Development of the Stock Synthesis model for Pacific saury in 2025

This paper documents the continued development of the age-structured assessment model in 2025 for Pacific saury in the Northwestern Pacific Ocean (WNPO) with Stock Synthesis 3.30. The model fits the historical catch, standardized catch-per-unit-effort (CPUE), fishery-independent index of abundance, and length composition data provided during SSC PS 15. This document describes the methodology on model structure and parameterization of the current base case model, followed by diagnostic procedures to determine goodness of fit to the data, estimability and stability of parameter estimates using jitter analysis and comparison of MCMC with maximum likelihood estimation, and predictive ability for short-term forecasting with retrospective analysis and hindcast validation of future index predictions in the first projection year. Likelihood profiles inform alternative choices on natural mortality (M) and steepness for sensitivity analysis. A reduction in growth has improved the fit to the length composition and reduced the retrospective pattern compared to previous models. However, predictive performance remains difficult for this short-lived species. Sensitivity analyses indicates difficulty estimating MSY reference points with the current M value in the base model. Further evaluation is recommended with respect to the choice of M in the base case model, as the current value does not provide practical management advice.