Tochigi Prefectural Fisheries Experiment Station, Ohtawara, Tochigi 324-0404, Japan
Discrimination of stocks of ayu Plecoglossus altivelis caught by angling in the Naka River, using an assignment method with multilocus microsatellite DNA markers, was conducted to estimate the ratio of stocked fish to wild fish in the catch. Substantial differences in allele frequencies and level of genetic diversity at seven microsatellite loci observed between wild populations and released stocks enable us to classify individuals caught by angling into those originated from the wild populations and those from the released stocks. Of a total of 326 individuals sampled in the upper, middle and lower reaches of the fishing ground of the Naka River, 88.4% of the individuals were classified as originating from wild populations and the remainder from released stocks. There were significant differences in the ratio of released fish in the catch among the reaches. Taking seasonal changes of the catch rate into consideration, our data suggested that wild fish recruited into the middle and lower reaches were abundant after midsummer. Proportional stock density differed among the stocks discriminated and among the reaches. These results indicate that there is a difference in properties of fishing ground among the reaches within the river that could affect efficiency of stocking and thus appropriate stocking methods that reflect the properties are needed.