Selectivity of 40 mm square and 50 mm diamond mesh codends for five species in the Eastern Mediterranean demersal trawl fishery

The aim of this study was to determine the selectivity of 40 mm square (40S) and 50 mm diamond (50D) mesh codend for five species, red mullet (Mullus barbatus), annular sea bream (Diplodus annularis), common Pandora (Pagellus erythrinus), axillary sea bream (Pagellus acarne) and blotched picarel (Spicara maena) in the Eastern Mediterranean. Selectivity data were collected by the covered codend method and analyzed. Mean selection curves were analyzed and compared using the between haul variations model for red mullet and annular sea bream. Pooled data were used for common Pandora, axillary sea bream and blotched picarel. In total, 22 valid hauls, 10 with 40S and 12 with 50D codends were carried out. The 50% retention lengths (L50) of 40S and 50D codend were found as 14.5 and 15.3 cm for red mullet, 9.5 and 11.3 cm for annular sea bream, 13.1 and 15.0 cm for common Pandora, 14.4 and 15.3 cm for axillary sea bream, 14.5 and 17.2 cm for blotched picarel, respectively. This study showed that codends with 40S and 50D L50 results improve the selectivity when considering commercially used 40 mm nominal diamond mesh codend in Turkish seas for given species. However, in multi-species, it seems likely that a separation of species system would potentially be of more value than size-selective systems. Therefore, different selective techniques such as grid and separation panel and behaviour of the species against the gear need to be investigated.Key words: Codend selectivity, 40 mm square mesh, 50 mm diamond mesh, Eastern Mediterranean, demersal trawl

Selectivity of 40 MM square- and 90º turned-mesh codend for the deepwater rose shrimp, Parapenaeus Longirostris (Crustacea), and greater forkbeard, phycis blennoides (Actinopterygii: Gadiformes: Phycidae), in the Eastern Mediterranean

Background. Bottom trawling is an important fishing technique for catching demersal fish, and selectivity is one of the most important tools for fishery management. This study aimed to determine the selectivity of 40 mm square- (40S) and 90º turned-mesh (40T90) codend for the deepwater rose shrimp, Parapenaeus longirostris, and greater forkbeard, Phycis blennoides (Brünnich, 1768). Material and methods. Fishing experiments were conducted on the international waters of the Aegean Sea by using a commercial vessel between 24 August and 13 September 2012. The hooped covered codend technique was employed for the estimated codend selectivity. The selectivity parameters were estimated by using CC 2000 software. Results. Nineteen successful hauls, 11 with 40S, and 8 with 40T90 codends, were performed. The mean L50 values (50% retention length) of 40S and 40T90 were found to be a 15.5 ± 0.1 mm and 14.8 ± 0.1 mm carapace lengths for the rose shrimp, and 15.3 ± 0.1 cm and 12.2 ± 0.1 cm total lengths for the greater forkbeard, respectively. Conclusion. In conclusion, 165 square meshes in a codend circumference with 40 mm mesh sizes provided higher L50 values than the 40 mm turned meshes, and 330 meshes around the codend circumference. Halving the number of meshes in the codend with larger turned mesh and different modifications should be investigated for further studies in order to appropriate the L50 values. © 2015, Scientific Society of Szczecin. All rights reserved

Selectivity of commercial, larger mesh and square mesh trawl codends for deep water rose shrimp Parapenaeus longirostris (Lucas, 1846) in the Aegean Sea [Selectividad para malla más grande y malla cuadrada de los copos comerciales en la pesca de la gamba blanca Parapenaeus longirostris (Lucas, 1846) en el mar Egeo]

We investigated the differences between size selectivity of a commercial codend (40 mm diamond mesh - 40D), a larger mesh codend (48 mm diamond mesh - 48D), and a square mesh codend (40 mm square mesh - 40S) for Parapenaeus longirostris in international waters of the Aegean Sea. Selectivity data were collected by using a covered codend method and analysed taking between-haul variation into account. The results indicate significant increases in L50 values in relation to an increase in mesh size and when the square mesh is used in the commercial trawl codend. The results demonstrate that the commercially used codend (40D) is not selective enough for P. longirostris in terms of length at first maturity. Changing from a 40D to a 48D codend significantly improves selection, with an increase of about 15% in the L50 values (carapace length 14.5 mm for 40D and 16.6 mm for 48D). Similarly, 40 mm square mesh, which has recently been legislated for EU Mediterranean waters, showed a 12.4% higher mean L50 value (16.3 mm) than 40 mm diamond mesh for this species. However, despite these improvements, the 48D and 40S codends still need further improvements to obtain higher selectivity closer to the length at first maturity (20 mm carapace length)

Alternative codend designs to improve size selectivity for Norway lobster (Nephrops norvegicus) and rose shrimp (Parapenaeus longirostris) in the Aegean Sea

Rose shrimp (Parapenaeus longirostris) and Norway lobster (Nephrops norvegicus) are the two main target crustaceans for the demersal trawl fleet in the international waters of the Aegean Sea. In the present study, selectivity data were obtained for these two species in commercial codends (300 MC is 40 mm polyethylene codend with 300 meshes on its circumference) and two newly designed types, known as a narrow codend (200 MC is 40 mm polyethylene codend with 200 meshes on its circumference - 33% reduced) and square mesh top panel codend (SMTPC is constructed as 150 diamond meshes on the lower and 75 square meshes on the top panel), respectively. These three types of codends were tested using the covered codend method. Trawling was carried out at depths ranging from 269 to 426 m onboard a commercial vessel in August 2004. Selection parameters were obtained by fitting a logistic equation using the maximum likelihood method. The rose shrimp, in terms of weight, was the most abundant marketable species, with catch per hour values of 19.1, 16.0, and 15.0 kg in 300 MC, 200 MC, and SMTPC, respectively. Results of the selectivity analysis indicate that the commercially used 40 mm nominal mesh size PE codend is rather unselective for these species. In general, narrow and square mesh top panel codends have relatively higher L50 values than the commercial codend. However, the differences are significant only for rose shrimp, but not for Norway lobster. It is concluded that despite the improvements in selectivity, the codend modifications 'narrow' and 'square mesh top panel' were not sufficient to release immature specimens of either species, at least not when using the 40 mm PE standard netting. © 2009 Brill Academic Publishers.NECESSITY/501605The authors thank the captain and crew of the commercial trawler “Niyazi Reis”. Thanks are also extended to Z. Tosunog^lu, M. Bilecenog^lu, S. Leblebici, I. Aydın, O. Soykan, and our Greek colleagues G. Petrakis and A. Chilary for participating in the sea trials, to R. Holst for guiding the stacked haul analysis method, and to C. Koçak for discussions on the subject. This study was financed by the Commission of the European Union (NECESSITY/501605). This paper, however, does not reflect the Commission’s view and does not anticipate its future policy in this area. -

Sorting grid trials to improve size selectivity of red mullet (Mullus barbatus) and annular sea bream (Diplodus annularis) in Turkish bottom trawl fishery

Sorting grids with two different bar spacings (12 mm and 14 mm) were tested to improve size selectivity of the commercially important fish species, red mullet (Mullus barbatus) and annular sea bream (Diplodus annularis), in Turkish bottom trawl fishery. Fishing trials were carried out with R/V 'Egesüf' between April and May 2003 in Izmir Bay, Aegean Sea, using a traditional bottom trawl. Selectivity data were collected by the top cover method and analyzed by means of a logistic equation with the maximum likelihood method. Selectivity parameters for individual hauls were obtained with the software program cc 2000. Mean selectivity was also estimated and compared using the EC Model software that takes between-haul variations into account. The codend catch size as an additional explanatory variable was used in the comparison. The Kolmogorov-Smirnov (K-S) test was also applied to detect differences between length-frequency distributions in the upper and lower bags of the 12 and 14 mm bar spacings for red mullet and annular sea bream. The mean L50 values of red mullet were 8.7 and 10.0 cm with the 12 and 14 mm grids, respectively. The L50 value of 14 mm was comparable with the L 50 value of the codend mesh size; however, the 12 mm value was rather low. The mean L50 values of 12 and 14 mm bar spacings were 8.8 and 10.4 cm for annular sea bream, respectively; the value of 14 mm bar spacing was very close to 50% size at sexual maturity of the species (10.5 cm). The K-S test indicated length distributions of red mullet and annular sea bream in the 12 and 14 mm upper and lower bags as significantly different (P < 0.05). These results show that improving the size selectivity in a multi-species fishery using a single selective device is rather difficult. However, higher size selectivity can be obtained when considering the minimum landing size or the 50% sexual maturity size for a given species. © 2008 Blackwell Verlag