ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on January 17, 2008
ICES Journal of Marine Science: Journal du Conseil 2008 65(2):148-154; doi:10.1093/icesjms/fsm189
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Minimum landing size for Northeast Atlantic stocks of deep-water red crab, Chaceon affinis (Milne Edwards and Bouvier, 1894)
SHELLTEC Research Centre, Galway-Mayo Institute of Technology, Dublin Road, Co. Galway, Ireland
tel: +353 91 742430; fax: +353 91 758412; e-mail: martin.robinson{at}gmit.ie
Robinson, M. 2008. Minimum landing size for Northeast Atlantic stocks of deep-water red crab, Chaceon affinis (Milne Edwards and Bouvier, 1894). – ICES Journal of Marine Science, 65: 148–154.Annual landings of the deep-water red crab (Chaceon affinis) in the NE Atlantic have fluctuated around 1000 t for much of the past decade, but they dropped significantly in 2006. No EU or National Member State legislation governs the harvest of the species, and because of the on-board processing, it is difficult to set a single minimum landing size (MLS) based on carapace width (CW) alone. As the claws are detached during processing and represent the most valuable component of the catch, a supplementary MLS based on claw length (CL) for onshore inspection and enforcement would seem appropriate. Physiological size-at-maturity and morphometric claw data were used to derive primary (CW) and secondary (CL) MLS. All males and females are mature at 110 and 125 mm CW, respectively, and 50% of females are mature at 109 mm CW. Because of a lack of information relating to the biology, distribution, and fishing mortality of the species, and a doubt as to real landing statistics, a conservative MLS of 120 mm CW and 72 mm CL is suggested, representing the first use of commercial fisheries data to suggest MLS for this otherwise unregulated fishery.
Keywords: Chaceon affinis, deep-water red crab, management, minimum landing size
Received 19 June 2007; accepted 1 December 2007; advance access publication 17 January 2008.
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Introduction |
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The brachyuran red crab (Chaceon affinis) is the largest known species of the family Geryonidae (Manning and Holthuis, 1989). It inhabits various substrata on continental slopes, seamounts, and deep-water banks between 400 and 1500 m deep in the Northeast Atlantic, between Iceland (64°N) and the Canary Islands (28°N). Commercial fishing vessels began to target the species in the NE Atlantic in 1995. Landings reported earlier than that were a result of bycatches in static net fisheries primarily targeting deep-water finfish. Most landings (>95%) reported for C. affinis in ICES Areas of the NE Atlantic are made by UK registered vessels and flagships (FAO, 2000), although some Irish vessels joined the fishery in 2002.
Combined annual landings of UK and Irish vessels (Table 1) fluctuated widely over the period 2000–2006, with a general decline over recent years. Reported landings for 2004 and 2005 were significantly lower than the 2003 high of 1132 t (D. Eaton, Cefas, pers. comm.), and they dropped further to just 220 t in 2006. The occurrence of C. affinis on more than one type of substratum has led to the development of two fishing methods to target it in ICES Areas. As static nets are used on soft substrata with relatively smooth topography, and pots are used on rougher, more irregular and coralline habitats, the two methods are not generally employed in the same fishing areas. The use of pots specifically to target the species commercially began in 2001, and then accounted for 26% of the total catch. This figure rose to 41% in 2003 as a strong pot fishery developed in ICES Divisions VIa and VIb (Table 1). Irish and UK national reports revealed no landings from pot fishing during 2004 or 2005, and just 132 t in 2006. From direct observations of port landings, it is clear, however, that there was a significant amount of pot fishing within this period, and it appears that there is a general failure to capture robust data relating to the exploitation of the resource. A contributing factor has been the novel nature of the fishery, which has led to some misreporting attributable to incorrect identification of the species by industry and port staff. Confusion with a smaller family member (Chaceon tridens Krøyer, 1837) is one source of error, but the species has also been incorrectly recorded as "king crab" or "crab royale" on many occasions.
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The greatest uncertainty in determining actual catch, however, is associated with the practice of processing at sea and the landing of only marketable body parts. This involves removing the claws and splitting the carapace down the midline, dividing the crab into two sections of equal size. The remaining carapace fragments, viscera and gills are discarded. Such processing necessitates the use of raising factors to estimate the weight of the original catch, but it is acknowledged that the raising factor used currently for any/all processed body parts landed (x4 is generally used) is inaccurate because of variation in the weight of product derived by sex and size, the type of product packed in each freezer carton (claws, leg clusters, legs), misreporting of the type and quantity of products landed, and a general lack of robust processing data on which to base calculations. Failure to identify and record body components as originating from C. affinis (e.g. the use of generic terms such as crab claws) also contributes to the doubt surrounding the validity of landings data. The lack of robust landings and corresponding effort data is of particular concern when the unregulated nature of the fishery is considered. Although management plans for several other global fisheries for Chaceon spp. are relatively well-developed (Le Roux, 2001; SAFE, 2004; NEFSC, 2006), there are currently no EU or EU National member state legislations relating to technical conservation measures, input or output controls for the fishery, and little is known of the biology, ecology (Hastie, 1995; Fernández-Vergaz et al., 2000; Pinho et al., 2001; Lopez Abellán et al., 2002), or stock resilience of the species under fishing pressure (Course and Lart, 1994; Hastie, 1995). Other Chaceon species have been shown to be K-strategists so can be considered to be vulnerable to overexploitation when poorly managed (Erdman and Blake, 1988; Hines, 1988).
The precautionary approach to fisheries exploitation and management dictates that measures be taken to ensure that it is harvested in a sustainable manner (Caddy and Seijo, 2005). Although setting a legal minimum landing size (MLS) would be considered an easily introduced and readily enforceable initial step for management of this species, the practice of processing at sea renders a traditional carapace-based regulation impractical for land-based inspection. More recently, some potting vessels have started to store the catch in on-board seawater tanks, and to land whole, live animals to onshore processing factories. Although these factories utilize the species to produce similar end-products to those processed offshore, the size of the crabs being processed can be inspected on landing. The current practices of making processed or live landings of C. affinis do not allow for the application of a single MLS without discriminating against one group of operators. If a MLS was established based on carapace width (CW) alone, it would only apply to crabs landed live, not to those processed at sea. Alternatively, to insist that whole animals only be landed would discriminate against vessels with freezing capacity, which process at sea because of the reduced storage capacity needed for marketable body parts and live storage. Claws are by far the most valuable component of the catch, so are ideally suited to the application of an additional MLS for inspection by shore-based officials; claws are usually proportional to body size. As measurement of CW is more easily and rapidly achieved than claw length (CL) when the claws are attached to the body of a crab, a MLS defined by the former parameter remains desirable for instances where there is no processing at sea (allowing such vessels to operate at their existing rate of efficiency).
The onset of targeted exploitation of C. affinis by Irish pot fishing vessels in 2002 provided scientists with direct access to commercial fishing data and catches. In that case, fishers agreed to adhere to a voluntary MLS based on the more conservative of (i) the size indicated by scientists to represent a safe biological limit, and (ii) the size specified by processors as the smallest size acceptable for marketing purposes. There are examples of fisheries where a legal MLS based on biological data is permanently or periodically below that dictated by local buyers and processors (Addison and Bennett, 1992). Here, I use maturity and morphometric data to produce a possible MLS for managing the NE Atlantic C. affinis fishery. I also make suggestions for a compulsory MLS based on biological parameters, and for a more conservative MLS should market requirements and management require it.
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Material and methods |
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Commercial trapfishing, using standard creel and inkwell pots, was conducted on the Anton Dohrn Seamount and Rockall Bank during the period 2002–2004. Each set of fishing gear consisted of
100 pots equally spaced along the length of a single rope or fishing "string", the vessel carrying between 600 and 800 pots. On-board observers recorded the size of a random sample of C. affinis from each fishing string. The entire catch from a randomly selected pot was recorded, 0–20 crabs, including those subsequently discarded as below commercially viable size. The process was repeated for a number of randomly selected pots along each fishing string. CW, the distance between the tips of the fifth lateral spines, was recorded to the nearest millimetre using callipers. Between May and July (the full season for Irish vessels), male and female crabs from each size class of 5 mm were retained for subsequent analyses of maturity. Total weight (TW) of each crab retained for maturity analysis was recorded to the nearest gramme, and the length of each propodus (CL) was measured to the nearest millimetre using callipers. Initial indications suggested that the CL was the most easily measured claw dimension when compared with the more ambiguous task of defining the point of maximum width or height.
Male physiological maturity was assessed by visual examination of the size, colour, and degree of coiling in the vasa deferentia. Males were deemed to be mature when relatively large, white, highly coiled vasa deferentia were visible to the naked eye and contained many spermatophores when examined microscopically (Haefner, 1977). The size, colour, and appearance of female ovaries were recorded. Ovaries were dissected out and weighed to an accuracy of 0.1 g.
Females were deemed to be immature if ovaries were cream or white in colour (Haefner, 1977; Erdman and Blake, 1988; Fernández-Vergaz et al., 2000) and represented <1% of TW, and mature when purple/brown and >1% TW. Pleopods were examined for evidence of previous spawning (eggs and adhesive residue). If present, crabs were deemed mature, regardless of relative ovary weight. Although this allowed for the categorization of functionally mature crabs that had reproduced and possessed spent/recovering ovaries of a relatively light weight, perhaps crabs with recovering ovaries but no signs of previous spawning may have been present in the samples. The categorization of females based on evidence of previous spawning and current ovary condition allowed for the best possible assessment of maturity in the absence of samples from other periods of the year. Because of the unavailability of samples from all seasons, it was not possible to determine whether crabs in those size classes where many were deemed immature would have matured by the end of the year. Using such criteria, males and females were assigned to immature or mature groupings by 5-mm CW size class. A maturity ogive was constructed for females using FISHPARM (Prager et al., 1989).
The claws of crabs dissected during maturity analysis were weighed separately, and the main body carcass was reduced to commercial "clusters" by separating the carapace and viscera from the leg sections. The resulting sections were weighed individually.
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Results |
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There was a strong indication of spatial segregation of size classes of C. affinis within each area fished, smaller crabs inhabiting deeper water. Figure 1 shows the pooled size frequency distribution of randomly subsampled CW measurements recorded from all fishing areas between the Anton Dohrn Seamount and South Rockall Bank. Except a small distinct grouping <100 mm CW, the size distribution was relatively normal. It is stressed that no distinction is made here between the sexes because the data are used subsequently only to determine the portion of the population discarded under varying management regimes. Sex ratio (F:M) varied between area, but approximated 1.5:1 overall. For crabs between 100 and 170 mm CW, males tended to be heavier than females of the same CW (Figure 2, Table 2).
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From examination of the vasa deferentia and spermatophores of 5–6 crabs per 5-mm CW class, male size-at-50%-maturity was estimated to be 94 mm CW, and no male >107 mm CW was immature. Values for male size-at-50%-maturity are listed in Table 2.
Of the 280 females examined, 58% showed signs of previous spawning and were deemed to be functionally mature. In all, 43 of the 117 crabs that showed no sign of previous spawning were deemed to be mature based on ovary condition. Corresponding ovary weights for immature and mature groups are shown in Figure 3. No female crab >125 mm CW was categorized as being immature, although some mature crabs of >120 mm CW with redeveloping ovaries of relatively low weight are evident on Figure 3, below the main cluster of data at this size. The maturity ogive derived from the proportion of crabs categorized as mature or immature in each 5-mm CW class is shown in Figure 4. Output from a Weibull cumulative function was used to estimate the proportion of mature crabs in each 5-mm CW size class (Table 2). In the 100-mm-CW size class, just 11% of the females were expected to be mature, rising sharply to 100% in the 125-mm-CW size class. Female size-at-50%-maturity was estimated at 109 mm CW.
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Much of the additional increase in mean TW of male crabs can be attributed to the disproportionate increase in claw growth after 80 mm CW. One claw was always slightly larger than the other, between 1% and 10% of length, in >95% of male and female crabs. Because of the variability in CL at body size and the discrepancy between claw sizes of a single crab, there was no significant difference between regressions lines fitted to larger and smaller claws separately, so data were pooled by sex only. The incidence of individuals with one or more regenerating claws, judged by a gross visual assessment of relative size, was <1% in the crabs from which size frequency data were collected (Figure 1). Figure 5 displays sex-specific relationships between body size and CL (propodus) for both claws of those crabs sampled. The relationships were used to derive the sex-specific mean CL for the smallest size in each 5-mm-CW interval shown in Table 2. Dimorphism in claw growth between sexes is clear, with a mean CL of 72 mm reached two size classes earlier in males, but increasing to three size classes earlier by 84 mm CL. As changes in overall claw allometry between sexes were relatively subtle, claw data are not suitable for retrospectively ascertaining sex-specific body size, so are used here only to determine suitable MLS.
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The right column in Table 2 shows the proportion of the catch presented in Figure 1 that would be discarded if a MLS was set at the proposed size. It provides some indication of the economic and biological impact of implementing various combinations of MLS. For example, if a MLS of 110 mm CW was implemented to coincide with the expected female size-at-50%-maturity stated earlier (109 mm CW), then just 13% of the total catch would be discarded as undersized.
Although not strictly appropriate for two independent variables, the relationship between claw weight and total body weight may be useful in terms of determining raising/conversion factors for processed fisheries landings. Figure 6 shows the positive relationship between the combined weight of both claws and total body weight for both sexes. Claw weight accounted for 15% of total body weight using the relationship derived here. When processed using an identical methodology to that of industry, leg clusters accounted for 30% of total body weight, with the remaining 55% being carapace and viscera (which were not retained). These data suggest that a raising factor of 6.66 be used for claw-only landings, and that one of 2.22 be applied to landings consisting of claws and leg clusters.
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Discussion |
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The data presented here provide a basis for formulating a MLS for NE Atlantic stocks of C. affinis. It is not possible to establish a single MLS for the species without discriminating against one of the two fishing methods currently used to fish the resource. Based on the results presented here, it is proposed that a primary and secondary MLS be established for the species, with the former overriding the latter always. The primary parameter for MLS suggested is CW (CW MLS), and it should apply to all crabs landed to which one or both claws are attached. As it is clear that males of this species mature before females, a MLS should be determined by data relating to the latter. The size-at-50%-maturity presented suggests that CW MLS should not be set less than 109 mm. Considering the novel nature of the fishery, doubts in the accuracy of landings statistics, and a lack of knowledge pertaining to stock size or biology, an initial precautionary CW MLS of 120 mm would seem worthy of consideration by managers.
In some cases, a conscious decision to remove claws from the body and to destroy the carapace at sea is made, although leg clusters are also retained. In this case, a secondary MLS could be relevant for detached CL (CL MLS), because as stated earlier, claws are by far the most economically valuable component of the catch. Removal and discarding of claws of undersized crabs and the retention of leg clusters alone would soon render the fishery uneconomical. It is not uncommon for some species of crab to possess claws that are unusually small relative to their body size, because lost appendages redevelop to full size over a period of several moults. The ability of discarded crabs to survive and to regenerate claws is the basis of fisheries with claw-only harvests (Wenner and Stokes, 1984; Culver and Kuris, 1992; Tully et al., 2006). The presence of a large number of crabs with regenerating claws in C. affinis stocks would allow for claims that undersized claws were in fact removed from a crab of a legal CW MLS. In such cases, it should be remembered that a conscious decision has been made to remove the claws, so the onus should be for the fisher grading the catch to change from applying a CW MLS to an appropriate CL MLS when de-clawing is being done. The minimal occurrence of crabs with one or more regenerating claws (<1%) in this study would also suggest that including undersized claws from crabs of legal-sized CW in the landings should not pose a problem currently. If the data relating to the incidence of regenerating claws in the fishery were updated regularly with independent on-board observations, this may also offer a baseline against which to establish an acceptable tolerance of undersized claws in commercial landings (Tully et al., 2006). Although the incidence of crabs with regenerating claws may change over time owing to natural antagonism between conspecifics (Moksnes et al., 1998; Fernandez, 1999), changes in fishing practices can cause more rapid and significant changes. De-clawing can cause dramatic changes (Patterson et al., 2007), but because the legs and leg clusters represent a significant percentage of the total post-processing weight of marketable body parts of C. affinis, this practice is unlikely to be seen in the fishery studied here.
Although applying a CL MLS is seemingly less practical than a CW MLS during commercial fishing operations, trials on board Irish vessels proved that the use of a fixed CL gauge was simple and effective, particularly when compared with those for legs. Further, a sex-specific CW could be employed for this species as differentiation of the sexes is unambiguous and rapidly achieved as a result of gross abdominal morphology. The true value of a CL MLS is in the onshore inspection of landings by port authorities, however. As male claw size at any given size is greater than that of females, a CL MLS can also be dictated by data based on females. Assigning a CL MLS based on male mean CL corresponding to a required female CW MLS size class would effectively increase the CL MLS of females from which the claws could be removed, so discriminating against vessels that process at sea. For example, if a CL MLS of 80 mm was set from male data to correspond to a CW MLS of 120 mm based on female size-at-50%-maturity, then because of sexual dimorphism in CL, claws could only be removed from females >130 mm CW. For this reason, the CL MLS should be determined by female data or based on a male CL several size classes below that corresponding to a CW MLS based on female maturity. The greater the number of size classes between these parameters, the less conservative would both MLS values become. Based on the data presented, it is suggested that a CL MLS be set at 72 mm, corresponding to a CW of 110 mm for males. This integrates with the recommendation made for a CW MLS >120 mm (based on female maturity), because the mean female CL at this size was 72 mm.
Although implementing this MLS would effectively reduce the size at which males can be retained from 120 to 110 mm CW, and may encourage some de-clawing of males around the size range, all males are mature at this smaller size, and they represent a very small component of overall catch. CL is selected as the secondary MLS over other claw dimensions because it is less variable than height or width for any given size class of crab. The decision to set a CL MLS on mean male CL at the smallest size in each size class was made because of the wide variability in CL with size. Although it would at first seem logical to set a CL MLS based on the lower 95% confidence interval (CI) of male CL at any given size interval, the overlap between size classes was significant. Use of the lower 95% CI would in this case have determined a CL MLS several size classes below that selected, effectively lowering the CW MLS for both sexes. The situation is similar if CL data for both sexes are combined, because the 95% CI again increases significantly, reducing the CW MLS. Given the lack of information about this stock, a conservative combination of MLS is considered appropriate. Although this may lead to a relatively small number of females of legal CW MLS being rejected by vessels that process at sea because of their undersized claws, the size composition of the catch suggests that the economic loss would not outweigh the biological benefit, especially when the mean CL at size is determined by the smallest CW in each size class.
On establishing the Irish C. affinis fishery in 2002, participants agreed to adhere to a MLS dictated by the biological parameters of the population, or that of processors based on market demand if greater in size. Although a CW MLS of 120 mm was suggested, as in the current study, processors required crabs of not less than 500 g live weight, corresponding to a CW MLS of 125–130 mm. As the Irish fishery only lands live, whole crabs, it has been possible to ensure 100% compliance to this voluntary MLS, which proved highly effective in minimizing wastage at processing and maximizing economic benefit. It is possible too that periodic or regional differences in market demand may require application of a MLS above that suggested on a biological basis. Other global fisheries incorporate management measures that remove the need to develop the type of dual MLS suggested here, including a requirement to land only whole animals (Taylor et al., 1994; Weinberg and Keith, 2003), animals whose bodies have been sectioned but not had the claws detached (Steimle et al., 2001), or strict limitations on the proportion of a vessel’s total landing weight represented by detached claws (Tully et al., 2006). Each of these methods would currently discriminate against vessels using one or the other of the two fishing methods for C. affinis, or render fishing uneconomical for both, however.
The landings of a vessel that has processed at sea could be expected to contain a claw to leg cluster product weight ratio of 1:2. Adding an additional 55% of the combined weight of these product components would give a reasonable approximation of the TW of whole animals removed from the fishery before processing. With knowledge of the approximate size structure of the population processed, possibly by examination of CLs, it could be possible to improve the accuracy of the current x4 raising factor used significantly. Although negligible in the context of the current study, knowledge of the proportion of crabs with regenerating claws within the fished stock would permit further refinement of future estimates of stock structure derived from claw data (Weinberg and Keith, 2003, 2005).
The shape of the size frequency distribution is typical of long-lived K-strategist crab species in that, after maturity, growth becomes so reduced (Hartnoll, 1974) and/or variable that subsequent cohorts blend into a normal distribution. The distribution presented is similar to that observed in studies of C. affinis off the Canary Islands (Lopez Abellán et al., 2002) and the Azores (Pinho et al., 2001). It is also representative of that obtained by pot fishing only, and may differ significantly from that obtained in static net catches. Static nets are far less selective than fishing pots and often cause crabs to be damaged or destroyed to disentangle them, and the presence of larger conspecifics in pots can deter smaller crabs from entering and result in commercial catches being unrepresentative of the true population size structure (Millar and Addison, 1995). The use of pots allows suitably small crabs to escape through the mesh surrounding the pot frame, or to be discarded alive and intact from the vessel deck. The addition of escape gaps further decreases the catch of sublegal crabs of certain species (Guillory, 1993; Tallack, 2007). Although pot escape gaps are not currently used in the C. affinis fishery, their use should be examined considering the depths involved and the fact that crabs may be preyed upon on their way back to the seabed. Recent work by Tallack (2007) has shown the positive benefits derived from the addition of pot escape gaps for Chaceon quinquedens in New England. Even if a discarded crab returns uninjured on its way back to the seabed, it is likely to be displaced horizontally, and perhaps may land on unfavourable substratum (Wigley et al., 1975). Biodegradable escape panels also reduce the incidence of ghost fishing, which can cause increased mortality of crab in lost/discarded pots (Godøy et al., 2003) and monofilament nets (Kaiser et al., 1996). As the fraction of the population targeted by the various fishing methods is poorly understood and may be complementary, research effort should be directed towards examining the relative impacts of static net and pot fishing on this species and on the ecosystem of which it is a part (Quinn and Collie, 2005).
Although the data presented here appear suitable for application in the study area and possibly the relatively nearby, newly established Faroese fishery, spatial variation in population parameters might render the results unsuitable for other fished populations of C. affinis in the Canary and Azores Islands (Fernández-Vergaz et al., 2000; Pinho et al., 2001; Lopez Abellán et al., 2002). The estimate of 109 mm CW for size-at-50%-maturity is close to the 108 mm CW cited by Fernández-Vergaz et al. (2000) as the size of female maturation, however, despite those authors reporting no female crabs with evidence of previous spawning at a CW <120 mm, unlike the 103 mm CW of this study. The size-at-first-maturity for females in the range 83–97 mm CW reported by Pinho et al. (2001) would also appear to agree with the data presented here. There appear to be a number of similarities between the reproductive development of crabs from these geographic locations. Genetic studies similar to those conducted for other geryonids (Weinberg et al., 2003) may be required to elucidate more fully the range and stock connectivity of C. affinis in the NE Atlantic.
The precautionary approach to fisheries management dictates that, at a minimum, regulation is necessary to ensure that species are harvested within biologically safe limits (Rosenberg et al., 1993). This ethos extends not only to the quantity of a species harvested, but also to the reproductive potential of the animals that form the catch (Hastie, 1995). Chaceon affinis is currently harvested in significant quantities in the absence of any regulation or control in the NE Atlantic. No study until now has presented commercial fisheries data and attempted to propose a MLS. The findings here represent a rational basis for the establishment of non-discriminatory technical conservation measures for the species. They are intended to act as a guide and stimulus to the development of management measures to conserve the species in the absence of another form of regulation, and until more robust, seasonally available biological data on which to base regulation become available.
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Acknowledgements |
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The project was funded by the Irish Government and part financed by the EU under the National Development Plan 2000–2006 through the programme for Innovation and Sustainability in the Fisheries Sector. I acknowledge the support and participation of the vessel owners, skippers, and crews of the MFV "Peadar Elaine" and MFV "Niamh Eoghan", as well as the support of Bord Iascaigh Mhara (BIM; the Irish Sea Fisheries Board). I also thank Derek Eaton of the Centre for Environment, Fisheries, and Aquaculture Science (Cefas, Lowestoft, UK) for providing summary landings data, and Oliver Tully (BIM, Ireland), Rick Wahle (Bigelow Laboratory for Ocean Sciences, Maine), and an anonymous referee for comments on early drafts that greatly improved the final manuscript. The views, however, are my own, not necessarily those of the agencies and people mentioned.
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References |
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