Spatio-temporal model for CPUE standardization: application to blue shark caught by Japanese tuna longline fishery in the north Atlantic from 1994 to 2021

Abundance indices of blue shark caught by Japanese longline fishery from 1994 to 2021 in the North Atlantic were estimated. 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 data was then standardized using the spatio-temporal generalized linear mixed model (GLMM) to provide the annual changes in the abundance. The author focused on interannual variations of the density in the model to account for spatially and annually changes in the fishing location due to the target changes of tuna and tuna-like species. The estimated CPUE revealed downward trend from 1994 to 2004, and then sharply increased until 2008. Thereafter the CPUE was stable until 2015 with annual fluctuations and then decreased in recent years. The estimated CPUE using the spatio-temporal model with a large amount of data collected in the wide waters in the North Atlantic is a very useful information about the abundance of blue sharks.