Size selectivity of hand and machine woven codends and short term commercial loss in the Northeastern Mediterranean
Introduction
Mersin Bay is one of the richest fishing areas and, due to its topographic structure, one of the most suitable areas for trawl fishing (Gücü and Bingel, 1994) in the eastern Mediterranean (Fig. 1). There is an intensive fishing pressure in the region and as a result of this pressure, very sharp declines in Catch Per Unit Effort (CPUE) and reductions in the lengths of commercial species were reported by Gücü (2000). Poor selectivity of nets used by the trawl fleet is a significant factor in these declines.
Demersal trawling in Turkish waters is carried out on grounds where more than 130 species encounter the gear (Özbilgin et al., 2013). In this multi-species fishery, bycatch and discarding is a significant problem for fisheries management. Consequently, more than fifty published demersal trawl codend selectivity studies have been carried out in the Aegean and the Mediterranean waters of Turkey since the mid 1980s (Özbilgin et al., 2011) and these studies are still continuing today.
European Commission regulations (EC, 1967/2006) require fishermen to replace the 40 mm diamond mesh with square mesh at the codend. If the ship owner makes a justified request, a 50 mm diamond mesh may be substituted in Mediterranean demersal fisheries (E.C., 2006). In preparation for Turkey's entry to the European Community, the potential implications of these regulations upon Turkish Fisheries must be considered (Düzbastılar et al., 2010a). Currently, Turkish Fisheries Regulations (TFR) (Anonymous, 2012) define minimum diamond mesh sizes of 40 mm for the Black Sea, and 44 mm for both the Aegean and the Mediterranean Seas. The use of 40 mm square mesh codend is left to fishermen's preference. However, 40 mm diamond mesh codends are also used in the fleet (Tokaç et al., 2010), and in most cases are assumed to be legal. Multiple studies have shown that the 40 mm diamond mesh codend is unselective for many commercially important species in the Mediterranean (Özbilgin and Tosunoğlu, 2003, Tokaç et al., 2004, Tokaç et al., 2010, Özbilgin et al., 2005, Özbilgin et al., 2007, Özbilgin et al., 2012, Guijarro and Massuti, 2006, Bahamon et al., 2006, Ordines et al., 2006, Tosunoğlu et al., 2007, Sala et al., 2008, Luchetti, 2008, Sala and Luchetti, 2010, Ateş et al., 2010, Aydın et al., 2011). The General Fisheries Commission for the Mediterranean (GFCM) has encouraged scientists to continue to conduct selectivity studies and has suggested that experiments investigate the biological and socio-economic effects of 40 mm square mesh and 50 mm diamond mesh in Mediterranean demersal trawl fleet (GFCM, 2010).
Studies have clearly demonstrated that square mesh codends capture fewer juveniles than diamond mesh codends (e.g., Bahamon et al., 2006, Sala et al., 2008, Özbilgin et al., 2012). Diamond-shaped mesh in trawl nets has a tendency to close as the codend fills, so diamond mesh codends retain smaller fish than a similar sized square mesh codend, whose meshes remain open during a haul (Robertson and Stewart, 1988). But only a few studies (Guijarro and Massuti, 2006, Bahamon et al., 2006, Luchetti, 2008) have assessed the potential economic loss that would be experienced by fishermen during a transition to 40 mm square mesh codends. These studies showed that in the Mediterranean shelf grounds, economic losses would be higher than in the slope fishing grounds due to the escape of a high number of commercial species (Bahamon et al., 2006).
Poor selectivity of demersal trawls targeting fish, crustaceans and cephalopods in Mersin Bay is a significant concern. The majority of the boats working in this trawl fishery use traditional gears with 44 mm mesh size hand-woven slack knotted codends in which approximately 50% of the catch by weight and 70% by numbers are discarded (Özbilgin et al., 2013). This study firstly compares the selectivity of 44 mm hand-woven slack knotted (“commercial”) and three machine woven (“fabricated”) codends: 40 mm square mesh, 44 and 50 mm diamond mesh. Then, it estimates the potential short term losses in landing values if the commercial codend is replaced with one of the alternative codends.
Section snippets
Sea trials
Sea trials were conducted on the commercial fishing grounds of Mersin Bay in the eastern Mediterranean (36°23′ N–34°20′ E; 36°27′ N–34°12′ E; 36°41′ N–34°48′ E; 36°44′ N–34°37′ E (see Fig. 1)), at depths ranging from 14.7 to 141.1 m, between 10 January and 16 December 2011. Experiments were paused between 15 April and 15 September due to the seasonal closure in fisheries. A total of 87 valid hauls were carried out onboard the commercial trawler ‘Azim’ (18 m LOA, 350 HP/261 kW engine power) with tow
Results
Mean values of 60 mesh measurements of each codend were calculated (with their standard errors) as 44.50 mm (0.31) for the CD44, 41.36 mm (0.20) for the S40, 42.03 mm (0.10) for the D44, and 51.14 mm (0.14) for the D50 codends. Total tow duration and catch per hour in each codend and the cover are given in Table 1.
During the study period a total of 91 species were caught and 31 of them were retained by fishermen to be marketed. Selectivity analyses were performed on seven commercial species which
Discussion
Differences between size selectivity of the commercial and three test codends were studied for seven commercial species. Selectivity parameters of the hand-woven commercial codend were investigated for the first time in the present study. Results demonstrate that this codend is significantly less selective than all the others. Seventeen out of 31 marketed species found no chance to escape from it (Table 3). Although the use of slack knots in this codend seems to have a main role in the poor
Acknowledgments
We would like to thank the captain and crew of the commercial trawler “Azim”. Thanks are also extended to students from Faculty of Fisheries in Mersin University for their help during the sea trials. We are also grateful to Michael Pol (Chair of the ICES-FAO Working Group on Fishing Technology and Fish Behaviour) for helpful revision of the manuscript. This study was financed by the Scientific and Technological Research Council of Turkey (TUBITAK 109O684).
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