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

Abstract

The blue shark, Prionace glauca, is a pelagic shark occasionally captured as bycatch by pelagic longline fleets in the Indian Ocean. The data collected by fisheries observers on board the Taiwan large-scale longline fishing vessels from 2004 to 2016 were analyzed to describe the size, distribution and the relative abundance of the blue shark in the Indian Ocean. To cope with the large percentage of zero catch, the blue shark CPUE standardization was performed using a delta-GLM approach. 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. Considerable variability was observed in the size distribution and sex ratio by region in the Indian Ocean. The distributional patterns and catch rates and size composition presented in this study provide a better understanding of the blue shark’s spatio-temporal dynamics in the Indian Ocean and can be used to improve management and conservation measures for this species. The results obtained in this study can be improved if longer time series observers' data are available.