marginal costs of
discarding are less than those of gear selectivity (evaluated at zero), in the absence of
regulatory measures, fishers will not employ more selective gear but opt to discard.
However, this equation does not include the external costs of discarding such as
environmental impacts and stock depletion by by-catch mortality of juvenile
commercial species. If these were assigned a value following an approach taken by
Constanza et al. (1997), thus internalizing external costs, fishers may be encouraged
to use more selective gear. On a local scale, the use of creels instead of Nephrops
trawls could be encouraged by monetary incentives to fishers. This could also help to
reduce overcapacity, redundancies and – owing to lower overall catches – lead to an
increase in value.
Figure 8.2. Sorting the catch.
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Input control measures such as gear restrictions are difficult to implement through the
CFP as they may affect different member states unequally. Consider, for example
that Nephrops in the Adriatic have a lower average size than stocks from Northern
waters. Gear restrictions, MLS would affect the fishing industries in both areas
unequally and therefore regionalized management, as presently exists, is essential.
8.5.4 Marine Protected Areas / No-Take Zones
In recent years, many scientists have called for the establishment of Marine Protected
Areas (MPAs) or No-Take Zones (NTZs) as a precautionary policy to guard against
overexploitation of target species and to prevent local decline and extinction (e.g.
Lindeboom, 1995; Watling & Norse, 1998; Roberts & Hawkins, 1999; Collie et al.,
2000a; Sumaila et al., 2000).
NTZs can help to meet many conservation and
management objectives but should be implemented in conjunction with other
management tools (Moore & Jennings, 2000). Philippart (1998) recorded a decrease
or total disappearance of at least 25 species in the Dutch North Sea between 1947 and
1981 and attributed these long-term changes to an increased fishing effort and gear
changes. It is thought that such areas could guarantee a target species brood stock
and nursery ground thus enhancing recruitment to adjacent fishing grounds (Anon.,
1999).
Although the advantage in the case of many migratory fish species is
questionable (Horwood, 2000), it could be beneficial to more sedentary species such
as scallops (Hall-Spencer et al., 1999), Nephrops or red king crabs (Loher &
Armstrong, 2000) and protect certain particularly sensitive benthic communities
(Moore & Jennings, 2000). Closed areas of almost any size have some potential to be
useful, but to have significant effects the total area may have to be a considerable
proportion of the fished area (Anon., 1999; Horwood, 2000). Although recent calls
for 20% of the fished areas to be protected may provide a useful reference point for
future considerations, the size of NTZs is subject to an ongoing debate and a
consensus has yet to be achieved (Anon., 1999; Moore & Jennings, 2000).
Closed areas could provide opportunities for studies on recovery from
exploitation (Engel & Kvitek, 1998; Thrush et al., 1998; Ball et al., 2000; Collie et
al., 2000a,b) although they may never return to their original condition. Hall-Spencer
& Moore (2000a), inter alia, have shown that the most profound changes to fished
marine ecosystems occur at first impact. Pristine areas would therefore particularly
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qualify for such zones that need to be established and managed in close collaboration
with stakeholders.
8.5.5. Banning discards – Norway
This section gives an account on one possible solution to alleviate discarding taken
forward by Norway and draws heavily upon the recent FAO report (1998) on the state
of the world fisheries.
In 1983, Norway imposed a system where the discarding of quota species,
including sizes that might otherwise have been discarded, was proscribed. Fishers
have to ensure that they have sufficient unfilled quotas to allow for any by-catch of
quota species when targeting other regulated species. They also have to leave a
fishing ground if there is a perceived risk of exceeding quotas or if there are abundant
juveniles.
This is policed by a surveillance programme involving chartered
commercial fishing boats. It is now standard practice for grounds to be closed to
trawling if the catch contains more than one juvenile cod and/or haddock per kilogram
of shrimp, if undersized shrimp account for 10% of its weight, or if catches contain
15% of juveniles of the target species. This aspect of the Norwegian approach has
provided a strong incentive for the development of more selective gears in recent
years. The most groundbreaking step, however, has been the ban on discards. Even
when caught unintentionally, fishers are obliged to land all catches of commercial
species, both mature and undersized fish. This 'illegal' catch is landed and deducted
from the TAC of the species concerned. In general the fishing industry supports the
ban on discarding, as experience has taught that high by-catch ratios lead to lower
future catches, though some discarding of Norwegian by-catch outside Norwegian
territorial waters may now occur.
8.5.6 Integrated fisheries management
As more and more scientific studies have pointed out the environmental impacts of
fishing, there has been ever louder calls for fisheries management to embrace
ecological considerations. Instead of elaborating on this complex socio-economic
issue I agree with Symes (2000) and conclude with his summary on integrated
fisheries management:
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"1. introduce a restrictive licensing system to achieve and maintain fishing
capacity at levels commensurate with health of the ecosystem;
2. replace the discredited system of catch quotas by transferable effort
quotas, allocated on a community or individual basis;
3. introduce a discard ban, following the Norwegian example;
4. devise regionally specific gear regulations, including mesh sizes, the
mandatory introduction of more selective gears and restrictions in the use
of certain forms of fishing gear;
5. introduce a more extensive and coordinated network of seasonal closures
to protect nursery grounds;
6. introduce emergency closures where the proportion of juvenile or
immature fish exceeds a given level; and
7. introduce a limited number of NTZs for the protection of designated
habitats and populations of species at risk from serious and irreversible
damage. "
8.6 FUTURE WORK
This thesis has identified invertebrates regularly discarded from Clyde Sea Nephrops
trawlers. It has not established, however, what fractions of the whole population are
caught and discarded. The catch efficiency of Nephrops trawls for the most common
invertebrates could be examined by estimating the population density of epifaunal
organisms using a towed underwater camera in comparison with commercial trawl
catches from the same ground.
In situ observations of surviving discards could help to gain more accurate
mortality estimates. This could prove difficult in practice as Nephrops grounds are
typically too deep for scientific SCUBA diving. Alternatively behavioural laboratory
experiments could be designed to investigate the ability of discarded organisms to
defend themselves against predators and compete for food or shelter.
Recent work by Marrs et al. (2000 and continuing) has provided invaluable
high resolution data on the distribution and intensity of fishing effort for Nephrops in
the Clyde Sea area. These data could be used as a backdrop to studies of the epifaunal
and infaunal communities in areas subjected to varying degrees of fishing effort in
combination with a closed area in a submarine exercise zone.
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Wieczorek et al. (1999) have estimated that the Clyde Sea Nephrops fishery
generates some 25 000 t y-1 discards, however, quantitative data on the food
requirements of soft-sediment epifauna are currently lacking. Food consumption
experiments similar to those by Fonds & Groenewold (2000) in the North Sea are
required in order to be able to evaluate the impact of discards and damaged carrion on
scavenger populations on soft-sediment habitats like the Clyde Sea.
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