Stock assessment of Southwest Pacific Shortfin Mako shark

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

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
patterns 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.