溯上アユの生息密度と淵の利用のしかた

抄録

A dense population of a salmon-like fish, Plecoglossus altivelis, or Ayu in Japanese, is found in the River Ukawa in the north-western part of Kyoto Prefecture. We have been studying the ecology of this fish from various viewpoints since 1955. This report concerns the change of the modes of utilizing the river-pools, which we observed during the course of our study. As we have already reported (KAWANABE, MIYADI, MORI, HARADA and OHGUSHI, 1956), there can be distinguished two kinds in the life of Ayu in pools, which are related to the topographical characteristics of the river-pools as well as to their adjoining riffles, i.e., using the pools as both feeding and resting places or as shelters only. By our recent observation it was discovered that the modes practically taken by Ayu might be changed according to population density. The population of Ayu by our estimation in 1956 was far less than that of 1955 (about one-sixth). The decrease in population in 1956 was far greater in the river-pools (about one-tenth of 1955) than in the riffles (about one-fourth of 1955). In 1956,when the density of Ayu in the river was low, the pools were utilized chiefly as shefters or resting places in the night-time, and the fish used to take their foods in the daytime in the adjoining riffles, where they could find better and richer food materials in the form of algae attached to stones than, in the pools. On the contrary, in 1955,when the density was high, the pools were utilized as feeding places as in the case of the riffles ; so, some individuals were found staying and feeding there both in the daytime and at night.