Standardized CPUE of shortfin mako sharks by the Taiwanese large-scale tuna longline fishery in the Indian Ocean

In the present study, the shortfin mako shark catch and effort data from the logbook data of Taiwanese large longline fishing vessels operating in the Indian Ocean from 2005-2018 were analyzed. Based on the effort distribution, 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 shortfin mako shark, as the number of fish caught per 1,000 hooks, was standardized using zero-inflated negative binomial model (ZINB) that allows for “extra” zeros. ZINB model includes the main variables year, quarter, area, hooks per basket (HPB), and CTNO. The standardized CPUE showed a stable and slightly increasing trend for shortfin mako sharks. The results obtained in this study can be improved if longer time logbook data are available and environmental factors are included in the model.