Analysing potential inputs to the 2024 stock assessment of Western and Central Pacific silky shark (Carcharhinus falciformis)

Silky shark (Carcharhinus falciformis) are caught in longline and purse-seine fisheries in the western and central Pacific Ocean (WCPO). Previous assessments had indicated that the stock was possibly overfished and that overfishing was occurring. However, these assessments also highlighted large uncertainties in inputs, with inconsistent longline indices, and inconsistent signals in other indices compounding uncertainties.

The present project endeavoured to thoroughly investigate potential methods for reconstructing overall catch, CPUE and length compositions for use in a potential assessment, which is scheduled for 2024. The present study represents year one of the two year project to assess silky shark.

We followed recent shark assessments and reconstructed catch of silky shark in the WCPO by extrapolating from a catch rate model for observer data. We used multi-model inference via Bayesian hierarchical stacking to derive a best estimate of catches in longline and purse-seine fisheries. To alleviate concerns that recent observed catch rates may be biased low due to non-retention measures for silky sharks, we also compared our predictions with predictions from a model that ignored a year effect, which may be subject to reporting bias. Both predictions led to similar declines in catch since 2013, and our comparative approach led us to conclude that the majority of this decline was due to changes in total effort and effort distribution, rather than reporting changes.

CPUE trends in longline fisheries were highly variable, and as for previous assessments, were not consistent with other analyses. A more gradual approach to the analysis suggested highly variable trends in longline CPUE for silky shark among observer programs. As a result, any aggregate trend was estimated to be nearly flat. We therefore concluded that longline indices are unlikely to be useful for silky shark with the present datasets.

Purse seine CPUE, on the other hand, was highly consistent among observer programs across the WCPO, and showed good consistency between free-school and floating object-associated sets, both of which showed substantial increases in catch rates in recent years across the WCPO. Size composition analyses supported this trend, with scaled model-predicted length compositions showing the progression of a likely strong 2012 cohort progressing through the fishery until at least 2020.

The following recommendations are made:

• We conclude that there are likely sufficient data and a sufficiently consistent signal in the different datasets, especially from purse-seine, to conduct a stock assessment.
• Recent CPUE shows an increasing trend across all purse seine fisheries, which mirrors declines in longline catch since 2013.
• We suggest that our analyses provide evidence that changes in reported interactions from longline vessels due to non-retention measures do not change over-all trends in interactions for silky shark, and do not change the conclusion that longline interactions with silky sharks have declined substantially from their peak in 2013.
• We suggest that a fully integrated assessment could be attempted, based on the consistency of datasets developed herein.
• As year effects are relatively minor in the longline fisheries, future catch reconstruction attempts could extrapolate interactions further back in time to avoid complications with assuming or estimating initial fishing mortalities in the assessments.
• Alternative assessment methods (for example spatial risk assessments) could be run in parallel with an integrated assessment, and provide an alternative approach that is independent of recent longline data, and allow for multi-model inference that can strengthen conclusions and potentially management advice from an integrated approach.