Size-selective Catch in Tropical Tuna Purse Seine Fishery in the Eastern Indian Ocean: Assessment on New Selectivity Model for Purse Seine Net

Abstract

A series of purse seine survey fishing was conducted in the Eastern Indian Ocean from 1995 to 2003 by M. V. SEAFDEC in order to assess the degree of selective capture in tropical tuna purse seining around drift fish aggregating devices. A new model of size selectivity of a purse seine net was developed as an analytical tool. The model considered retention probabilities on fish encountering a net. The feasibility of selective capture was evaluated by comparing size distributions of skipjack, yellowfin, and bigeye tunas in the catches and selectivity curves for the three species estimated on the model. Size distributions were affected by the selectivity, and sizes at 50 % selectivity were larger than industrially marketable size. Most catch in the three species were commercial sizes, which were selectively retained; however, large proportions of immature yellowfin and bigeye tunas were included in the catch. The exclusion of immature tunas by size selectivity is difficult in multi-species tuna purse seine fishery for commercial purposes. The size selectivity of a net is insufficient as a tool for tuna resource management in multi-species tuna purse seine fishery. The selective capture by other fishing techniques should be pursued for resource management.