Spatio-temporal model for CPUE standardization: application to blue shark caught by Japanese tuna longline fishery in the SOUTH Atlantic from 1994 to 2021
Abundance indices of blue shark caught by Japanese longline fishery from 1994 to 2021 in the South 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 of blue sharks in the South Atlantic. 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 an upward trend from 2005 to 2011, and then downward trend until 2015. Thereafter the CPUE slightly 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 South Atlantic is a very useful information about the abundance of blue sharks.