Cod-end selectivities of a modified bottom trawl for three fish species in the Aegean Sea

Abstract

Recently, the catch per unit effort of conventional bottom otter trawl nets used in the Aegean Sea has decreased and the mean body length of the major species has become smaller. The reason may be partly the poor selectivity of conventional trawl nets. It may be possible to modify trawl net design to improve selection and protect demersal fish stocks. This study presents selectivity results for the modified design. Cod-end selectivity of red mullet ( Mullus barbatus L.), annular sea bream ( Diplodus annularis L.) and axillary sea bream ( Pagellus acarne Risso) were measured using the hooped covered cod-end method. Cod-end mesh selectivity experiments were carried out using 36, 40, 44 and 48 mm diamond and square mesh cod-ends both in Turkish and international waters of the Aegean Sea in 1995 on board R/V “Egesüf”. The cover mesh size was 24 mm. A total of 85 successful tows were made, the raw data were assessed haul by haul and the valid hauls for each cod-end were combined. The selection of cod-ends was found from the linear logistic function and the selection curves and parameters were derived from these combined data by the maximum likelihood method. The square mesh cod-ends (36, 40 and 44 mm) improved selectivity for red mullet. Fifty percent retention length and selection factors were higher for square mesh cod-ends than diamond mesh cod-ends. However, this did not apply to annular sea bream or axillary sea bream. For these species, it was found that there was no significant difference between diamond and square mesh cod-ends and the diamond mesh cod-ends even gave a slightly higher 50% retention length and selection factor than square mesh cod-ends for some mesh sizes. This result may be explained by the different body shapes and behaviour of the individual species.