Stock assessment of Southwest Pacific shortfin mako shark

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.

The stock assessment was set up in Stock Synthesis as a two-fleet model. The fisheries were structured into a low latitude high seas fleet (between 15 and 35^∘south) and a high latitude fleet (between 35 and 45^∘south) based on several observations that suggest: spawning may occur more often in higher latitudes; there may be lower catchability of smaller individuals in the warmer surface water in lower latitudes; and potential species identification issues in the most southern part of the fishery. The model was run for a 26 year period from 1995 to 2020, with the start year taken to be 1995 due to highly uncertain catches prior to 1995. The catches were reconstructed from observer data, producing relatively high catches between the mid-1990s and early 2000s, with relatively strong reductions in catch since about 2010. The catch reconstruction model also produced high uncertainties in catch between the mid-1990s and early 2000s, and in the early to mid-2010s.

Two CPUE series, one from New Zealand, representing high latitude fisheries capturing young-of-year and juvenile fish, and one from Japan representing low latitude fisheries on juvenile (mainly age 1+ but sub-mature) individuals, were used as indices of abundance. The high latitude index suggested a decline in the late 1990s, with subsequent increase since the early 2000s, and relatively variable, yet over-all flat trends in recent years. The low latitude index suggested a time-lagged decline compared to the New Zealand high latitude index in the later 1990s and early 2000s, but did not show a subsequent increase. Corresponding length frequencies appeared relatively consistent with indices: a decline and subsequent recovery in mean lengths for high latitude mean lengths, and relatively stable mean lengths for low latitude fisheries.

Despite numerous attempts, very few of our attempted models yielded plausible outcomes. In the diagnostic model, initial fishing mortality F_init was estimated from assumed equilibrium catches prior to the start of the time series in 1995. The resulting estimation uncertainty was large, leading to very large uncertainty around unfished biomass and stock status. The model also showed strong retrospective patterns, with only the addition of recent data providing signal to estimate scale parameters (R₀). The estimated initial equilibrium fishing mortality was largely driven by length composition data. Alternative assumptions about catch or biological parameters (e.g., M) often lead to implausible estimates for initial fishing mortality (i.e., near zero). CPUE indices appeared in conflict for both the estimation of R₀ and F_init. In addition, the model required highly correlated recruitment deviations to explain changes in abundance indices, suggesting that the assumed catch history alone was insufficient to explain early declines in abundance indices.

Together, these patterns suggest that the model inferences are highly dependent on assumptions and input data, and that the model solution for the diagnostic model is not stable. As result we suggest that the assessment model, while delivering information on stock biomass and fishing mortality trends, is not robust enough for providing management advice.

Despite the documented shortcomings, we suggest that the present assessment delivers some useful metrics. Fishing mortality and associated reference point metrics, for example, were consistently estimated (Table 2). The assessment therefore provides preliminary indications that recent fishing mortality may have declined below critical (i.e., F_crash,AS) levels. However, due to the inherent instability of the present model, we did not explore the sensitivity of these estimates to uncertainty in life-history, and the catch and discard assumptions. Our models used an estimate of natural mortality from New Zealand studies, that was noted as being high. As a consequence, F based reference points derived here may be overly optimistic. As alternative model runs did not succeed in providing plausible outputs, we therefore caution that the present analysis is preliminary and only gives ranges of values from a single assumption of life history, and our most-likely catch and discard scenarios only.

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 time-frames 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.

• Additional tagging should be carried out using satellite tags in a range of locations, especially known nursery grounds off southeast Australia and New Zealand, as well as high seas areas to the north and east of New Zealand, where catch-rates are high. Such tagging may help to resolve questions about the degree of natal homing and mixing of the stock.

• Tagging may also help to obtain better estimates of natural mortality, if carried out in sufficient numbers. This could be taken up as part of the WCPFC Shark Research Plan to assess the feasibility and scale of such an analysis.

• Additional growth studies and validation of aging methods from a range of locations could help build a better understanding of typical growth, as well as regional growth differences. Current growth data are conflicting, despite evidence that populations at locations of current tagging studies are likely connected or represent individuals from the same population.

• Genetic/genomic studies could be undertaken to augment the tagging work to help resolve the stock/sub-stock structure patterns. To support this work, a strategic tissue sampling program for sharks is recommended with samples to be stored and curated in the Pacific Marine Specimen Bank.

• Aggregated data are currently submitted as annual totals for the WCPFC area only, making them uninformative for a stock specific assessment. Therefore, shortfin mako shark aggregated data (and probably other Key Sharks) should be reported by ocean area not simply as WCPO and, where possible, these data should be retrospectively corrected. As such we propose that paragraph 1 bullet point 3 of the Scientific Data to be Provided to the Commission should include the following sentence: ”For Key Sharks, estimates of annual catch should be separated into catch north and south of the Equator. The WCPFC secretariat should work with CCMs to get these data retrospectively corrected where possible.”