Update on the CPUE standardization of the blue shark (Prionace glauca) caught by Indonesian tuna longline in the eastern Indian ocean

The blue shark or BSH (Prionace glauca) is commonly caught as bycatch in tuna longline fishery. It is vulnerable as a consequence of the increasing intensity of tuna harvesting. Despite this species categorized as well-studied compared to other shark species, an update on its abundance is essential for stock assessment and fishery management. This study provided an update on the CPUE standardization of the blue shark as a proxy of relative abundance by removing possible factors that influence the CPUE using a Generalized Linear Model (GLM). The fishery-independent data was gathered through the Indonesian onboard scientific observers program operated in the eastern Indian Ocean from August 2005 to December 2019. Due to the large proportion of the zero catch of blue shark (~62%), the CPUE was standardized using a delta-lognormal model. In general, an increase-fluctuated trend of the CPUE was observed in the last decade. The standardized CPUE of the blue shark as a proxy of its relative abundance decreased during 2006 and to 2011 and showed an increasing trend thereafter and peaked in 2018. The positive catch of blue shark was significantly affected by the variables of year, quarter, and latitude, where the blue shark is more abundant in high latitude waters.