Data-limited approach to the management and conservation of the pelagic thresher shark in the Northwest Pacific

The pelagic thresher shark is among the most heavily exploited shark species in the commercial fisheries of the tropical Indo-Pacific oceans. Despite this severe exploitation, little is known about pelagic thresher population dynamics, particularly the species' life history traits, and overall stock status. The pelagic thresher exhibits slow growth and extremely low fecundity, indicating the need for reassessment of the population status of this globally threatened shark. Because information on catch and effort of this bycatch species is scarce, several data-limited methods - quantitative demographic methods, per-recruit analysis, and risk assessment - were employed in this study to provide an accurate measurement of the status of pelagic thresher shark stocks. For each sex, the composite risk assessments indicated that the probability of the current fishing mortality increasing beyond any level of yield-per-recruit (YPR) was approximately zero given the biological reference points (BRPs). Moreover, the current spawning potential ratio (SPR) was found to be significantly below the target reference point of SPR60%, but slightly above the limit reference point of SPR40%. However, because SPR-based BRPs are much less sensitive to input parameter uncertainties than YPR-based BRPs, results derived from the SPR model are more suitable for species management. Management strategy simulations based on demographic two-sex models indicated that without mortality from fishing, female pelagic threshers have a higher population growth rate than males. However, the stock status of female pelagic thresher shark is poorer than that of males under current conditions. The findings suggest that sex-specific management decisions must be made to achieve the sustainable utilization of this species. Overall, both the sex-specific per-recruit and demographic models determined that recruitment overfishing has occurred in the female pelagic thresher population of the Northwest Pacific Ocean, indicating that close monitoring of female sharks is urgent and necessary to ensure that stocks remain sustainable.