Standardized catch rates of blue sharks caught by the Taiwanese longline fishery in the Atlantic Ocean

In this document, the blue shark catch and effort data from observers’ records of Taiwanese large longline fishing vessels operating in the Atlantic Ocean from 2004-2012 were analyzed. Based on the shark by-catch rate, five areas, namely, A (north of 20ºN), B (5ºN-20ºN), C (5ºN-15ºS), D(15ºS-50ºS, west to 20ºW) and E (15ºS-50ºS, 20ºW-20ºE), 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 approach that treats the proportion of positive sets and the CPUE of positive catches separately. Standardized indices with 95% bootstrapping confidence intervals are reported. The standardized CPUE showed a stable trend for blue sharks with a peak in 2006 for the South Atlantic and two peaks (2005 and 2006) for the North Atlantic. The results suggested that the blue shark stock in the North and South Atlantic Ocean were likely at the level of optimum utilization in recent years. The results obtained in this study can be improved if longer time series observers' data are available.