Demographic Analysis of the Shortfin Mako Shark, Isurus oxyrinchus in the western North Pacific Using Stage-Based Models

Abstract

The shortfin mako shark (Isurus oxyrinchus) is a large shark species that grows slowly, matures late, and generates few offspring. This species is vulnerable to overexploitation yet its population status is still poorly known. In this study, deterministic and stochastic stage-based models were constructed to assess the stock status of the shortfin mako shark in the Northwest Pacific. Based on the best biological information available, the life history of the shortfin mako shark was represented as four stages: neonates, juveniles, subadults, and adults. Two reproduction trails - two-year and three-year reproductive cycle coupling with various natural mortality (M) and age at maturity were simulated in this study. The results of three-year reproductive cycle model with M following normal distribution indicated that the mean annual population growth rate of the stock (λ) = 1.059 yr^(-1), 95% CI = 1.023-1.098 yr^(-1) without fishing mortality. If the fishing mortality was introduced into the model, the population growth rate decreased apparently (λ = 0.942 yr^(-1), 95% CI = 0.854-1.021 yr^(-1)) and declined to 68.2% in the 20 yr projections. The shortfin mako shark produced higher potential of population growth with two-year reproductive cycle than that with three-year reproductive cycle. All simulations with various size-limit management strategies except the two-year reproductive cycle scenarios showed that the shortfin mako shark stock is shrunk under current condition. A catch quota of 265 MT equals to a 57% reduction of fishing effort is believed to be the optimum management measure. However, λ is sensitive to the survival of juveniles and adults, which implies that close monitoring and reducing fishing pressure on these stages are needed to ensure sustainable utilization of the stock.