Assessment of the Impact on 20 Pelagic Fish Species by the Taiwanese Small-Scale Longline Fishery in the Western North Pacific Using Ecological Risk Assessment

SIMPLE SUMMARY: In this study, a semi-quantitative ecological risk assessment (ERA) was used to evaluate the ecological risks of fishing impact on 20 pelagic fish species by the small-scale longline fisheries in the western North Pacific Ocean. More than 2.38 million individual landing records at Nangfangao and Hsinkang fishing ports, eastern Taiwan from 2001–2021 were used in this study. The productivity was estimated based on the mean ranking (high, median, and low) of seven life history parameters and the susceptibility was calculated by the multiplication of the catchability, selectivity and post-capture mortality. The ERA results indicated sharks have higher ecological risk than those of tunas and billfishes, except yellowfin tuna (Thunnus albacares). The shortfin mako shark (Isurus oxyrinchus) and dusky shark (Carcharhinus obscurus) have the highest risk. Yellowfin tuna, other shark species, and sailfish (Istiophorus platypterus) have medium risk. While the striped marlin (Kajikia audax), and albacore tuna (T. alalunga) have the lowest risk. Although ERA cannot replace the conventional stock assessment methods that can produce solid management information on catch and effort, yet it can provide useful information for precautionary management measures. ABSTRACT: Ecological risk assessment (ERA) has been applied on assessing the relative risk of bycatch species in recent years. ERA index is calculated by productivity of species and susceptibility of fisheries on fish species. In this study, a semi-quantitative method was used to evaluate the risks of exploitation for 20 pelagic fish species by the small-scale longline fisheries in the western North Pacific Ocean. The productivity was estimated based on the ranking (high, median, and low) of seven life history parameters. The susceptibility was calculated by the multiplication of the catchability, selectivity and post-capture mortality. The ERA results indicated the risks of sharks are higher than those of tunas and billfishes, except yellowfin tuna (Thunnus albacares). The shortfin mako shark (Isurus oxyrinchus) and dusky shark (Carcharhinus obscurus) have the highest risk. Other shark species, yellowfin tuna, and sailfish (Istiophorus platypterus) have medium risk. While the striped marlin (Kajikia audax), and albacore tuna (T. alalunga) have the lowest risk. Stock assessment and rigorous management measures such as catch quota and size limit are recommended for the species in high or medium ecological risk and a consistent monitoring management scheme is suggested for those in low ecological risk.