Stock assessment of landlocked ayu Plecoglossus altivelis altivelis in Japan through length-based models

Ayu Plecoglossus altivelis altivelis is a key commercially and culturally important freshwater osmeroid in Japan. Its native population is mostly an amphidromous form migrating between rivers and the sea, and not only native but also artificially landlocked forms are found in lakes and dam reservoirs. This study was undertaken to execute population feasibility and maximum sustainable yield (MSY) analysis of an artificially landlocked form of ayu during January 2018 to December 2020 in the Haidzuka reservoir and its connected Tabusa River, western Japan. FAO-ICLARM Stock Assessment Tools-II and empirical models were employed to estimate the growth function and population parameters. The estimated asymptotic length was 19.50 cm, and the growth coefficient was 0.73 year^-1 with a growth performance index of 2.443. The length at first maturity and length at optimum yield were calculated as 10.77 cm and 12.63 cm, respectively, which were lower than the length at first capture (L_c = 13.15 cm), suggesting the mesh size of fishing gear favoring the sustainability of the reproductive potential of this population. The calculated total, natural, and fishing mortalities were 2.45, 1.19, and 1.26 year^-1, respectively. The recruitment pattern was continuous round the year with two pulses where the peak was during July. The current level of exploitation (0.51) was slightly higher than the maximum exploitation rate (0.45), indicating a little overharvesting. The MSY of ayu in the Haidzuka reservoir was estimated to be 211 metric tons if the recommended L_c is maintained. Results of this study illustrate the first information on population characteristics of landlocked ayu and will help the development of suitable management policies for ayu fishery in lakes and dam reservoirs.