Application of Virtual Population Analysis (VPA) and State-space Assessment Model (SAM) to the Shared Data of Chub Mackerel in the Northwest Pacific

The Technical Working Group for Chub Mackerel Stock Assessment in NPFC has decided to use an operating model (OM) for comparing the performance of different four assessment model candidates. Japan has proposed tuned virtual population analysis (VPA) and state-space assessment model (SAM) as candidate stock assessment models. In this paper, we report the results on application of the two models to the merged data that has been shared among the members for OM development. Although the past estimates were close between VPA and SAM, the recent abundance estimates in VPA were greatly higher than those in the SAM and the recent fishing pressure was greatly lower in the VPA. Both models did not show serious retrospective biases and it is difficult to determine which model is better from this result. The simulation testing will be important for the choice of best stock assessment model and we should discuss specific scenario settings in the operating model framework.