Identifying appropriate reference points for elasmobranchs within the WCPFC - 2019

Elasmobranch species are bycatch in fisheries managed by the Western and Central Pacific Fisheries Commission (WCPFC). These species have limited fishery-dependent data and biological information. Traditional stock assessment cannot be performed for most of the stocks. Assessment using alternative approaches has become a priority research project. Recently, Clarke and Hoyle (2014) reviewed appropriate limit reference points (LRPs) for WCPFC elasmobranchs and provided a conceptual framework for selecting appropriate LRPs. In 2015 an expert panel held a workshop to identify the most appropriate life history data to be used in calculating the risk-based LRPs. The panel compiled and reviewed over 270 studies worldwide on 16 WCPFC elasmobranch stocks.

The current study continues the previous work. The report contains several components related to reference point development. In the first section, we apply a total of four methods and use the data in the expert panel report to estimate fishing mortality-based reference points (FRPs). As natural mortality M is a key variable in three of the four methods, we start with M estimation by using six M estimators as well as adopting M values from the literature. Comparison among the seven M estimators shows that the estimator based on maximum life span tmax and the estimator based on the von Bertalanffy growth function (VBGF, K and Linf) differ markedly from other estimators for most stocks. On average, M from tmax is 1.45 times higher than the mean value from all seven approaches. In contrast, M based on VBGF is only 0.73 times of the average.