Spatio-temporal changes in catch rates of pelagic sharks caught by Japanese research and training vessels in the western and central North Pacific

Key pelagic shark species are caught in the North Pacific by Japanese longline fisheries as bycatch. These species include blue shark (Prionace glauca) and shortfin mako (Isurus oxyrinchus). In this study, fishery-independent data was leveraged by utilizing a generalized linear mixed model to standardize these data and identify changes in spatio-temporal catch rates. For blue shark, catch was recorded from temperate waters of the western and central North Pacific. Shortfin mako were distributed in coastal and offshore waters along the Kuroshio-Oyashio transition zone and the Mixed Water Region. The yearly changes in the catch per unit effort (CPUE) of blue shark illustrated a downward trend until 2008 and an upward trend thereafter, whereas those of shortfin mako exhibited a flat trend with large fluctuations from 1994 to 2008, and a down-and-up trend thereafter. Successfully leveraging this otherwise unreliable dataset suggest it can now be utilized to improve the stock assessment and management of these shark species.