Estimating the composition and capture status of bycatch using Chinese longline observer data in the Indian Ocean

From 2012to 2015, five Chinese tuna longline observer trips were conducted in the Indian Ocean (N10°35′-S33°20′, E40°58′-E89°54′). Bycatch and capture status from these trips were analyzed in this report. A total of 4,463individuals among 52bycatch species were captured from 911,718 hooks deployed, including tunas (39.4%), billfishes(12.0%), sharks (12.4%), rays (2.4%), sea turtles and seabirds(0.09%), and miscellaneous species (33.8%).The dominant bycatch species were yellowfin tuna (Thunnus albacores), swordfish (Xiphias gladius),Indo-pacific blue marlin (Makaira mazara), blue shark (Prionace glauca), pelagic stingray (Dasyatis violacea), longnose lancetfish (Alepisaurus ferox), escolar (Lepidocybium flavobrunneum), and opah (Lampris guttatus).The capture status for bycatch species, i.e. dead, hurt, alive, retained, or discarded, was also analyzed. The capture fate of dead, hurt, and alive accounted for 44.7, 12.7, and 42.6percent of the total bycatch, respectively. The proportion of retained and discarded bycatch was 76.2% and 23.8%. Blue shark and shortfin mako were mostly retained when captured, whereas other sharks (oceanic whitetip shark, pelagic thresher, bigeye thresher, etc), rays and sea turtles were almost all discarded or released. This report provides important information for the understanding of bycatch composition and discards for the Chinese longline fleet in the Indian Ocean during that period.