Stock assessment of bigeye tuna in the western and central Pacific Ocean: 2023. J. Day, A. Magnusson, 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  bigeye  tuna Thunnus  obesus 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 applies the same 9–region model structure that was used for management advice from the 2020 assessment. New developments to the stock assessment include:

• Conversion from a catch-errors to a catch-conditioned modelling framework, and the inclusionof 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 using the Lorenzen functional form for natural mor-tality at age.
• Additional procedures adopted to achieve more reliable model convergence, including exten-sive jittering and checking the 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 used in the structural uncertainty grid.
• Use of conditional  age-at-length data, and  internal  estimation  of  growth, with  alternative weighting of these data included in the structural uncertainty grid.

This assessment is supported by the analysis of catch and effort data for longline fisheries to provide regional abundance indices (Teears et al., 2023), revised analysis of tagger effects and tag reporting rates  (Peatman  et  al.,  2023a;  Peatman,  2023),  size  composition  data  analyses  and  preparation(Peatman  et  al.,  2023b),  improvement  to  data  for  length-weight  conversion  factors  (Macdonald et al., 2023b) and developments to MFCL software (Davies et al., 2023).