LAUSR

Abstract Chub mackerel (Scomber japonicus) in the northwest Pacific Ocean is generally divided into the Tsushima Warm Current (TWC) and Pacific Ocean (Pacific stock) stocks, based on habitat and migratory patterns. Since the 1970s, the abundance of the Pacific stock has undergone more extreme fluctuations than the TWC stock, which decreased sharply in the late 1970s and remained at low levels until it began to recover after the 2000s. In this study we focused on the Pacific stock, and analysed the variability in its abundance based on time series data for catch, biomass and recruitment. Sea surface temperature is considered a major environmental factor that affects the wintering and spawning of chub mackerel. Consequently, we used wintering and spawning ground indices, including the average sea surface temperature (SST), the meridional positional deviance of the 15 °C (MPD15) and 18 °C (MPD18) isotherms and the SST-suitability weighted size of the potential spawning ground (WSSG) in Kuroshio Current waters to analyse the impacts of oceanic environmental conditions in the wintering and spawning grounds on variations in chub mackerel abundance. The wintering and spawning ground indices showed clear decadal variability, with abrupt changes evident in the late 1970s, the late 1980s and the 2010s. These were correlated with major fluctuations in the abundance of chub mackerel. A combination of correlation and gradient forest analyses showed that the thermal environment in the wintering ground were relatively important predictors of chub mackerel abundance; these predictors (with a 1- or 2-year time lag) may better explain the variations in abundance after the 1970s. Variations in the SST and MPD18 during the spawning period correlated well with the recovery of chub mackerel abundance after the 2000s. Both the Asian Monsoon Index and Southern Oscillation Index correlated with the wintering and spawning ground temperature conditions, indicating that large scale climatic factors have a major influence on variability in temperature in the wintering and spawning grounds of chub mackerel. We conclude that wintering and spawning ground indices for the Kuroshio Current region may be useful for explaining the variations in chub mackerel abundance.