1Faculty of Applied Biological Science, Hiroshima University, Higashi-hiroshima, Hiroshima 739-8528, 2Kochi Prefectual Freshwater Fishies Experimental Station, Tosayamada, Kochi 782-0016, 3Faculty of Bioresources, Mie University, Tsu, Mie 514-8507, Japan
A stock separation study on ayu Plecoglossus altivelis captured from the Ioki and the Monobe rivers was conducted using the otolith Sr/Ca concentration ratio method. Discrimination among amphidromus, hatchery-cultured and landlocked populations was possible by comparing with chart patterns during early life stage, because the rearing salinity of hatchery-cultured fish is intermediate between those of amphidromus and landlocked populations. When stock separation was carried out by both otolith Sr/Ca ratio and mark-releasing with 85 individuals, disagreement between them occurred for only 9 individuals. It is indicated that stock separation by the method of otolith Sr/Ca ratio is as effective as the conventional mark-releasing method.