Estimation of Natural Mortality and Possible Shapes of Distributions for Japanese Eel Anguilla japonica by Empirical Methods

Instantaneous natural mortality rate (M) is one of the most important parameters governing the population dynamics of the fish, and is frequently necessary in fishery assessment and management. Japanese eel (Anguilla japonica) is an important eel species widely distributed in Northeastern Asia. Its population suffers greatly depress in recent decades, but lack of regional specific information of M may impede the effective management across the distribution range. In this study, life-history parameters are collected from available literatures and seven empirical methods are applied to estimate M, standard error and distribution of Japanese eel from regions in Japan, Mainland China and Taiwan. The sex-pooled mean (± SD, year^-1) of M is 0.429 ± 0.213 for Kaoping River in Taiwan, 0.420 ± 0.194 and 0.482 ± 0.269 for Pearl River and Yangtze River in China, respectively. In Japan, it is 0.527 ± 0.186 and 0.468 ± 0.331 for Mikawa Bay and Hanaka Lake, respectively. Estimates of M are normally distributed (Kolmogorov-Smirnov Test, all p > 0.103 except in Pearl River, p = 0.027) or log-normally distributed (all p > 0.173). Therefore, normal or log-normal distributions with mean and standard errors above can be applied in assessment models for the Japanese eel in Taiwan, China and Japan, respectively. The distributions of M from empirical methods can be further updated when direct estimates and associated likelihood are available by Bayesian approaches.