Stock assessment of Southwest Pacific Shortfin Mako shark

Citation
Large K, Neubauer P, Brouwer, Stephen (2022) Stock assessment of Southwest Pacific Shortfin Mako shark. In: WCPFC Scientific Committee 18th Regular Session. WCPFC-SC18-2022/SA-WP-02, Electronic Meeting
Abstract

This analysis assesses the southwest Pacific shortfin mako shark stock in the Western and Central Pacific Ocean (WCPO) hereafter referred to as the Southwest Pacific.

South Pacific mako sharks have been caught in longline fisheries since their inception in the 1950s but have only been reported in catch records since the 1990s. They are thought to consist of two stocks, a southwest and southeastern stock which are both separated from those in the north Pacific at the Equator. Shortfin mako sharks in the north Pacific have been assessed and that stock is currently considered not to be overfished and overfishing is not taking place. This is the first attempt at undertaking an assessment of the southwest Pacific stock.

Main Assessment Conclusions • The assessment was un-stable, with high estimation uncertainty and sensitivity to a range of inputs. We therefore consider this assessment preliminary and suggest it should not be used for providing management advice. • Poor representation of mature females in commercial fishing data suggests that all inferences for this important partition of the stock are derived from assumptions and estimates of biological and fisheries parameters, with no direct observations to assess the appropriateness of these assumptions/estimates. In the absence of alternative data sources on trends in this component of the stock, this issues will likely remain in future, and alternative assessment approaches should be explored. • Relatively consistent estimates of fishing mortality and related reference points suggest that recent declines in catch may have been sufficient to reduce fishing mortality below critical levels. However, we note that these statistics are based on a single set of assumptions, and further work will be required to test the robustness of these preliminary statistics.

Given some of the fundamental uncertainties highlighted above, we recommend: • Future assessments should spend increased effort to reconstruct spatio-temporal abundance patterns for shortfin mako, and develop a better understanding of how these pa􀄴erns drive regional abundance indices. • Providing more time, either as inter-sessional projects, or by extending timeframes for shark analyses will allow more thorough investigation of input data quality and trends, which shape assessment choices. In addition, this approach would allow input analyses to be completed in time to be presented to the March pre-assessment workshop prior to the stock assessment commencing. Moreover, this will provide more time for the assessments themselves allowing a more thorough investigation of alternative model structures or assessment approaches. • Increased effort should be made to re-construct catch histories for sharks (and other bycatch species) from a range of sources. Our catch reconstruction models showed that model assumptions and formulation can have important implications for reconstructed catch. Additional data sources, such as log-sheet reported captures from reliably reporting vessels, may be incorporated into integrated catch-reconstruction models to fill gaps in observer coverage.