Comparison of a rigid grid to a square-mesh panel in size selection for commercial fishing in the Persian Gulf shrimp trawl fishery

Abstract

Two bycatch reduction devices (BRDs): a juvenile and trash fish excluder device (JTED) and a square-mesh panel (SMP), were tested in the Persian Gulf shrimp fishery from a small-scale shrimp trawler in October–November 2012. Each device was alternatively tested by using a small-mesh cover net to retain individuals that escaped the BRDs. Three valuable fish species—tigertooth croaker (Otolithes ruber), silver pomfret (Pampus argenteus) and silver sillago (Sillago sihama)—were selected for detailed analyses. Average escapement rates by number for commercial species in the trawl nets with either JTED or SMP were calculated as 24% and 33%, respectively. There were significant differences in escapement rates for Otolithes ruber and Sillago sihama between JTED and SMP (p < .05). In both BRDs, Pampus argenteus had the smallest escapement rate. A two-sample Kolmogorov–Smirnov test detected a significant difference (p < .05) in the length-frequency of Sillago sihama, and Otolithes ruber captured in the codend and cover net equipped with JTED. Selectivity curves of both BRDs indicated that for each species, the length of 50% retention probability (L50) was substantially smaller than the length at maturity (Lm), except for Otolithes ruber. From an ecological perspective, the codend equipped with SMP was superior to the codend equipped with JTED because it retained substantially fewer under-sized fish and had the smallest selection range. It therefore seems that replacing the standard codend with a codend equipped with SMP provides the best outcome for the fishery whereby fishermen will increase their catches of larger fish while simultaneously reducing their under-sized fish catches.