Evaluation of Biological Reference Points for Conservation and Management of the Bigeye Thresher Shark, Alopias superciliosus, in the Northwest Pacific

Abstract

Full stock assessment of sharks is usually hindered by a lack of long time-series catch and effort data. In these circumstances, demographic and per-recruit analyses may provide alternate approaches to describe population status because these methods can be applied to estimate biological reference points (BRPs) for shark stocks. However, the appropriate level of BRPs for sharks is difficult to determine, given the expected low reproductive rates. To determine which BRPs are most appropriate for the CITES-listed species-bigeye thresher shark, Alopias superciliosus, a stochastic demographic model with Monte Carlo simulations and per-recruit models were used to estimate BRPs in this study. The results indicated that conventional fishing mortality-based BRPs (F-BRPs) derived from per-recruit models may result in a clear population decline. Our analyses also demonstrated that the bigeye thresher population in the Northwest Pacific will stabilize only if demographic-based F-BRP is implemented. The F-BRP estimated based on the stochastic demographic model was 0.079-0.139 y(-1), which was equivalent to SPR = 50-70%. The findings strongly suggested that more conservative threshold F-BRPs should be implemented to ensure sustainable utilization of the bigeye thresher stock. The present study provides new and strategically important information on the population dynamics of the bigeye thresher in the Northwest Pacific, which can be used to help fishery managers to adopt more efficient management measures for this stock. It is also suggested that this approach can be applied to other shark species with limited catch and effort data.