Identifying appropriate reference points for elasmobranchs within the WCPFC - 2018

Citation
Zhou S, Deng R, Hoyle S, Dunn M (2018) Identifying appropriate reference points for elasmobranchs within the WCPFC - 2018. In: WCPFC Scientific Committee 14th Regular Session. WCPFC-SC14-2018/ MI-WP-07, Busan, Republic of Korea, p 54
Abstract

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 a worldwide database of over 270 studies 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. The four methods are: an empirical relationship between FRPs and life history parameters, demographic analysis, the intrinsic population growth rate from literature, and the spawning per recruit (SPR) approach. 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. We provide three reference points, Fmsm, Flim, and Fcrash. As expected, the estimated values are similar between multiple methods (i.e. 2 to 4 methods depending on available data) in some stocks but vary considerably in other stocks. Because of a lack of selectivity and maturity information, the SPR approach is applied to only three stocks. It is difficult to determine what percentage of SPR is appropriate for elasmobranchs and how it corresponds to the three FRPs, so this approach has limited value. Given the fact that the WCPFC has adopted a benchmark 20%SBdynamic10, unfished as the limit biomass reference point for target species, we recommend using the FRPs combined from the three methods and adopted combined Flim (cFlim) as LRP for elasmobranchs.
In the second section, we review some potential methods for estimating fishing mortality for datapoor species, including formal stock assessment, area-based ERA methods, age-based methods, and length-based methods. We focus on the area-based methods, as varying versions, tailored for varying data availability, have been developed and have been applied to three WCPFC species. This group of methods can be flexibly modified to suit the available data. To be consistent, this method is recommended for other data-poor WCPFC species.
In the third section, we briefly review other potential management procedures for WCPFC elasmobranchs. As a wide range of assessment methods and management procedures have been developed for data-poor fisheries, and several comprehensive reviews have already been completed, we only discuss three procedures that are potentially promising for WCPFC bycatch. These procedures include catch-rate approaches, length-based traffic-light approaches, and catchonly methods. We suggest that before adopting a particular approach, it is essential to check the data inventory against the key assumptions required by the method, and keep in mind the merit of consistent methodology across multiple species.