Size selectivity of diamond and square mesh cod-ends for Nephrops norvegicus in the Aegean Sea

Abstract

Trawl cod-end mesh selection for Nephrops norvegicus was studied in two experiments in the western Aegean Sea using the covered cod-end method (cover: diamond-shaped, 10 mm). The following cod-end mesh sizes were used: 14 mm (knot-to-knot) diamond-shaped (14D), presently operated in the Hellenic commercial trawl fishery; 20 mm square-shaped (20S); 20 mm diamond-shaped (20D), this being the mesh size proposed by the European Union (EU) for the entire Mediterranean. Selectivity estimates were derived from pooling the data over all hauls and sampling dates. The results showed that the proportion of the population retained was higher for 14D than for 20D and 20S (0.99, 0.79 and 0.84, respectively). The direct estimation of the 50% retention length for 14D was not possible because of the very small number of specimens escaping through that cod-end. The results also suggested that 20S has a higher and sharper selectivity than 20D (higher 50% retention length and selection factor, lower selection range) and retains fewer undersized individuals than 20D. Yet, the 50% retention lengths for 20D and 20S were both lower than the length at 50% maturity, a fact indicating that even the use of 20S and 20D may provide little opportunity for reproduction.