Updated standardized catch rates for blue shark caught by the Taiwanese large-scale tuna longline fishery in the Indian Ocean

The catches and efforts of the blue shark in the Indian Ocean were estimated based on the observers’ records (2004-2019) of Taiwanese tuna longline fisheries. To cope with the large percentage of zero shark catch, the catch per unit effort (CPUE) of blue shark, as the number of fish caught per 1,000 hooks, was standardized using a two-step delta-lognormal model (DLN) that treats the proportion of positive sets and the CPUE of positive catches separately. The standardized CPUE showed a stable increasing trend for blue sharks from 2008 to 2014 (the peak), although decreased in 2015, it increased again in 2016. Overall, the standardized CPUE series of the blue shark caught by Taiwanese longline fishery showed a stable trend. The stable trend suggested that blue shark stocks in the Indian Ocean seems at the level of optimum utilization.