Abstract
We investigated upstream migration of ayu Plecoglossus altivelis, by attaching PIT tags to 300 wild individuals captured at Naga Dam, approximately 30 kilometers away from the river mouth of Miya River, Mie Prefecture, Japan. The tagged individuals were released on 25 April, 2018, and their upstream migrations were recorded by using a PIT tag reader set up at a fish way in Takihara Dam, located in the lower reach of a tributary, Ouchiyama River, flowing into the middle reach of Miya River. During 3 May to 5 July, 2018, a total of 24 individuals （8 % for the tagged individuals） were recorded by the recorder. The earliest ascending was observed 8 days after releasing, while the latest was observed 71 days later. A generalized additive model revealed that the ascending of ayu was positively related to the water level of Miya River and the amount of water taken from Takihara Dam. These findings suggest that most of the upstream migrants pass by the junction of Ouchiyama River and the mainstream of Miya River, and stray into an upstream pool located directly beneath of Misedani Dam which fragments Miya River for electricity generation. Since no fish way is constructed in Misedani Dam, most stock of ayu seem to have not been utilized as fisheries resources. Artificial transfer to the upper stream of Miya River and/or Ouchiyama River may be beneficial to utilize the stock. Leading the fish up to Ouchiyama River may also be profitable.
