Size selectivity of gillnet fishery for Nemipterus virgatus in the South China Sea based on SELECT method

We carried out fishing experiments with gillnets of five different mesh sizes(40 mm, 45 mm, 50 mm, 55 mm and 60 mm)in fishing grounds of the northern South China Sea in May 2008, to investigate the size selectivity of gillnets for Nemipterus virgatus. Three function models (Normal, Gama and Bi-normal) were used to calculate the selectivity of gillnet for N. virgatus based on SELECT (Share Each LEngthclass′s Catch Total) method. By comparing the AIC (Akaike′s Information Criterion) value, the best fit model was chosen. The results show that 185 specimens of N. virgatus were collected, and their modal lengths group increased as the mesh size increased. As with the lowest AIC value (466.55), the estimated split model of Gama was regarded as the best fit model. The descending order of relative fishing intensity (P_i) of gillnet for N. virgatus was 40 mm>60 mm>55 mm>45 mm>50 mm. The selectivities of 150 mm-length N. virgatus were 98.39%, 80.94%, 39.30%, 13.70% and 3.87% for gillnets with 40 mm, 45 mm, 50 mm, 55 mm and 60 mm, respectively. Finally, the minimal mesh size of gillnets for N. virgatus was suggested as 55 mm for protection of fisheries resources.