Updated and revised standardized catch rate of blue sharks caught by the Taiwanese longline fishery in the Indian Ocean

The blue shark catch and effort data from observers’ records of Taiwanese large longline fishing vessels operating in the Indian Ocean from 2004-2016 were analyzed. Based on the effort distribution and fishing grounds of the target species, four areas, namely, A (north of 10ºS, east to 70ºE), B (north of 10ºS, 70ºE-120ºE), C (south of 10ºS, 20ºE-60ºE), D (south of 10ºS, 60ºE-120ºE) were categorized. 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 that treats the proportion of positive sets and the CPUE of positive catches separately. Each model includes the main variables year, quarter, area, hooks per basket (HPB), and all two-way interactions between quarter, area and HPB. Standardized indices with 95% bootstrapping confidence intervals were reported. The standardized CPUE showed a stable trend for blue sharks from 2004 to 2008 and increased steadily thereafter with peaks in 2014. The results obtained in this study can be improved if longer time series observers' data are available.