Long-term changes in maturity at age of chub mackerel (Scomber japonicus) in relation to population declines in the waters off northeastern Japan

The Pacific stock of Japanese chub mackerel Scomber jaoponicus has been heavily exploited and its abundance has declined since the 1970s. A marked increase in the mean length at a given age and a decline in the age and/or size at maturity were coincidentally recognized with this population decline; however, the long-term changes of age and/or size at maturity have not been evaluated. We examined the fork length, body weight, and gonad weight of 21,048 individuals of chub mackerel ranging in age from 1 to 4 years from data recorded during 1970–1998. We found a clear trend for the proportion of mature females at given ages to increase, and the length and age at 50% maturity to decrease. Changes in the proportion of mature females at ages 1–4 years were mainly explained by changes in length at a given age. The sea surface temperature (SST) of the spawning ground also affected the maturation of young females (1 year). We constructed a model that included the effects of length at a given age, biomass during the spawning season, and the SST of the spawning area; the model well described the changes in the proportions of mature females at given ages over the 29 years examined. This model will be useful to predict future proportions of mature females at given ages in projection models for fishery management if changes in age and size at maturation are phenotypically plastic responses.