Stock assessment of yellowfin tuna in the Western and Central Pacific Ocean: 2023. A. Magnusson, J. Day, T. Teears, J. Hampton, N. Davies, C. Castillo Jordán, T. Peatman, R. Scott, J. Scutt Phillips, S. McKechnie, F. Scott, N. Yao, R. Natadra, G. Pilling, P. Williams, P. Hamer
Executive Summary
This paper describes the 2023 stock assessment of yellowfin tuna (Thunnus albacares) in the western and central Pacific Ocean. An additional three years of data were available since the previous assessment in 2020, and the model extends through to the end of 2021. The assessment moved to a new 5 region spatial structure with improved convergence properties and which achieved a positive definite Hessian solution, which was a requirement for future assessments from SC18. This change was made during the stepwise model development process. The 5 region model, given its superior convergence properties to the original 9 region model, is used as the basis for the structural uncertainty grid and stock status conclusions. Additional new developments to the stock assessment, many of which have emanated from the independent peer review of the 2020 yellowfin assessment, include:
- Conversion from a catch-errors to a catch-conditioned approach, and the inclusion of a likelihood component for the CPUE from the index fisheries.
- Change from using VAST to sdmTMB to standardise the input CPUE series and the inclusion of additional covariates in the CPUE model.
- Different CPUE variances used for the CPUE associated with each index fishery, applying anew approach to estimate these variances.
- Internal estimation of natural mortality and application of the Lorenzen form of natural mortality at age.
- Additional procedures implemented for achieving more reliable model convergence, including jittering and checking positive definite Hessian status for all grid models.
- Integration of parameter estimation uncertainty with model-based uncertainty across the model grid for the key management reference points.
- Additional size composition filtering.
- Modifications to selectivity estimation settings, changes to fisheries with non-decreasing selectivity.
- Adoption of revised tagger effect modelling framework, reverting to assumptions similar to those used in 2017.
- Changes to size data weighting and downweighting the conditional age-at-length data for internal growth estimation.