Update on the CPUE standardization of the shortfin mako shark caught 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 2005-2020 were analyzed. Based on the effort distribution, four areas, namely, (1) Northwest Indian Ocean (north of 10°S, east of 70°E); (2) Northeast Indian Ocean (north of 10°S, 70°E-120°E); (3) Southwest Indian Ocean (south of 10°S, 20°E60°E); (4) Southeast Indian Ocean (south of 10°S, 60°E-120°E) 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, HPBF, CTNO, and Cluster. The standardized CPUE showed a stable and slightly increasing trend for shortfin mako sharks. The stable trend suggested that shortfin mako shark stocks in the Indian Ocean seems at the level of optimum utilization.