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 trips 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.