Changes in vertical distribution of masu salmon during high flow in a large pool just below the Jin-san Dam in the Jinzu River.

Abstract

Vertical distribution of masu salmon, Oncorhynchus masou masou, during high flow was investigated by using the archival tags in a large pool (ca. 70 m long, ca. 150 m wide and Max. 13.7 m depth) just below the Jin-san Dam in the Jinzu River in 2009. The masu salmon, resided in the surface layer （ca. 1.0 m below the surface） under low flow conditions, moved to a deeper layer while repeating up-and-down movement with the increase in flow. It is supposed that the masu salmon took refuge in the middle layer （2-4 m）of weak current near the river bank aside to avoid surface and bottom of center layers (5-14 m) of rapid current when the flow has increased to the maximum level. And the masu salmon moved to a shallower layer again while repeating up-and-down movement with the decrease in flow. After the flow came to an ordinary level, masu salmon returned back to the surface layer. These results suggest that deep pools, having a variety of spaces ideal for creating weak currents, play an important role as refuge in case of high flow.