Standardization of blue shark Prionace glauca catch rates of the Japanese-flagged component of the South African large pelagic longline fleet based on observer records

The blue shark Prionace glauca is caught as bycatch in the large pelagic longline fishery in South Africa. The fleet includes a domestic component with varying but increasing degree of observer coverage, and a foreign-flagged component of Japanese vessels that operate under joint venture agreements with South African Right Holders. Japanese flagged vessels have been operating under a mandatory 100% observer coverage since 2007. The catch and effort data include consistent records of bycatch species in numbers caught per set. We investigated blue shark abundance by standardising the Catch per Unit Effort (CPUE) in numbers from Observer data for the time series 2007 to 2019. To do this, we applied a Generalised Additive Mixed Model (GAMM) with a Poisson error distribution. Explanatory variables of the final model included year, month, grid (lat, long) with the number of blue shark caught in a set offset by the number of hooks set, so as to maintain a count distribution. Vessel was included as a random effect. Despite a period of relatively low catch rates (2009-2012) followed by a period of relatively high catch rates (2015-2017), the results indicate that blue shark CPUE in the south-western IOTC area has been stable overall. Our dataset is unique in that the joint-venture Japanese flagged vessels have required 100% observer coverage since 2007. Given the increasing stricter catch regulation on shark species, our observer dataset may be the most appropriate dataset to accurately represent trends in abundance of blue sharks in the south-western IOTC region.