Age, growth, and estimation of the age-length key for Japanese flounder Paralichthys olivaceus in the southwestern Sea of Japan

Japanese flounder Paralichthys olivaceus is one of the most important coastal fish species in Japan. However, its catch in the southwestern Sea of Japan has been decreasing. Therefore, there is a need to improve the accuracy of stock assessment. Catch-at-age, which is the basis for stock assessment, is calculated based on the length–frequency distribution using age-length keys. It is recommended that this basic information is revised annually. However, the information for this species has not been updated for a relatively long period. In this study, growth curves were estimated and compared with those recorded from other areas. A multinomial logistic age-length key was generated and compared with the classical catch-at-age calculation method to examine potential errors. On the basis of the results obtained, a sex-specific von Bertalanffy growth curve was selected using the Bayesian information criterion. A model of sex and season was chosen for the multinomial logistic age-length key. The relative error was smaller and the estimated age composition was more accurate with the multinomial logistic age-length key, even when data with missing values were used. The improved accuracy of catch-at-age achieved due to the superior characteristics of the multinomial logistic age-length key is expected to be useful for the advancement of stock assessment.