Upstream migration and summer stay of adult masu salmon in the Jinzu and Shou Rivers of the Hokuriku region

Abstract

The upstream migration and the summer stay of adult masu salmon Oncorhynchus masou were investigated in the Jinzu River and the Shou River of the Hokuriku region from 1991 to 1995. The period of upstream migration estimated from the monthly fluctuation of the catch data of masu salmon in the Jinzu River was from February to June, and the peak was in May. The mean fork length of the migrants was 58.5±4.9 cm and the mean body weight was 2.8±0.8 kg, and the female occupied 74.7% of the total. In the fishing surveys using the drift net in the Shou River, the number of adult masu salmon caught at the pools of the middle reaches occupied 96.4% of total catch. Most of fish (80.2%) caught from the the Jinzu River had empty stomachs. These findings indicate that adult masu salmon migrates to a river and stays in the pools of the middle reaches during summer season, feeding rarely. For effective increase of masu salmon resources, the preservation and/or the reconstruction of larger and deeper pools in a river are very important.