© 2005 International Council for the Exploration of the Sea
Selectivity of diamond- and square-mesh codends in the deepwater crustacean trawl fishery off the Balearic Islands (western Mediterranean)
IEO, Centre Oceanogràfic de les Balears PO Box 291, 07080 Palma de Mallorca, Spain
*Correspondence to B. Guijarro: tel: +34 971401561; fax: +34 971404945. e-mail: beatriz{at}ba.ieo.es.
An analysis of 38 bottom trawl hauls at depths of 251–737 m off the Balearic Islands (western Mediterranean) during autumn 2002 and spring 2003 was used to compare the species and size selectivity of 40-mm diamond- and square-mesh codends under commercial conditions. There was no difference in the catch composition or the yield that could be attributable to mesh shape, although the percentage of total and commercial species discarded with a diamond mesh was higher than with a square mesh. At least in the short term, the escapement ratio and the economic loss with the diamond mesh were lower than with square mesh, but economic efficiency was no different between them. For all the main species compared, except one flatfish, size selectivity parameters were lower for the diamond-than for the square-mesh codend. Selectivity values for the one flatfish species were similar. From the results it is concluded that, within the context of precautionary management, introduction of a 40-mm square mesh in the codend could be an appropriate and plausible measure to improve the state of the resources exploited by the deepwater crustacean trawl fishery of the upper slope off the Balearic Islands, and to reduce the impact of the fishery on the ecosystem.
Keywords: Balearic Islands, bottom trawl, codend selectivity, deep water, discards, diamond and square meshes
Received 1 April 2005; accepted 25 August 2005.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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Knowledge of gear selectivity is crucial to good fisheries management. Its improvement contributes to minimizing the capture of juveniles by regulating the size at first capture, increasing the yield per recruit of targeted species, and reducing the discards and hence the impact of fishing on ecosystems (Armstrong et al., 1990; MacLennan, 1992), some of the principles implicitly enshrined in the Code of Conduct for Responsible Fisheries (Garcia, 2000). Recently, the General Fisheries Commission for the Mediterranean (GFCM) has stressed the incongruence between the minimum legal size for hake (Merluccius merluccius; 20 cm total length, TL) and Norway lobster (Nephrops norvegicus; 20 mm carapace length, CL), established by European legislation, and the estimated length of first capture (12 cm and 16 mm, respectively), given the legal minimum 40-mm diamond mesh in force for trawling in the Mediterranean (GFCM, 2000), and has encouraged studies aimed at improving the selectivity of trawls, in trying to eliminate these contradictions and reducing discards (GFCM, 2001).
Size and shape of the mesh in the codend have been demonstrated as the main factors influencing the selectivity of trawl catches (e.g. Robertson and Stewart, 1988; Reeves et al., 1992). Diamond-shaped mesh in trawlnets stretches under tension during the haul and has a tendency to close when the codend fills, thus reducing its effective selectivity compared with square mesh, which remains open during a tow (Robertson and Stewart, 1988). For this reason, many studies on square mesh have been carried out recently (e.g. MacLennan, 1992; Campos et al., 2002).
Decapod crustaceans, specifically N. norvegicus and Aristeus antennatus (red shrimp), are the prime target of the deepwater bottom trawl fishery carried out in the western Mediterranean (Sardà, 1998; GFCM, 2004). The fishery is well developed on the upper slope off the Balearic Islands (Merella et al., 1998; Carbonell et al., 1999; García-Rodríguez and Esteban, 1999), and landings in Mallorca, the biggest island in the Archipelago, from which about half the trawl fleet operates, are estimated at 10–20 and 100–200 t, respectively, 4–8% of the total landings and 30% of the total revenue. Stock assessments of these two and other target species of the Mediterranean demersal trawl fisheries (e.g. M. merluccius) suggest overfishing (Sardà, 1998; García-Rodríguez and Esteban, 1999; GFCM, 2004).
Catch composition of the deepwater crustacean fisheries off the Balearic Islands have been analysed by Moranta et al. (2000), revealing that discards, mainly fish (73%) and crustaceans (16%), represent 42% of the total catch. However, some differences are evident by depth. In the depths at which N. norvegicus is targeted, fish constitute 60–90% of the catch and 70–80% of the discards. The fish discarded are mainly undersize commercial species such as Helicolenus dactylopterus, Micromesistius poutassou, Phycis blennoides, and M. merluccius. In the depths at which A. antennatus is mainly caught, landings comprise both crustaceans (60%) and fish (40%), although fish (mostly unmarketable species) constitute 70% of the discards.
In the Mediterranean, studies on trawl selectivity using square mesh in the codend have been undertaken mainly in the eastern basin, and have focused on catch composition and discarding (Stergiou et al., 1997b), as well as selectivity parameters of target species (Petrakis and Stergiou, 1997; Stergiou et al., 1997a). In the western basin, a few studies have compared the effects of diamond mesh of different sizes in the codend, for both fish and crustaceans (Sardà et al., 1993; Ragonese et al., 2001, 2002), but to our knowledge, only two studies have assessed the influence of square mesh on target fish on the shelf (Mallol et al., 2001; Sardà et al., 2004).
The current study was initiated to compare, under commercial fishing conditions in the trawl fishery for crustaceans off Balearics, catch composition, commercial yields, retention efficiency, discards, and size selectivity parameters, using the 40-mm "traditional" diamond-mesh codend mentioned in European legislation, and an "experimental" square-mesh codend of similar mesh size. The main objective is to analyse the effect of the introduction of such a codend as a possible management measure to improve the state of the resources and to reduce the impact of the fishery on the ecosystem.
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Material and methods |
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Sampling was conducted on the main fishing grounds on the continental slope south of Mallorca during September and October 2002 (autumn) and May and June 2003 (spring), on board the commercial bottom trawler FV "Moralti Nou" (length 22 m; 59 grt; nominal engine power 365 hp), which traditionally operates in the area. A conventional "huelvano"-type trawl was used (Figure 1). Two codends of 40 mm nominal mesh size but different mesh shapes were used, employing the covered codend method. The cover was a net of 20-mm diamond mesh, attached directly to the funnel end of the net. In order to maintain a good flow of water and to avoid masking the codend meshes, the cover was 1.5 m wider and longer than the two codends. This method has been considered appropriate where catches are not very large (Wileman et al., 1996), and it has been used in most of the trawl selectivity studies in the Mediterranean.
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, diamond mesh; 
, square mesh; ø, diameter).In all, 38 hauls were carried out in daylight (18 in autumn and 20 in spring) between 251 and 737 m, following commercial fishing procedures, but trying to make the same number of trawls with each shape of mesh and at similar depths (Table 1). The average duration and speed of trawls were 4.5 h and 2.5 knots, respectively. Each codend was used on the same gear and changed weekly. After each haul, catches in the codend and the covernet were sorted by taxonomic and commercial (landings and discards) categories, counted and weighed separately. TL and CL of fish and crustaceans, respectively, were measured.
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The PRIMER package was used to analyse the standardized biomass matrix of species by trawl (kg h–1 retained in the codend). After square-root transformation, cluster analysis was applied to assess the different fishing strategies, choosing Bray–Curtis as the similarity index and UPGMA to link samples. Species recorded in fewer than 5% of the samples were omitted from the analysis. A similarity percentage analysis (SIMPER) was also applied to estimate the dissimilarity between groups and the contribution of main species to each.
Two-way analysis of variance (ANOVA) was applied to test differences between mesh shape and season in the commercial yields (kg h–1 retained in the codend) of those species whose cumulative contribution, by SIMPER analysis, was >90%. It was also used to analyse the total catch, the catch of commercial species, total discards, and the discards of commercial species retained in the codend. Data were transformed to 
or log(x + 1) as necessary and checked for normality, and Cochran's test was applied to test the homogeneity of variance. In some cases, transformation did not produce homogeneous variance, but ANOVA is considered a robust analysis when sample sizes are equal (Zar, 1996).
To assess economic performance in the short term of both mesh shapes, the escapement ratio (ER: proportion of the catch escaping, as kg h–1, in relation to the total catch), the economic loss (EL: proportion of the value of fish and shellfish escaping, as 
h–1, in relation to the total value), and the economic efficiency (EE: kg–1of the retained catch in relation to the total weight captured) were estimated by depth range, season, and mesh shape. To test differences between mesh shape and season, two-way ANOVA was applied. To transform the proportional data (ER and EL) to a normal distribution, the procedure was:
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For species in which ANOVA was applied and specimens were present in the covernet, length frequency distributions in the codend and the cover were calculated by haul, season, and mesh shape. Size selectivity was modelled using the generalized logistic curve
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This logistic model was applied haul by haul. A mean selectivity curve, taking into account between-haul variability, was estimated using ECWeb (ConStat), a software that follows the methodology proposed by Fryer (1991). As in some hauls the number of retained and escaped fish was insufficient for estimating the selection curve, the logistic model was also applied on the basis of pooled data from all hauls.
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Results |
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In total, the catch was 6029 kg with an economic value of
3685 (Table 2). Teleosts were the most important group in terms of weight (55%) and second in economic value (32%); crustaceans were the most important group by value (64%) and the second by weight (28%). Elasmobranchs and cephalopods were less important, constituting 14% and 6% by weight, respectively, and 1–3% by economic value. Only two species appeared in all hauls (M. merluccius and P. blennoides), both constituting >5% of the total weight, but the most important species were M. poutassou, Galeus melastomus, A. antennatus, and Parapenaeus longirostris, which constituted 15%, 9%, 7%, and 8% by weight, respectively. The biggest contributions in terms of economic value were A. antennatus (32%), followed by P. longirostris (18%), and M. merluccius (10%). Although N. norvegicus comprised just 3% by weight, its importance in economic terms was great (9%).
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In the similarity dendrogram (Figure 2), a first cluster separated trawls in 399–483 m (SL1) from those in 644–737 m (SL2), with two hauls (<300 m) not taken into account for further analysis. A second cluster separated trawls made during spring (SL1-S and SL2-S) from those made during autumn (SL1-A and SL2-A). There was no association attributable to differences in mesh shape. The results of SIMPER confirmed these clusterings of trawls (Table 3). The greatest dissimilarity was when depth intervals were compared, because they showed a different species composition: M. merluccius, P. longirostris, M. poutassou, P. blennoides, N. norvegicus, and Lepidorhombus boscii were the dominant species in SL1, whereas SL2 was dominated by A. antennatus, G. melastomus, P. blennoides, Geryon longipes, and Plesionika martia. Seasonal differences were less noticeable, a result attributable to distinct contributions by some of these species.
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as kg h–1 retained in the codend); average similarity (
i); s.d., standard deviation; percentage contribution to the similarity (%
i.Comparing yields of the main species and different catch categories revealed that most differences were seasonal (Table 4). Total catch, catch of commercial species, total discards, and yields of A. antennatus, P. longirostris, and M. poutassou obtained in SL1 during spring were higher than in autumn, whereas yields of P. martia and G. longipes in SL2 and of H. dactylopterus, M. merluccius, and P. blennoides in SL1 during autumn were higher than in spring. The only differences related to mesh shape were observed in SL2, where total discards and the discards of commercial species were higher in the diamond mesh than in the square mesh. The interaction of both factors (season and mesh shape) was significant only for the catch of commercial species and for H. dactylopterus and M. poutassou in SL1.
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Catch composition by commercial and taxonomic categories (Figure 3) revealed that the percentage of catch discarded from the diamond-mesh net (18–45%) was greater than from the square-mesh net (6–18%). Such a reduction with the square mesh was also observed in the results for commercial species discarded (7–17% with the diamond mesh, 2–7% with the square mesh). Elasmobranchs and teleosts were the dominant commercial species discarded (31–91% and 6–33%, respectively), followed by crustaceans (7–43%) and cephalopods (
2%).
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A clear increase in the escapement ratio from diamond to square mesh was observed (Figure 4). This is reflected in the economic loss, which is significantly higher with the square than with the diamond mesh. By contrast, there were no differences in economic efficiency between mesh shapes, with season as the only significant factor. With both factors combined (mesh and season), there was no significant difference. No saturation was detected, because the relationships between the escapement ratio and the retained catch did not fit a linear regression: p = 0.322 for diamond mesh, p = 0.624 for square mesh.
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The lengths of the retained and escaped animals by mesh shape and season are summarized for the main species in Table 5. Their selectivity parameters and curves, calculated by mesh shape, season, and both seasons combined, taking into account between-haul variability and pooled data, are shown in Tables 6 and 7, and Figures 5 and 6. For those species where it was possible to estimate size selectivity with both methods, values were similar. In all species, there was an increase in the length at first capture from diamond to square mesh, the only exception being for L. boscii, for which the length at first capture was similar for both mesh shapes.
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, average; s.d., standard deviation) from the length frequency distribution (fish, total length in cm; crustaceans, carapace length in mm) for the main species captured with diamond and square meshes by season.
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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In this work we have compared the selectivity of the "traditional" diamond and an "experimental" square mesh in the codend, under commercial conditions, in the deepwater crustacean trawl fishery off the Balearic Islands. In contrast to other studies comparing both meshes in the Mediterranean (Stergiou et al., 1997a, b; Petrakis and Stergiou, 1997), we took into account not only the size selectivity parameters for the main species but also the selectivity in relation to catch composition, commercial yield, and discards. These are important aspects in fisheries management.
No differences between mesh shape were observed in the species composition of catches (Figure 2). By contrast, bathymetric differences in catch composition confirmed two fishing strategies, previously described for the trawlfishery of the study area targeting N. norvegicus between 350 and 600 m (Merella et al., 1998) and A. antennatus deeper than this (Carbonell et al., 1999). Besides these two crustaceans, other bycatch species were also captured (Table 2). In the first fishing strategy, M. merluccius and P. longirostris, two species with high market value and yields greater than N. norvegicus, can be also considered target species. In the second strategy, G. melastomus is the only bycatch species with yields bigger than A. antennatus, but this elasmobranch cannot be considered as a target species because of its low market value (Table 1).
No differences between mesh shape were observed by comparing commercial yields for the main species (Table 3), although the comparison showed some seasonal differences for P. longirostris, H. dactylopterus, M. merluccius, M. poutassou, P. blennoides, A. antennatus, G. longipes, and P. martia, probably related to their time of recruitment, which in the western Mediterranean is clearly seasonal for some (Massutí et al., 1996, 2001; Company and Sardà, 1997; Recasens et al., 1998). The only differences in yield between the mesh shapes were obtained for total discards and discards of commercial species for hauls targeted on A. antennatus, in both cases discarding being higher with diamond than with square mesh. Such a reduction was also observed in the percentage of the discarded catch, both total and commercial species, using square mesh in each fishing strategy (Figure 3). For total discards it varied between 8% and 36%, depending on depth interval and season. These two factors affect the abundance and size range of the exploited species in this fishery and they are on the basis of temporal variations observed in catches, landings, and discards (Moranta et al., 2000). The reduction of 4–11% of commercial species discarded mainly affected the elasmobranchs Scyliorhinus canicula and G. melastomus, the teleosts P. blennoides, H. dactylopterus, and L. boscii, and crustaceans of the genus Plesionika. On the whole, such a reduction could benefit the environment by decreasing the impact of the fishery upon such a fragile ecosystem (Moranta et al., 2000), one that is particularly important for elasmobranchs because fishing affects this group more than it does most teleosts (Stevens et al., 2000). The Balearic Islands has one of the most diverse and abundant elasmobranch populations in the western Mediterranean and, for this reason, harvesting strategies should be linked to conservation of species in the area (Massutí and Moranta, 2003).
According to Fryer (1991), the selectivity of a net can vary between hauls, so the estimation of selection curves from a model based on between-haul variation provides more realistic parameters than consideration of pooled data over hauls. In the Mediterranean, the small catches mean that the number of fish and shellfish retained and that escape per haul are small too, precluding estimation of selectivity parameters that take into account between-haul variability (Petrakis and Stergiou, 1997). In fact, such information is scarce (Ragonese et al., 2002) and, in our study, estimation of selection curves in this manner was not possible for M. merluccius and G. longipes, and is limited to diamond mesh. In any case, previous and current results (Tables 5 and 6) show similar values for selectivity parameters estimated from both methods.
Selectivity parameters were clearly lower for the diamond than for the square mesh, except for L. boscii, for which values were similar. However, the same parameters estimated for this species in the eastern Mediterranean are higher for diamond than for square mesh (Petrakis and Stergiou, 1997). In general, square mesh is more selective than diamond mesh of similar size for roundfish (Robertson and Stewart, 1988), but the opposite pertains to flatfish (Millar and Walsh, 1992). Although nothing is known in the Mediterranean about the survival of fish and shellfish after escapement from codends, estimates for other areas range between 48% and 89% for gadoids through diamond-mesh codends (Sangster et al., 1996). In the case of elasmobranchs and decapods, their tougher skin and rigid integument, respectively, should allow better survival.
The estimated length at first capture (L50) with 40-mm square mesh for M. merluccius is similar to that obtained for the same species in the Aegean Sea (15 cm TL; Petrakis and Stergiou, 1997). For N. norvegicus, the L50 with the square mesh in the current study is equal to that estimated by Stergiou et al. (1997a) for the eastern Mediterranean with the same size mesh (24 mm CL), but larger than the various values reported for diamond mesh: (i) 23 mm CL with 40-mm mesh in the eastern Mediterranean (Stergiou et al., 1997a); (ii) 18 and 20 mm CL with 40- and 48-mm mesh, respectively, in the same area (Mytilineou et al., 1998); and (iii) 15 and 21 mm CL with 38- and 42-mm mesh, respectively, in the northwestern Mediterranean (Sardà et al., 1993). In any case, square mesh is clearly more efficient than diamond mesh when trawling for N. norvegicus.
Introduction of a 40-mm square mesh codend to the deepwater trawl fishery off the Balearic Islands would improve the exploitation pattern of the main target species, which currently show clear symptoms of overexploitation (Sardà, 1998; García-Rodríguez and Esteban, 1999; GFCM, 2004), by reducing the fishing pressure on small fish, generating improvements in the state of these resources and benefits in their yield per recruit. Although such a change in codend mesh would produce a small, but significant, increase in the escapement ratio (5–15%) and an economic loss (1.4–2.4%; Figure 4), the yields of main species, in terms of biomass, and the economic efficiency would be maintained. In part, this is because escaping individuals are small and of relatively low value. Further, even given small economic loss in the short term, there may be recovery in the medium and longer term, as demonstrated for the P. longirostris fishery off the Gulf of Cádiz (Sobrino et al., 2000). In addition, an increase in L50 with the use of a square mesh would remove some of the contradictions in management of the Mediterranean trawl fishery. For N. norvegicus, the estimated L50 with square mesh is greater than the minimum legal landing size (20 mm CL), which contrasts with the pessimistic perspective of Sardà (1998), who concluded that "fisheries regulations relating to mesh trawl selectivity for Nephrops are not efficient and should be abandoned". For P. longirostris and A. antennatus, species without a minimum legal landing size, the L50 with square mesh is at the size at first maturity, estimated at 20–22 mm CL (Mori et al., 2000; Ben Meriem et al., 2001) and 16–29 mm CL (Carbonell et al., 1999; García-Rodríguez and Esteban, 1999), respectively. Although effective, a square mesh will not allow such a target objective to be attained for M. merluccius, for which the legal landing size is 20 cm TL and the length at first maturity estimated to be 32 cm TL (Oliver, 1993). Reducing the capture of undersized marketable species, which only influences fishing mortality without yielding economic benefit, could help the fishery by minimizing the handling and sorting time of catches and improving the quality of landings.
In conclusion, within the context of precautionary management, introducing a 40-mm square mesh in trawl codends could be an appropriate and plausible measure to improve the state of the resources exploited by the deepwater crustacean trawl fishery off the Balearic Islands, and concomitantly reduce the impact of the fishery on the ecosystem.
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Acknowledgements |
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We thank Joan Jesús Vaquero, Damià Gómez, óscar Fernández, Manuel Salvà, Juan José Picó, and Vicente Sempere, the captain and crew of the FV "Moralti Nou", for their help during the surveys, and Biel Pomar, Ramón Mas, Maria Magdalena Guardiola, and José Luis Pellicer, of the Centre Oceanogràfic de les Balears, for their assistance in both field and laboratory. We also thank Chris Rodgers, John Gordon, Andrew Revill, and an anonymous reviewer for their valued comments on an earlier draft. The study was financed by the Spanish Ministry of Fisheries (RAI-AP-22/2001 and RAI-AP-6/2002).
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References |
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