Stock assessment of oceanic whitetip shark in the Western and Central Pacific Ocean 2025

The present analysis assessed the oceanic whitetip shark (Carcharhinus longimanus; OCS) stock in the Western and Central Pacific Ocean (WCPO), marking the third stock assessment of this stock. The analysis incorporated updated data inputs through 2023 and used methodologies building upon recent WCPFC shark stock assessments to address the challenges and uncertainties inherent in assessing OCS and sharks in general. A central challenge, acknowledged throughout the assessment process, is the paradoxical effect of the primary conservation measure CMM-2011-04 which, while intended to reduce mortality through a non-retention policy, has simultaneously degraded the quality of the scientific data required to monitor its effectiveness.

For this assessment, all data inputs were re-evaluated, and redeveloped. The historical catch series supporting the model was reconstructed using a refined approach for imputing hooks-between-floats (HBF), a critical proxy for fishing depth. Previous assessments treated reported zero-HBF values as true data, which likely inflated early catch estimates. By treating these zeros as missing data, the updated catch history for the early period was markedly lower and less variable than catch estimates used for the previous stock assessment for OCS. Despite improvements in methodologies, a conflict persisted between the standardised catch-per-unit-effort (CPUE) index, which shows a steep historical decline, and the length-composition data, which did not show the expected corresponding decline in the mean size of caught sharks.

In recognition of this data conflict and other structural uncertainties, the 2025 assessment applied a dual-model approach to ensure the robustness of its conclusions. The primary assessment was conducted using an ensemble over integrated, age-structured population models in Stock Synthesis (SS3), which built on the framework of the 2019 assessment. In parallel, a more parsimonious dynamic surplus production model (DSPM) was used. The DSPM relied primarily on catch and CPUE time series, and did not use the conflicting length-composition data. This approach served as a crucial structural sensitivity analysis, and provides an additional perspective on stock status. This multi-model inference strengthens the scientific basis for management advice in a data-limited context.

The multi-model analysis showed that the OCS stock remains in a severely depleted state but is showing signs of recovery. The stock synthesis ensemble estimated that the stock biomass reached a low point around 2013–2014, at approximately 4% of its unfished level. Since then, the biomass was estimated to have experienced a subtle but steady increase, reaching approximately 6% of the unfished level in recent years (i.e., 2022–2023). This trajectory aligned with expectations from previous projection studies, and indicates that the steep decline observed in prior decades has likely been halted. Nevertheless, fundamental uncertainties remain, and recent signs of improvement need to be considered with caution given the subtlety of the estimated increase.

Considering fishing pressure, the largest historical source of fishing mortality was estimated to be from longline fisheries. The significant reduction in interactions resulting from changes in fishing practices over the last decade appears to have been effective in reducing this pressure. The assessment concluded with high confidence that recent fishing mortality has been below biological limit reference points that would preclude stock rebuilding. The ensemble of models indicates that recent fishing mortality rates are below both F_lim and F_crash (the fishing mortality that would lead to long-term extinction), and the probability of exceeding these limits was near zero in recent years (i.e., 2022–2023) under the considered models.

Main assessment conclusions

• Based on the precedent of using SS3 for the OCS assessment, and on advances in Bayesian methodologies used for the present assessment (relative to the 2024 silky shark assessment), we suggest that the ensemble of SS3 models be used for management advice.

• The multi-model approach for assessing OCS resulted in a low stock status, but with high confidence that recent fishing mortality is below levels that would preclude stock rebuilding.

• The largest fishing mortality of OCS was estimated to be in longline fisheries. Reductions in OCS interactions as a result of changes in fishing practices over the last decade may have substantially reduced this source of mortality, likely halting the previously observed steep decline, and possibly leading to some (albeit slow) rebuilding.

• Recent fishing mortality rates were below biological limit reference points for the ensemble (Diagnostic F_recent/F_crash: 0.54 [0.37–0.74]; P(F_recent/F_crash>1)=0; P( F_recent/F_lim>1)=0).

• Recent biomass was estimated to have had a subtle increase from a low-point in 2013–14 near 5% of unfished biomass, to 6% of unfished biomass in recent years (2022–23).

Given some of the fundamental uncertainties highlighted above, we recommend:

• Improve observer data protocols: To counter the degradation of data quality resulting from the non-retention conservation measure (CMM-2011-04), it is recommended that longline observer programmes implement clear and consistent directives for recording all capture events, especially unobserved “discarded-cut-free” (DCF) individuals. Furthermore, recording approximate length measurements for sharks released in the water, a practice already in place in some programmes, should be standardised across the Western and Central Pacific Ocean.

• Prioritise research on stock structure and connectivity: Fundamental uncertainty remains regarding the stock structure of OCS in the Pacific Ocean. It is recommended that CCMs prioritise and support planned work under the WCPFC’s Shark Research Plan (SRP) involving satellite tagging and expanded genetic/genomics studies to address questions of regional residency, mixing, and stock boundaries.

• Resolve conflicting life history parameters: The significant divergence between available growth studies remains a considerable factor for the uncertainty in stock productivity. To build a more robust understanding of this species’ life history, it is recommended that work scheduled under the SRP to conduct additional growth studies and validate ageing methods from a range of locations be prioritised.

• Continue multi-model assessment frameworks: Given the persistent conflict between CPUE and length data, it is recommended that future assessments continue to use multi-model approaches. The use of simpler models, such as the Dynamic Surplus Production Model, alongside integrated age-structured models, provides a vital cross-assessment, and ensures management advice is robust to structural uncertainty.

• Refine historical catch estimates: Although progress has been made, the catch history for the longline fishery remains uncertain with considerable discrepancies between studies. It is recommended that shark catch reconstructions be reviewed, and these discrepancies be explored to gain an improved understanding of core uncertainties.

• Review and document recent improvements in shark assessment methodologies: With the present assessment, a full cycle of assessments has now been undertaken for blue, mako, silky and oceanic whitetip sharks, using consistent assessment methods, consistently refined over time. A review workshop and summary paper to capture recent progress and outstanding challenges is recommended to provide a solid basis for upcoming work, and provide an opportunity to share these advances across RFMOs. We recommend this workshop be considered by the Informal Small Working Group: Sharks for inclusion and prioritisation in the Shark Research Plan update at SC21.