アユの社会構造と生産II : 15年間の変化をみて

抄録

The immigration number from the sea in spring, and population density, body-length distribution and social behaviour during the settling season (summer) of the Ayu-fish (Plecoglossus altivelis) were investigated from 1955 to 1969 in the River Ukawa. The population density varied between 0.03 and 5.5 indiv./m^2,but the natural mortality from spring to summer was stable being about a half to onethird. It was confirmed that the social behaviour was changed by its population density and that the growth was not limited directly by algal production but mediated by its own social structure. When overall population density was about four times to that the all fish had their own territories, territorial structure was established only in certain types of river-bed. The difference resulted from the relation between the value of the feeding or resting site and its closedness against the invasion of non-territorial ones. Territorial structure of Ayu had probably evolved as a self-regulatory process but was not so distinct as at the present time.