Review of the maturity criterion using gonad index in Pacific stock chub mackerel

Japan have been using gonad index (KG) based maturity criterion for female chub mackerel of the Pacific stock with KG=3 as a sign of maturity. The present paper aimed to review the maturity criterion by estimating maturity probability curves based on KG and GSI using chub mackerel collected in 2013-2023, determining the maturity probability at KG=3, and comparing the annual changes in the threshold values for maturity based on KG and GSI. The review has found that KG=3 is a good indicator to extract only matured fish, however, its strict criterion labeled many individuals in early stages of yolk accumulation as immature. The 50% maturity KG showed a lower trend during 2019-2022 than during 2014-2018, whereas the 50% maturity GSI exhibited a relatively stable trend, hence GSI is considered a more appropriate maturity criterion. The 50% maturity GSI ranged from 1.6-1.8 in 2014-2023 and was 1.6 in the 2013-2023 integrated. This paper suggests that, in order to improve maturity-at-age (MAA) data for future stock assessment, it is more appropriate to use 50% maturity, notably GSI of 1.6 as the maturity criterion for stock assessment purpose, while noting KG=3 accurately designates maturation.