The discards and bycatch of Chinese tuna longline fleets in the Pacific Ocean from 2010 to 2018

A substantial level of non-target species are often caught in tuna longline fisheries, which can therefore impose a heavy ecological burden on pelagic fish communities. However, limited studies have quantitatively examined the scale of bycatch varying across fishing fleets and areas in regional tuna longline fisheries, as data are often sparse. We used scientific observer data to analyze the discard and bycatch patterns of Chinese tuna longline fleets in the Pacific Ocean by applying general statistical analysis and spatial hotspots analysis. A total of 9971 sets were observed, representing 1.56% of the total efforts. Of 435,156 individuals, 30.4% were recorded as bycatch, and 15.8% were discarded. The majority of bycatch was other bony fishes (15.5%) and sharks (7.3%). Bycatch and discard rates of the bigeye tuna (Thunnus obesus, BET) fleet were larger than that of the albacore (Thunnus alalunga, ALB) fleet. The highest bycatch rates of other bony fishes and elasmobranchs most often occurred in the western and central temperate Pacific and the eastern tropical Pacific, respectively. Most marine turtles and mammals were caught by the BET fleet in the Tropical Pacific, and the bycatch of seabirds often occurred in the Temperate Pacific. Low bycatch rates but high discard mortality of marine mammals, turtles, and sea birds were found. Bycatch rates were estimated by fleet, seasons, and areas, with the rates of marine turtles ranging from 0 to 0.024 per 1000 hooks, and those of sea birds ranging from 0 to 0.228 per 1000 hooks. Six bycatch hotspots, identified in this study, should be considered in future management and conservation planning. Implementing measures such as live releasing bycatch, reducing the fishing efforts in the area with high bycatch rate, optimizing monitoring programs, and allocating observers more widely in areas where there is more bycatch are necessary.