Survival of fish after escape from a 40 mm stretched diamond mesh trawl codend in the Aegean Sea

Abstract

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.

Determining the severity of skin injuries of red mullet, Mullus barbatus (Actinopterygii: Perciformes: Mullidae), inflicted during escape from trawl codend

“…Breen (2004) reported that haddock were more likely to be seen at the bottom of the cages, whereas whiting preferred the upper cage section. Düzbastılar (2010a) observed that species such as annular seabream, two-banded seabream, Diplodus vulgaris (Geoffroy Saint-Hilaire, 1817), common pandora, and brown comber were seen to quickly adapt to their new environment, however damaged fish as blotched picarel, red mullet, and black goby attempted to disassociate themselves from the main shoal.…”

“…Previous survival studies carried out at the same site (Metin et al 2004, Düzbastılar et al 2010a, 2010b, 2010c did not investigate the role of scale and skin damages on fish mortality. Metin et al (2004) performed only a visual post-mortem examination for red mullet within the first 24 h following capture.…”

“…However, the 40S and 50D mesh codends had lower mortality rates (26.3% and 27.3%, respectively). Düzbastılar (2010a) reported a low mortality rate (<5%) for red mullet escaping from 40S mesh codend. Variability in the environmental conditions or the state of the fished stock could Düzbastilar et al Survival studies have shown that there was no significant relation between codend catch sizes and escape mortality .…”

“…However, there have hitherto been only a few studies (Metin et al 2004, Düzbastılar et al 2010a, 2010b, 2010c attempting to estimate the mortality of fish escaping from the trawl codend in the Mediterranean. Thus, there is little available information on how escape mortality during commercial bottom trawling is affected by factors such as: codend mesh size and shape, fish size, species-specific variation, and the type of skin injury (Suuronen 2005).…”

“…Thus, there is little available information on how escape mortality during commercial bottom trawling is affected by factors such as: codend mesh size and shape, fish size, species-specific variation, and the type of skin injury (Suuronen 2005). However, a number of studies have addressed escape mortality for some demersal fishes from the Aegean Sea, such as: red mullet, Mullus barbatus Linnaeus, 1758; annular seabream, Diplodus annularis (Linnaeus, 1758); blotched picarel, Spicara maena (Linnaeus, 1758); brown comber, Serranus hepatus (Linnaeus, 1758); and black goby, Gobius niger Linnaeus, 1758; and the causative role of several factors such as the mesh size and shape, fish size, species-specific variation, and water temperature (Metin et al 2004, Düzbastılar 2010a, 2010b, 2010c.…”