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This study investigates the catch components of a commercially used demersal trawl cod end in Izmir Bay, and reveals some of the morphological characteristics of nine commercial ¢sh species; red mullet (Mullus barbatus), hake (Merluccius merluccius), whiting (Merlangius merlangus euxinus), poor cod (Trisopterus minutus capelanus), tub gurnard (Trigla lucerna), common pandora (Pagellus erythrinus), axillary sea bream (Pagellus acarne), picarel (Spicara smaris) and annular sea bream (Diplodus annularis) in relation to mesh size and shapes. Furthermore selectivity of 40-mm polyethylene netting cod end for red mullet, hake, common pandora, axillary sea bream, picarel and annular sea bream is presented. The results are discussed in a way to understand the probable e¡ects of ¢sh body shape on mesh selectivity.

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Currently, traditional Mediterranean trawls are generally made with non-selective netting and the fishing boats are involved in multi-species fisheries. As a result, most near-shore stocks are over-exploited. Weather permitting, the demersal trawl fleet tends to fish in relatively deeper, international waters of the Aegean Sea, where the catch is usually higher. Therefore, the need for evaluation of the codends used in this fishery and the potential improvements to their selectivity are of prime importance. In the present study, selectivity data were collected for hake (Merluccius merluccius), blue whiting (Micromesistius poutassou), greater forkbeard (Phycis blennoides), blackbelly rosefish (Helicolenus dactylopterus dactylopterus) and fourspotted megrim (Lepidorhombus boscii) in commercial (300 MC) and square mesh top panel (SMTPC) codends. Trawling was carried out at depths of 274-426 m onboard a commercial vessel chartered for a 15-day sea trial in August 2004. Selection parameters were obtained by fitting a logistic equation using a maximum likelihood method. Results of the selectivity analysis indicated that the commercially used 40 mm nominal mesh size PE codend was rather unselective for the species investigated in this study. In general, the square mesh top panel codend has relatively higher L 50 values than the commercial codend. However, except for blue whiting, even this codend is rather unselective when 50% maturity lengths (L M50 ) are considered.

Farmed gilthead sea bream, Sparus aurata, frequently escape from the sea cages and interact with wild populations. The impact of these interactions on the wild populations will depend, in part, on di¡erences in performance of the bream. This study compared the swimming performance of the wild and farmed ¢sh in a current channel. The absolute critical swimming speed (U crit ) increased with increasing size while the relative U crit decreased. Even at the same length there were noticeable performance di¡erences between the individuals. The wild sea bream have signi¢cantly higher (Po0.05) absolute U crit performance (0.86 AE 0.01m s À1 ) than the farmed ¢sh (0.79 AE 0.01m s À1 ) and signi¢cantly higher (Po0.05) relative U crit performance (4.52 AE 0.05 BL s À1 ) than the farmed ¢sh (4.21 AE 0.05 BL s À1 ). The present study suggests that cultured sea bream may not have the ability to compete with wild sea bream in native seawaters.

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