Bycatch and discards by Taiwanese large-scale tuna longline fleets in the Indian Ocean

Conservation of ecologically related species and understanding the discard composition of fisheries are major concerns for marine ecosystem conservation. However, high sea longline fisheries data are insufficient because of difficulties in deploying observers for data collection. Observer data collected from 77 trips on Taiwanese large-scale longline fishing vessels in the Indian Ocean from June 2004 to March 2008 were used to estimate the scale of the bycatch. At least 40 species were recorded. Albacore, bigeye, yellowfin, and southern bluefin tuna were the major species recorded and comprised over 73.30% of the total retained catch. Major bycatch species were swordfish, blue shark, sailfish, pomfret, and escolar. The average discard rate was 14.09%, ranging from 3.20% for the yellowfin tuna fleet to 18.09% for the bigeye tuna fleet. In total, 0.80% of the catch of the albacore, 4.74% of the bigeye, and 2.32% of the yellowfin tuna were discarded. There were significant differences among seasons and areas for the discard rates of the bigeye and yellowfin tuna. The discard rates of the bigeye and southern bluefin tuna were positively correlated to the catch per unit effort. The depredation percentage of tuna by cetaceans was from 0.7% to 12.3% of total discards for the different fleets. The high discard and cetacean depredation rates showed that major possible reasons for discards are depredation by cetaceans, economic factors, and quota limitations. Regarding other species, 61 seabirds and 84 sea turtles were a part of the bycatch. The major species were Indian Yellow-nosed Albatross in the southern Indian Ocean and Olive Ridley turtles in tropical areas. The estimated annual incidental catch numbers were 715 to 311 seabirds and 1856 to 1127 sea turtles from 2004 to 2007. For conservation, this discard information could be used to assess tuna stocks. Mitigation measures, including the live release of small-sized fish, and the use of bird-scaring lines and circle hooks, are required to minimize the bycatch.