Size selectivity of codend-mesh size in single boat bottom otter trawls for Nemipterus virgatus in South China Sea

Threadfin bream (Nemipterus virgatus) is an important species in the trawl fishery of the South China Sea. In order to improve the size selectivity and exploitation pattern of single boat bottom otter trawls for N. virgatus, we tested four experimental codends with mesh sizes of 30, 35, 40 and 45 mm by applying the covered codend method in the South China Sea. We analyzed the catch data with double-bootstrapping technique to estimate the selectivity parameters, selectivity curves, delta selectivity curves and exploitation pattern indicators. The results demonstrate that with increasing mesh sizes in codends, 50% retention length (L50) increased, confidence intervals of the selectivity curves became sharper, and retention fraction of undersized individuals decreased. However, when considering the minimum conservation reference size of the target species (15.0 cm), the L50 values of codends tested were less than 15.0 cm, and their discarded percentages were all over 67%. Though the study shows that increasing mesh sizes will improve size selectivity, it should be further improved to achieve a sustainable trawl fishery for threadfin bream.