Seasonal Variation in Trawl Codend Selectivity for Annular Sea Bream (Diplodus annularis L., 1758)


Abstract: This study investigated the selectivity of a 40 mm mesh size polyethylene codend commonly used by Turkish demersal trawlers. Seasonal selectivity data were collected for annular sea bream (Diplodus annularis), which is the most abundant discard species in the catch composition in Gülbahçe Bay. Four sets of data were collected in spring (4-18 April 2002), summer (10-25 July 2002), autumn (26 September-2 October 2002) and winter (22-23 January 2003). The selectivity of the same codend was tested under very similar conditions except for the seasonal variables. The data were obtained using the covered codend technique, and analysed by logistic equation with the maximum likelihood method. Parameters of mean curves for each trip were estimated using Fryer's between haul variation model. The results show that although there is a pattern of decrease in selectivity from summer (L_{50} = 9.3 cm, SR = 0.9 cm) to autumn (L_{50} = 9.2 cm, SR = 0.9 cm), winter (L_{50} = 8.9 cm, SR = 1.1 cm) and spring (L_{50} = 8.7 cm, SR = 1.1 cm), seasonal differences in selectivity for annular sea bream are not very high.

Keywords: Seasonal variation, trawl selectivity, the Aegean Sea, Annular sea bream (Diplodus annularis)

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