アユの社会構造と生産 : 生息密度と関連づけて

抄録

A dense population of a salmon-like fish, Plecoglossus altivelis, or"Ayu"in Japanese, is found in the River Ukawa on the Japan Sea coast of Kyoto Prefecture. Members of our research group have been studing the ecology of this fish from various viewpoints since 1955. This paper concerns the change of the social behaviour and the mode of production. As already reported by our group, the common habit of this fish is territorial in the river-rapids and schooling in the river-pools. But on certain occasions, it shows no territorial behaviour, even in the river-rapids. By my recent observation, the social type of this fish appears to change with reference to the population density, and this change is accompanied by the difference in the body-length distribution. Five types were recognized in its social behaviour : schooling, solitary travelling, non-territorial solitary residential, solitary territorial and aggregated. The schooling is a social type in which all the members of a group show common behaviours, swimming in the same direction and feeding in similar manner. Solitary traveller is an independent passenger, having no home, but it may be united with a school or may be separated again. Non-territorial resident is a solitary dweller having its own range but does not show the territorial or attacking behaviour to the nearby individuals. In the territorial solitary type each fish has its own territory. An aggregation means that its members are aggregated within a certain area but do not show the common behaviour, and this type of behaviour is observed when they are resting or sleeping. The population of this fish, by our estimation, was far less in 1956 than in 1955(about one sixth), and the social relationship seemed to have been influenced by it. In 1955,when the density was high, most fishes showed the schooling behaviour and the territorial ones were very scarce. On the contrary, in 1956,when the density was low, many fishes behaved as settled solitaries and territorial individuals were not scarce. The stability of the school and the territory differs according to the change of population density. In the high density, the schooling is the common and more stable behaviour type of this fish and no difference is seen in the bodylength between the schooling and terrtorial ones. In the low density, however, the territorial behaviour is more stable, and there occurs a difference in the body-length, i.e. the territorial fishes are much larger than the schooling ones. In the case of the social structure in which the fishes are stabilized with territoriality, the growth of the non-territorial fishes seems to be checked because the latter cannot invade the bottom of the first class, which is defended by territorial fishes.