CPUE STANDARDIZATION AND CATCH ESTIMATE OF BLUE SHARK BY TAIWANESE LARGE-SCALE TUNA LONGLINE FISHERY IN THE NORTH PACIFIC OCEAN

The blue shark, Prionace glauca, is a pelagic species with a circum-global distribution in tropical to temperate waters. It is the top by-catch shark species for the Taiwanese large-scale tuna longline (LTLL) fishery. However, its population status in the North Pacific Ocean is still little known. In the present study, the blue shark catch and effort data from observers' records of Taiwanese LTLL fishing vessels operating in the North Pacific Ocean from 2004 to 2012 were analyzed. Due to the large percentage of zero blue shark catch, the catch per unit effort (CPUE) of blue shark, as the number of fish caught per 1,000 hooks, was standardized using a delta-lognormal model. The standardized CPUE of blue sharks, as an estimate of relative abundance of blue shark, decreased from 2005 to 2009 and showed an increasing trend thereafter. Back-estimated historical blue shark by-catch of Taiwanese LTLL fishery in the North Pacific Ocean ranged from 0.4 MT in 1973 to 1,447 MT in 2002. The results obtained in this study can be improved if longer time series observers' data are available and environmental factors are included in the model.