抄録
The influence of channel works on spawning habitat of masu salmon (Oncorhynchus mason BREVOORT) was studied in the Shakotan River of Central Hokkaido, Northern Japan. The meandering channel was straight-ened by enbankment and the river bed was protected by gabion works about 20 years ago. The spawned sites were widely distributed throughout the artificial altered section and natural tributary with density of 0. 5 per 100 m distance. The spawned sites were densely made in the transition from pool to rapid riffle. The gravel used for spawning in altered section were larger than natural river (mode 8-10 cm in diameter), and the velocity was relatively faster than natural (mean 50-60 cm/sec) . Vegetation cover was also additional factor to be chosen for spawn. These facts indicates that the masu salmons were forced to spawn in worse condition in altered section than natural. However, there was remarkable difference on the spawning density depending on the protection ratio of river bed. The spawning sites in altered section were limited in the downstream of falling works with 1 m height where the gravel layer with favorable water depth and velocity was provided. For reproduction of masu salmon in artificially altered river, it is important to allow minor gravel scouring and sedimentation along channel. They can be revived by creating variable stream flow and gravel deposition place.
