Evaluation of possibility of estimating systematic changes in catchability in the state-space production model

The paper aims at evaluating possibility of estimation of systematic change in the catchability coefficient over time within the interim stock assessment model, BSSPM, used in Pacific saury stock assessment in the NPFC. For conditioning the simulation models, the parameters including in the time-varying catchability coefficient were firstly estimated according to the three base case scenarios agreed in the last stock assessment. The simulation data were then generated and applied to the BSSPM models. The results showed that such a dynamic change in catchability might be estimated well for some scenarios within their models if the assumption in the relative biomass is correct. However, if such an assumption is not correctly hold, there may be some potential bias in the estimation of catchability and biomass. This sort of exercise should be more paid attention before finalizing 2020 stock assessment. Otherwise, a safer option might be to exclude Japanese early CPUE from the assessment.