abstract:The pelagic thresher shark, Alopias pelagicus, owns the characteristics of large sharks and produces low number of offspring (2 embryos/litter). However, there is no accurate information on its population status. To reduce the uncertainties in simulations, a stochastic stage-based birth-flow model was constructed to assess the stock status of the pelagic thresher in the Northwestern Pacific. Based on the best biological information available, its life history was represented as four stages: neonates, juveniles, sub-adults, and adults. The results indicated that, without mortality from fishing, the stock would clearly increase (mean annual population growth rate (λ) = 1.058 yr-1, 95% CI = 1.014－1.102 yr-1). When current fishing mortality was taken into account, a decrease in population was indicated (λ = 0.979 yr-1, 95% CI = 0.921－1.030 yr-1) and a projected reduction of 34.3% over 20 years. These results suggest that the stock is over-exploited. Simulations using various management measures showed that the population will remain steady under these protection options. However, this species is extremely vulnerable to over-exploitation and is especially sensitive at the juvenile and adult stages, implying that nursery closures or size limit management measures are urgently needed to ensure the sustainable utilization of the stock.