Standardizing monthly egg survey data as an abundance index for spawning stock biomass of chub mackerel in the Northwest Pacific

We estimated a relative abundance index of spawning stock biomass (SSB) for the Pacific stock of chub mackerel with monthly egg density data obtained by research surveys. The monthly egg surveys have been conducted from 2005 to 2020 off the Pacific coast of Japan to cover the spawning ground of chub mackerel. We applied the vector-autoregressive spatio-temporal (VAST) model to the survey data to derive the index of egg abundance, which should represent relative SSB. This document provides important references and diagnostics on this standardization according to the “CPUE Standardization Protocol for Chub Mackerel”. Since we found no serious problems in the diagnostics and confirmed the convergence of the spatio-temporal model, we suggest the estimated index can be utilized as an SSB abundance index for the forthcoming stock assessment of chub mackerel in the Technical Working Group for the Chub Mackerel Stock Assessment.