Updated standardized CPUE and catch estimation of the blue shark caught by the Taiwanese large scale tuna longline fishery in the North Pacific Ocean

Abstract

In the present study, the blue shark catch and effort data from observers’ records of the Taiwanese large-scale longline fishing vessels operating in the North Pacific Ocean during the period of 2004-2018 were analyzed. The catch per unit effort (CPUE) of blue shark, as the number of fish caught per 1,000 hooks, was standardized using delta lognormal approach. The standardized CPUE of blue shark showed a stable increasing trend. The results suggested that the blue shark stock in the North Pacific Ocean seems at the level of optimum utilization. The blue shark by-catch was estimated using the area-specific nominal CPUE multiplying the fishing effort and accounting for the coverage rate. Estimated blue shark by-catch in weight ranged from 1 ton in 1973 to 1,247 tons in 2020.