Comparing the biological characteristics, length structure and capture status of bycatch in the Chinese longline fishery targeting different species in the Indian Ocean

From October 2013 to January 2018, twelve Chinese tuna longline observer trips were operated in the Indian Ocean, seven targeting Bigeye tuna (N10°14′ - S22°47′, E23°12′ - E89°54′) and the other six targeting Albacore (N0°11′ - S34°37′, E25°19′ E89°54′) including one trip changing the targeting species. Regarding observer trips targeting Bigeye tuna (Thunnus obesus), a total of 11,293 individuals among 49 bycatch species were observed from 2,178,636 hooks, including tunas (42.36%), billfishes (17.29%), sharks (12.26%), rays(3.19%) , dolphins and turtles (0.13%) ,and other species (24.77%). Major bycatch species (above 4% of total individuals) were as follows: Yellowfin tuna (Thunnus albacares), Longnose lancetfish (Alepisaurus ferox), Swordfish (Xiphias gladius), and the Blue shark (Prionace glauca). Regarding observer trips targeting Albacore (Thunnus alalunga), 7,860 individuals among 40 bycatch species were observed from 1,454,153 hooks, including tunas (40.01%), billfishes (5.22%), sharks(5.52%), rays (2.23%) , turtles (0.03%) ,and other species (47.00%). Major bycatch species were as follows: Bigeye tuna, Longnose lancetfish, Opah (Lampris guttatus), Escolar (Lepidocybium flavobrunneum), Dolphinfish (Coryphaena hippurus), Skipjack tuna (Katsuwonus pelamis), Yellowfin tuna and the Blue shark. The fate and condition of the capture status for bycatch were also analyzed. This report also compared the length frequency of major bycatch species between longline fishing vessels targeting bigeye tuna and albacore with Chinese scientific observer data.