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Knowledge on selectivity is considered essential for an adequate management of the trawl fishery. This paper compares the selectivity of conventionally used codends of 44 mm diamond mesh, 40 mm square, and hexagonal mesh for Atlantic horse mackerel, European hake and greater forkbeard in the eastern Mediterranean. Data were collected using the covered codend method and analyzed using the logistic equation with the maximum likelihood method. Mean selection curves were analyzed and compared using the betweenhaul variations model. The mean L 50 values of diamond, square and hexagonal mesh codends are 14.7, 15.9 and 17.1 cm total length (TL) for horse mackerel, 10.4, 14.4 and 11.0 cm TL for hake and 12.2, 15.8 and 12.7 cm TL for greater forkbeard, respectively. For horse mackerel, the differences among the codends are statistically significant (P < 0.05); however, the L 50 values for all three meshes are higher than the minimum landing size (13 cm TL). For hake and greater forkbeard, the L 50 value for the square mesh codend is significantly different from the values for diamond and hexagonal meshes (P < 0.01), whereas there is no significant difference between diamond and hexagonal meshes (P > 0.05). Regardless of the mesh shape, L 50 values were substantially lower than the minimum landing size or size at first maturity for hake and greater forkbeard. To enable more effective solutions, research on species-selective trawls is required for the Mediterranean demersal trawl fishery.

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This research investigated effects of changing the diamond mesh size on codend selectivity in Mediterranean fisheries. The selectivity of a typical 50-mm diamond knotless polyethylene (PE) codend used in the Turkish fishery in the Aegean Sea was measured for commercially important species, in particular hake (Merluccius merluccius), horse mackerel (Trachurus trachurus), anglerfish (Lophius piscatorius) and John Dory (Zeus faber). Fishing trials were carried out on the commercial trawler ÔHapulog˘luÕ between 9 and 12 December 2006 using a modified trawl net. Selectivity data were collected by the covered codend method and analysed by means of a logistic equation (Maximum Likelihood Method). The mean selectivity curve was estimated from individual hauls, taking betweenhaul variations into account. Mean mesh size of the codend was 49.4 mm as measured by digital calliper. Mean values for 50% retention length of hake and horse mackerel were estimated to be 11.4 and 15.6 cm total length; corresponding selection ranges were 4.1 and 5.5 cm respectively. The 50-mm diamond mesh codend showed adequate selectivity compared to the minimum landing size (MLS) for horse mackerel, while for hake it selected specimens in a size range far lower than the MLS. No selectivity values could be determined for anglerfish or John Dory. To design a more selective codend for the Turkish demersal trawl fishery, not only mesh size regulations but also other codend characteristics and netting material properties must be urgently considered.

This study aimed to determine the survival rates of three fish species, the brown comber (Serranus hepatus), black goby (Gobius niger) and annular seabream (Diplodus annularis), after their escape from a 40 mm stretched diamond mesh polyethylene (PE) codend. Experiments were carried out in the eastern Mediterranean in September 2007 using a conventional bottom trawl with 600 meshes around the mouth. A constant 15 min towing duration was used for all hauls. The towing speed varied between 2.0 and 2.5 knots. Codend covers, supported by two hoops, were used to retain escaping fish. At the end of each haul, these covers were detached from the codend, fixed to the sea floor at depths of 19 to 28 m, and then observed by the divers for a period of seven days. On the eighth day, all of the covers were lifted up and the survivors and mortalities were counted and measured. The mean survival percentages of open codend and experimental cages were found to be 97.1% and 98.3% for brown comber, 69.0% and 77.2% for black goby, and 97.5% and 98.6% for annular seabream respectively. Keywords: survival rate, escapes, trawl codend, discard and by-catch, Aegean Sea. RESUMEN: Supervivencia de peces después de escapar de una red de arrastre con copo de 40 mm de malla romboidal en el mar Egeo.-Este estudio tuvo como objetivo determinar las tasas de supervivencia de tres especies de peces, cherna afanecada o merillo (Serranus hepatus), chaparrudo (Gobius niger) y raspallón (Diplodus annularis), después de escapar de un copo con mallas de 40 mm romboidales de polietileno (PE). Los experimentos fueron realizados en el Me-diterráneo Oriental, en septiembre de 2007 utilizando una red de arrastre de fondo convencional con 600 mallas alrededor de la boca. La duración estándar fue de 15 minutos de arrastre para todos los lances. La velocidad de arrastre varió entre 2.0 y 2.5 nudos. Los peces que escapaban del copo eran retenidos por un sobrecojo, el cual quedaba separado del copo mediante aros que facilitan el escape en buenas condiciones entre ambos paños. Al final de cada lance, las cubiertas del sobrecopo se separaban del copo y se fijaban al fondo del mar a profundidades entre 19 y 28 m y fueron observados por buceadores durante los siete días siguinetes. Al octavo día se contaron y midieron los sobrevivientes. Los porcentajes de supervivencia medios de control y en las jaulas experimentales resultaron ser del 97.1% y 98.3% para la cherna afanecada; 69.0% y 77.2% para el chaparrudo y 97.5% y 98.6% para el raspallón. Palabras clave: tasa de supervivencia, escapes, red de arrastre, descartes y capturas acompañantes, mar Egeo.