Selectivity of PE and PA material codends for rose shrimp (Parapenaus longirostris) in Turkish twin rigged beam trawl fishery

This study investigated the differences in the selective properties of 32 mm nominal polyethylene (PE) and 32, 36, 40 and 56 mm nominal polyamide (PA) codends in beam trawl fisheries targeting rose shrimp, Parapenaeus longirostris, in the Sea of Marmara, Turkey. Data were collected in spring 2005 onboard a commercial trawler, on commercial fishing grounds. Selectivity data were collected using the covered codend method in two twin beam trawls in every haul. Using this method, four selection curves could be obtained in one haul. Selectivity parameters were calculated by fitting a logistic function using maximum likelihood. The L50 values of 32, 36 and 40 mm PA codends were found to be 9.1, 10.2 and 12.6 cm, respectively. Due to retention of only a few specimens, realistic selectivity parameters could not be calculated for the 56 mm PA codend. Mean L50 decreased with a change in mesh material from PA to PE. The lowest L50 (8.0 cm) was found for the 32 mm PE material codend. This study has shown that for rose shrimp the 32 mm PA diamond mesh codend is not appropriate for the first maturity size (FMS) of 10 cm for rose shrimp in the Turkish twin rigged beam trawl fishery. In addition, the PE netting material provides a significantly lower L50 value than the PA netting material.