Quantifying bycatch risk factors for the Chinese distant water fishery

Mitigating bycatch of non-target fish species is a common objective in fisheries management that may be supported by the gathering of data from fishery observer programs and quantitative analysis of bycatch risk factors. We build three GLM models based on Chinese tuna longline fishery observer data in terms of analysis of total bycatch rate (TB rate), total bycatch ratio (TB ratio), and species-specific bycatch rate, respectively. The positive log-linear models assumed a Gaussian observation error model and a linear combination of categorical independent variables, including area, year, month, depth, and bycatch species. Results show that distributions of TB rate and TB ratio followed different trends and a latitudinal decrease was observed from both the northern and southern hemisphere of the equator. Comprehensively, the Pacific is a better place to fish compared to the Indian and Atlantic Oceans in terms of relatively lower TB rate and TB ratio. Fishing in open oceans can somehow avoid a high TB ratio than fishing in coastal waters. As a result, we recommend area 2SW, 2SE, 2 NW, 14SW, 14SE, and 14 NW as appropriate fishing ground for Albacore (Thunnus alalunga) while area 11N,  11S, 18SW, and 18SE to be appropriate fishing ground for fishing bigeye tuna (Thunnus obesus). Setting fishing gears deeper than 500 m would also help to get a low TB rate and TB ratio.