Spatio-temporal model for CPUE standardization: Application to blue shark caught by Japanese offshore and distant water shallow-set longliner in the western North Pacific

This working paper provides a standardized CPUE of blue shark caught by Japanese offshore and distant-water shallow-set longline fishery from 1994 to 2020 in the western North Pacific Ocean. Since the catch data of sharks caught by commercial tuna longline fishery is usually underreported due to discard of sharks, the author filtered the logbook data using the similar filtering methods applied in the previous analysis. The nominal CPUE of filtered shallow-set data was then standardized using the spatio-temporal generalized linear mixed model (GLMM) to provide the annual changes in the abundance of blue sharks in the northwestern Pacific. The author focused on seasonal and interannual variations of the density in the model to account for spatially and seasonally changes in the fishing location due to the target changes between blue shark and swordfish. The estimated annual changes in the CPUE of blue shark revealed an upward trend from 1994 to 2005, and then downward trend until 2008. Thereafter the CPUE gradually increased until 2015 and then slightly decreased in recent years. The estimated CPUE trends from the spatio-temporal model with a large amount of data collected in the most abundant waters in the North Pacific Ocean is a very useful information about the abundance of North Pacific blue shark.