Stock assessment for oceanic whitetip shark in the Western and Central Pacific Ocean

This is the second stock assessment for oceanic whitetip shark in the WCPO following that of Rice & Harley (2012), and the first since CMM2011-04 became active in 2013, enacting a no-retention measure for this species for WCPFC Members, Cooperating Non-Members and Participating Territories (CCMs). This assessment for oceanic whitetip shark was performed in the Stock Synthesis modelling framework (Methot Jr & Wetzel 2013), an integrated age-structured population model previously used to assess the status of sharks in the Pacific Ocean and elsewhere. The population dynamics model was informed by three sources of data: historical catches, time series of catch-per-unit-effort and length frequencies. We maintained the four-fleet structure used in the previous stock assessment, splitting the longline fishery into bycatch and target fleets, and the purse-seine fishery into fleets of associated and unassociated sets.

A new development in this assessment was the inclusion of discard mortality (DM) scenarios in the historical catches. This was a key step to account for the potential impacts of the no-retention measure for oceanic whitetip sharks.Three scenarios were used assuming 25%, 43.75% and 100% mortality on the discards, accounting for mortality at different stages of the discarding process from the catch event and crew handling to post-release mortality. In addition, results from two new WCPO growth studies predicted a much less productive profile for the stock than what had been assumed previously. Because growth was a key uncertainty in this assessment, we use two growth and fecundity assessment ‘profiles’ to reflect the differences between growth studies, and ensure that conclusions about stock status were robust to the uncertainty about life-history parameters.

We developed a diagnostic case for the assessment based on the model with the best overall diagnostics, an informative likelihood profile, and the most reasonable assumptions about biology and fleet settings based on current knowledge about oceanic whitetip shark and the fisheries that catch this species. Based on the results from the one-off sensitivities from the diagnostic case and previous discussions at the Pre-Assessment workshop (Pilling & Brouwer 2019), a set of uncertainty axes for the model was defined outlining alternative values for key uncertainties and influential model or biological parameters. The combination of all levels across axes forms the structural uncertainty grid with a total of 648 individual model runs.

Stock status was obtained by summarizing reference points over all grid runs to account for the assumptions about life-history parameters and impact of fishing underpinning the assessment. We estimate the stock to be overfished and undergoing overfishing based on SB/SB_MSY and F/F_MSY reference points and assuming equal weightings for grid levels. This overall conclusion is the same as that from the previous assessment, despite a wider range of uncertainties being considered, notably in the growth and fecundity parameters. In terms of the depletion of the spawning biomass, most model runs predict SB/SB₀ to be below 0.05, and all model runs predict SB/SB₀ to be below 0.1. Given the high levels of depletion, we focus on SB/SB₀ as a biomass-based reference point throughout.

We found that F-based reference points improved in the period since CMM2011-04 became active, which covers the last 4 years of the assessment’s time-span (2013–2016). Notably, F/F_MSY is predicted to have declined by more than half from 6.12 to 2.67 (median) for the last year of the assessment when the impact of CMM2011-04 on survival is accounted for under the 25% and 43.75% discard mortality scenarios. F levels relative to two alternative reference points, F/F_lim,AS and F/F_crash,AS, follow similar trends following the adoption of the measure.

All catch scenarios accounting for DM <100% showed a very slight increase in spawning biomass since 2013, but final levels of depletion (SB/SB₀) remain very low over all grid runs (median: 0.0367, 95%CI: 0.021–0.061). Given the assessment assumes oceanic whitetip sharks to become mature after 4 or 8 years, stock recovery should be expected to be slow in the period following the conservation measure while the spawning biomass rebuilds. Despite the relative improvements in F-based reference points since 2013, the median value of F/F_crash over all 648 grid runs for 2016 remains above 1 (median: 1.41, 95%CI: 0.98–2.15), indicating that the population should go extinct on the long-term under current levels of fishing mortality.