Standardizing abundance indices for recruitment and spawning stock biomass of the chub mackerel in the Northwest Pacific

This document introduces three analyses of CPUE standardization for Pacific chub mackerel according to standardization protocol by NPFC. The year trends for the recruitment were extracted from standardized CPUEs by applying the data of surface trawl surveys during summer (Jun. to Jul.) and autumn (Sep. to Oct.) to the delta-gamma model with the GLM-tree. The year trends for the spawning stock biomass were also extracted from the standardized CPUE by applying the catch-and-effort data of commercial dip-net fishery around main spawning grounds of the Pacific chub mackerel to the delta-gamma model. These standardized CPUEs have been used as abundance indices for tuned virtual population analysis in the latest Japanese stock assessment of Pacific chub mackerel. Because those abundance indices are primary information sources for the population trend of the chub mackerel, this document provides fundamental information for the discussion in the Technical Working Group for the Chub Mackerel Stock Assessment (TWG CMSA) for further review and model improvement.