Compariosn of physical characteristics of the spawning redds between the fluvial Japanese charr Salvelinus leucomaenis and the masu salnon Oncorhynchus masou masou in the headwaters of the kinu river, central Japan

摘要

Water depth, current velocity, substrate size, stream bed type, and presence of cover at spawning sites of the fluvial Japanese charr Salvelinus leucomaenis and the masu salmon Oncorhynchus masou masou were studied in the headwaters of the Kinu River, central Japan, in 1991 and 1996. Water depth and median diameter of substrate were similar at both charr and salomon redds. However, current velocities were significantly slower at the redds of the charr than those of the salmon. Many salmon redds were observed at the pool ends, flat riffles, and rapid riffles where water currents were faster and direct. By contrast, charr redds were observed not only at such sites but also at pool shores and backwaters of obstructions (Boulders and bedrocks) were observed at the salmon redds. Relative wide spawning site selectivity of the charr may play an immportant role on the population continuance in the headwater reaches and small tributaries where the suitable spawning sites were scarce because of the steeper stream bed gradient. Also, the different spawning site selectivities between both species may play a role on the co-existence of both populations in their sympatric areas.