Estimation of the Number of Native Amphidromous Ayu, Plecoglossus altivelis, in the Yoshino River

Abstract

The stock of ayu, Plecoglossus altivelis in the Yoshino River consists of three types, the amphidromous hatchery form, landlocked form derived from Lake Biwa and the native amphidromous form. The number of the native amphidromous form was estimated by analyzing the frequency distribution of the rank line scale number of the upper lateral line. Each frequency distribution of the three types was normally distributed and each average was significantly different from the other types. By fitting the normal distributions to the frequency distribution of the rank line scale number of the caught samples in which the fish from three types were mixed, the ratio of the native amphidromous form was calculated. The Marqualdt method was used for fitting the normal distributions. The number of the native amphidromous form was calculated from the ratio and the number of released hatchery form and landlocked form. The estimated number of the native amphidromous form from year 1998 to 2002 was 3, 938, 000, 17, 147, 000, 16, 934, 000, 24, 630, 000, and 32, 628, 000, respectively. The method adopted here is considered to be effective for the stock abundance estimation in big rivers such as the Yoshino River.