22nd Regular Session of the Scientific Committee

Review of the 2026 bigeye tuna stock assessment

SC22 will review the results of the 2026 bigeye tuna stock assessment, consider new findings, suggestions, and future research needs, including budget implications, and provide management recommendations to the Commission.

• The last stock assessment for bigeye tuna was conducted in 2023 and reviewed at SC19. The 2023 bigeye tuna stock assessment concluded that the stock is very likely not experiencing overfishing (100% probability Frecent<FMSY) and is not in an overfished condition (0% probability SBrecent/SBF=0<LRP). To support further development and improvement of the WCPO bigeye tuna stock assessment, SC19 in 2023 identified key priority areas, including strengthening research and data inputs through improved biological sampling, particularly age composition, expanded tagging programs, and review of size composition data; advancing model development through refinement of CPUE indices, consideration of effort creep, improved estimation of growth and mortality, and evaluation of spatial structure; and improving uncertainty analysis through targeted sensitivity testing of key assumptions, including tagging processes and data weighting, and addressing inconsistencies among data sources. SC19 also highlighted the importance of enhanced tagging diagnostics and supporting analyses. 

• Building on these areas, SC21 identified additional issues directly relevant to the next assessment. Indicator analyses presented to SC21 showed a sustained decline in mean weight across fisheries, linked to increased catches of small fish, which affects key model inputs and has contributed to higher estimated fishing mortality relative to FMSY. SC21, therefore, recommended a detailed analysis of mean weight trends by space, time, and fleet to inform model assumptions. SC21 also highlighted the need to resolve key data uncertainties, including issues related to small bigeye classification, and supported continued improvements in biological sampling, size data reconciliation, and conversion factors, as well as ongoing refinement of model assumptions and uncertainty related to growth, mortality, movement, and CPUE, to improve the robustness of the 2026 stock assessment.