Comparing redd distributions of chum and masu salmon in the Kotonihassam River, Hokkaido.

Abstract

We studied the distribution and physical environmental characteristics of chum (Oncorhynchus keta) and masu salmon （O. masoui） redds within the alluvial fan of the Koonihassam River. After the spawning period, we evaluated the differences between surface and subsurface water temperatures during the winter to determine the presence or abence of underground water discharge. Our results indicated that the redds of masu salmon were mainly distributed in the upstream section and those of chum salmon were only distributed in the downstream section of the river. Chum salmon redds were characterized by significantly larger grain sizes than those of masu salmon. There were no significant differences between surface and subsurface water temperatures of masu salmon redds in the up-stream section. However, the subsurface water temperature of chum salmon in the down-stream section was significantly higher than the surface water temperature, suggesting the presence of underground water discharge. Chum salmon redds were largely distributed at the lower edge of the alluvial fan, where underground water tends to discharge from the river bed. In contrast, masu salmon redds were mainly found in the upper area of the alluvial fan, and underground water discharge from the river bed was not confirmed. Our results suggested that the spawning site selection of masu salmon is independent of the presence or absence of underground water discharge. These results suggest that suitable spawning habitats for these species are likely affected by multiple environmental conditions, including geomorphological features, underground water discharge, and riverbed composition.