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Öğe Seasonal variation in trawl codend selection of northern North Sea haddock(Oxford Univ Press, 2006) Ozbilgin, H.; Ferro, R. S. T.; Robertson, J. H. B.; Holtrop, G.; Kynoch, R. J.We investigate the variation in three separate months of trawl codend size selection for haddock, paying attention to changes in the length/girth relationship, and size of the total catch in the codend. Three cruises were conducted on the same fishing grounds using the same fishing gear on board the same commercial trawler in April 1995, September 1995, and February 1996. The periods coincided with postspawning in April, post-summer feeding in September, and prespawning in February. The seawater temperature at the seabed on the fishing grounds was near its annual maximum in September (12.2 degrees C) and annual minimum in February (7.2 degrees C) and in April (7 degrees C). There was significant variation in selectivity of haddock with month and total catch in the codend. Fish length and, for two of the three cruises, fish maximum girth were used as variables in selectivity analysis. The probability of being retained was lowest in September, when the fish were in good condition and water temperature was highest, but girth was also largest (50% retention length L50 = 33.5 cm; girth at L50 = 172 mm). The retention probability was intermediate in February when the water was cold and the fish were prespawning, again with relatively large girths (L50 = 31.3 cm; girth at L50 = 158 mm). The retention probability was highest in April when the fish were postspawning, the water was cold, and the fish had the smallest girth (L50 = 27.3 cm; girth at L50 = 129 mm). Although care was taken to maintain similar conditions during all trips, there were some differences between April and the other months. Overall, retention was not related simply to girth and length. In September and February, at constant girth, codend retention increased with length, suggesting that within a given month, fish condition has an effect on selectivity. However, at constant length an increase in girth in February led to an increase in retention, but in September to a decrease in retention. A possible explanation is that, in February, the girth of a fish increases owing to its sexual maturity, whereas in September, increased girth is due to maximum volume of somatic tissue. Water temperature and volume of fish muscle affect the maximum rate and power of swimming. Swimming ability may be a significant factor in determining selectivity. (c) 2006 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.Öğe Selectivity of commercial compared to larger mesh and square mesh trawl codends for four fish species in the Aegean Sea(Wiley-Blackwell, 2012) Ozbilgin, H.; Tokac, A.; Kaykac, H.Differences in size selectivity of commercial (40 mm diamond mesh, 40D), larger mesh (48 mm diamond mesh, 48D), and square mesh codends (40 mm square mesh, 40S) for hake (Merluccius merluccius), greater forkbeard (Phycis blennoides), blackbelly rosefish (Helicolenus dactylopterus dactylopterus) and four-spot megrim (Lepidorhombus boscii) were investigated in the Aegean Sea. The study was conducted using the covered codend method. Data were analysed taking between-haul variations into account. Results showed that changing from a 40D to 48D codend significantly improved mean L50 values, with increases of about 22% for hake, 8% for greater forkbeard, 20% for blackbelly rosefish, and 11% for four-spot megrim (P = 0.000). A change from diamond to square mesh configuration with the same 40 mm netting also significantly increased the mean L50 values, with 45% for hake, 36% for greater forkbeard, and 25% for blackbelly rosefish (P = 0.000). For four-spot megrim the mean L50 value was about 10% lower, but this difference was statistically insignificant (P > 0.05). Despite the increases in L50 values, this study concludes that the selectivity of the 48D and 40S codends is still not sufficient to release fish smaller than length at first maturity for these four species.Öğe Selectivity of conventional and alternative codend design for five fish species in the Aegean Sea(Wiley-Blackwell, 2010) Tokac, A.; Ozbilgin, H.; Kaykac, H.P>Currently, traditional Mediterranean trawls are generally made with non-selective netting and the fishing boats are involved in multi-species fisheries. As a result, most near-shore stocks are over-exploited. Weather permitting, the demersal trawl fleet tends to fish in relatively deeper, international waters of the Aegean Sea, where the catch is usually higher. Therefore, the need for evaluation of the codends used in this fishery and the potential improvements to their selectivity are of prime importance. In the present study, selectivity data were collected for hake (Merluccius merluccius), blue whiting (Micromesistius poutassou), greater forkbeard (Phycis blennoides), blackbelly rosefish (Helicolenus dactylopterus dactylopterus) and fourspotted megrim (Lepidorhombus boscii) in commercial (300 MC) and square mesh top panel (SMTPC) codends. Trawling was carried out at depths of 274-426 m onboard a commercial vessel chartered for a 15-day sea trial in August 2004. Selection parameters were obtained by fitting a logistic equation using a maximum likelihood method. Results of the selectivity analysis indicated that the commercially used 40 mm nominal mesh size PE codend was rather unselective for the species investigated in this study. In general, the square mesh top panel codend has relatively higher L(50) values than the commercial codend. However, except for blue whiting, even this codend is rather unselective when 50% maturity lengths (L(M50)) are considered.Öğe Size selectivity of three diamond mesh codends for the European hake (Merluccius merluccius) and the tub gurnard (Trigla lucerna) in the Sea of Marmara, Turkey(Blackwell Publishing, 2007) Deval, M. C.; Bok, T.; Ates, C.; Ozbilgin, H.The effects of an increase in the codend mesh size from 44 to 56 and 80 mm on size selectivity of the two by-catch species (the European hake Merluccius merlucius and the tub gurnard Trigla lucerna) commonly captured in the shrimp fishery in the Sea of Marmara, were evaluated. The parameters of the logistic selectivity model were estimated by maximum likelihood. Length at 50% retention, L-50, was found to increase with mesh size for both species. The L-50 values of 44, 56 and 80 mm were, respectively, 19.8, 21.5 and 33.4 cm for European hake, 17.9, 17.7 and 25.6 cm for tub gurnard. The main conclusions are that 44 mm and 56 mm diamond mesh codends retain a considerable amount of undersized hake and tub gurnard as by-catch in the shrimp fishery. With 80 mm mesh size codend, most juvenile European hake are released. For the tub gurnard, however, even this mesh size does not provide a good enough selection. From the biological point of view, it is concluded that the legal 44 mm diamond mesh codend does not provide satisfactory selection for European hake and the tub-gurnard in trawl fisheries of the Sea of Marmara.
