An evaluation of the contribution of stocks in the supplementation of ayu Plecoglossus altivelis in the Tohoku area, using allozyme markers

Abstract

This study evaluated the contribution of released stocks of ayu, Plecoglossus altivelis, to a target population in the Tohoku area of northern Honshu Island, using allozyme markers. Both natural and hatchery-reared ayu in this area were genetically characterized using 17 gene loci. Samples were taken from one landlocked population from Lake Biwa, from four natural amphidromous populations, and from one hatchery-reared population from Fukushima Prefecture. Two rivers in this region, the Sakari River and the Hienuki River, which had received numerous stocks from all three sources, were examined for stock composition using a maximum likelihood procedure. Significant allele frequency differences were found among the three sources at three of the 17 loci analyzed which are GPI-1*, MDH-1* and MPI* loci. A combination of differences in frequencies of GPI-1*, MPI* and MDH-1* alleles among three released stocks served as natural genetic markers which can identify the source of released stock after stocking. In the Sakari River, which carries an abundant amphidromous ayu population, we observed little influence from either the Fukushima or Lake Biwa stocks. However, in the Hienuki River, which has no native ayu stock, the maximum likelihood evaluated the proportional contribution of released stocks as 36 ± 10% from the Fukushima stock, 42 ± 12% from the Lake Biwa stock, and 22 ± 10% from the amphidromous stock, which corresponds well with the actual proportions of released stock, suggesting that the three released stocks survive and contribute throughout the harvest. The implications of estimates of the stocked contribution are discussed.