Size-selective catch by fishing operation technique in tropical tuna purse seine fishery in the Western Indian Ocean: Feasibility of free school operation for skippers

Size-selective fishing operation types and skippers’ fishing strategies in operation type combinations were analyzed using fishing data from Thai tuna purse seiners in the Western Indian Ocean during 2005–2007. Catch species and size compositions of tropical tuna, i.e., skipjack, yellowfin and bigeye tunas, were compared among operation types, including free school (FS), fish aggregating device, natural log, and other floating objects operations. FS operation was found to be the most size-selective technique, and caught the fewest small-sized individuals, while the associated operations were less size-selective. Fishing strategy analysis showed that success rates and differences between optimistic and actual values represent economic risk. FS operation holds the highest risk; however, it represents potentially high-revenue fishing because of its ability to catch large-sized individuals and high-priced species. Skipper skills are believed to affect a skipper’s fishing strategy, and specialist and generalist skippers were both identified in this analysis. The specialists achieved high revenue by overcoming the risks of FS operation via their skills, while generalists distributed fishing efforts over operation types to avoid risks. Simulation results suggested that high- and moderate- skilled skippers can shift to FS operations with no revenue decline to respond to the policies of the Tuna Regional Fishery Management Organizations, which increasingly promote FS operation.