Winter habitat environment for juvenile masu salmon (<i>Oncorhynchus masou</i>) Focusing on Stream hierarchical structures

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Other Title
  • 河川の階層構造に着目したサクラマス幼魚の越冬環境―越冬環境を考慮した川づくリの提言―
  • 河川の階層構造に着目したサクラマス幼魚の越冬環境--越冬環境を考慮した川づくりの提言
  • カセン ノ カイソウ コウゾウ ニ チャクモク シタ サクラマス ヨウギョ ノ エットウ カンキョウ エットウ カンキョウ オ コウリョ シタ カワヅクリ ノ テイゲン
  • ―越冬環境を考慮した川づくりの提言―

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Abstract

The purpose of this research is to explain the relationship between the winter habitat for juvenile masu salmon and three hierarchical structures: channel unit, subunit and microhabitat. The research was conducted at the Makomanai River and Shakotan River, southwest part of Hokkaido, from the middle to the end of December 2001. The natural stream reaches of both rivers are assigned as segment one. The three hierarchical stream structures are designated at the lower hierarchy under stream reaches. The channel unit is at the uppermost part, follows by subunit and then microhabitat. The results show that winter season environment of the juvenile masu salmon depends on two hierarchical stream structures, subunit and microhabitat. The winter season environment is related to combination of subunits close to the riverbank of the pool and glide that have relatively slow current velocity and small Froude number (Fr < 0.125), and the microhabitats with cover that have even slower current velocity. Not only is the microhabitat significant to winter season environment for juvenile masu salmon, but also the subunits around the microhabitat that have slow current velocity and small Froude number. We considered the subunits and microhabitats are very important habitat for the juvenile masu salmon before winter season in addition to the period during winter season. It is, therefore, suitable to regard the hierarchical characteristic of stream structure as contiguous habitat that shows seasonal change. Furthermore, we proposed river management methods with consideration of the winter season environment based on this research results as follows. We should be aware of conserving and creating channel unit with subunit and microhabitat for winter season by river management in the scale of the stream reach. In terms of microhabitat, we should pay attention to microtopography, vegetation and coarse woody debris.

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