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8. GENERAL DISCUSSION _____________________________ 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 149 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 150 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 151 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 152 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 153 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). 154 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 155 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 156 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., 157 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 158 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 159 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 160 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). 161 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 162 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. 163 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 164 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: 165 "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. 166 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. 167