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

The upstream migration and the summer stay of adult masu salmon Oncorhynchu s masou were investigated in the Jinzu River and the Shou River of the Hoku riku region from 1991 to 1995. The period of upstream migration estimated f rom the monthly fluctuation of the catch data of masu salmon in the Jinzu R iver was from February to June, and the peak was in May. The mean fork leng th 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 cau ght 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 f indings indicate that adult masu salmon migrates to a river and stays in th e pools of the middle reaches during summer season, feeding rarely. For eff ective increase of masu salmon resources, the preservation and/or the recon struction of larger and deeper pools in a river are very important.