Pacific-wide sustainability risk assessment of bigeye thresher shark (Alopias superciliosus) - Prepared for the WCPFC

NIWA (2016) Pacific-wide sustainability risk assessment of bigeye thresher shark (Alopias superciliosus) - Prepared for the WCPFC. Wellington New Zealand

The bigeye thresher shark, Alopias superciliosus, has been identified as one of the least productive pelagic sharks and there is concern about its conservation status. Although it is one of three thresher sharks designated by Western and Central Pacific Fisheries Commission as key shark species, no Pacific Ocean stock assessment has been conducted. Information gaps and changes in reporting and observer coverage over time and space, make traditional approaches to stock assessment impractical. As an alternative and to gain new insights into the sustainability status of bigeye thresher shark, this study applies a spatially explicit and quantitative sustainability risk assessment to available data. The analytical framework evaluates sustainability risk as the ratio of current impacts from fisheries (spatially-explicit and cumulative fishing mortality F) to a maximum impact sustainable threshold (MIST) reference point based on population productivity. This approach differs from traditional stock assessment because it evaluates F in terms of whether the population’s ability to withstand fishing pressure is exceeded, rather than evaluating biomass (B) and whether the population is overfished. Key components (and analytical procedures) included: 1) estimation of the species distribution or relative abundance in space; 2) calibration of population and fishery groups catchability; and 3) estimation of the maximum intrinsic population growth rate r for the species, using available life history data. The first two components were used in conjunction with commercial effort (logsheet) data to quantify fishing impact. The third was used to define the MIST reference point. A scenario-based approach to sustainability risk evaluation was implemented, with scenarios ranging from more to less precautionary and representing different species distribution, initial population status, maximum density and post-capture survival assumptions. This approach served to cope with currently high levels of uncertainty in population status, movements and biology, and limited information about some aspects of the available datasets. Observer data from the Pacific Community (SPC), United States (US) and Japan were standardized with two models, a zero-inflated negative binomial (ZINB) model and a geo-statistical delta-generalised linear mixed (delta-GLMM) model, which permitted derivation of spatial indices of relative abundance over different but overlapping areas. Population catchability (q) was statistically calibrated using a Bayesian state-space biomass dynamics model (BDM) fitted to time series of relative abundance and annual catch estimates obtained from a representative subset of the observer data. This approach assumed that although the available data were insufficient to estimate absolute catchability, they could be used to calibrate a relative catchability parameter for use in spatially-explicit impact estimation. A range of plausible q values were estimated, with uncertainty, and adjusted spatially by fishing season and catch group (i.e., ‘fishery groups’), as well as for the occurrence of post-capture survival. Fishing mortality was calculated as the sum product of total effort and fishery-group specific catchability in 5x5 degree cells, weighted by the relative density of bigeye thresher shark in each cell, as obtained from the spatial standardization. The distribution of the maximum population growth rate r had a median value of 0.03, which is higher than previously reported for the species, and was used to define the MIST. Analyses performed assuming 100% capture mortality produced median F values ranging from 0.02 to 0.04 among base case scenarios for the period 2000-2014. Sustainability risk, corresponding to the ratio of total impact to the MIST, ranged from 0.6 to 1.2. The average probability that fishing impact exceeded the MIST was 0.4 across years and scenarios. Analyses performed assuming a range of post-capture survival rates produced median F values ranging from 0.01 to 0.03 and median sustainability risk between 0.4 and 1.0, with an average probability of 0.20 of total fishing impact exceeding the MIST. 2 Pacific-wide sustainability risk assessment of bigeye thresher shark Earlier studies indicated that the species is vulnerable to exploitation owing to limited productivity, even at relatively low levels of fishing mortality. Sustainability risk results presented here, which incorporate considerable uncertainty both within and among scenarios, are not inconsistent with this view. They suggest that total impacts from pelagic longline fisheries in the Pacific since 2000 are generally low (