Estimation of the mixed ratio of native and released ayu Plecoglossus altivelis altivelis caught in the Takatsu River, Shimane Prefecture, Japan

摘要

Mixed ratios of native and released ayu Plecoglossus altivelis altivelis caught in the Takatsu River were estimated for years 2009 to 2012. The stock of ayu in this river consists of three types: native amphidromous and two released hatchery-reared forms bred at different hatcheries. Mixed ratios were estimated by fitting a multiple normal distribution to the frequency distribution of the scale number above the lateral line using the maximum likelihood method. The data of scale numbers above the lateral line of ayu caught in the upper and lower waters of the dam in this river were resampled by the bootstrap method at 2,000 repetitions, and then each bootstrap distribution was fitted to the multiple normal distribution with two peaks. The native ayu ratios were estimated at 26.3-58.8. and 82.6-100., respectively. These estimates reflected upstreaming conditions of native ayu, and were considered to be proper values.