Growth related to behaviour of ayu, Plecoglossus altivelis , reared under low density

Abstract

(1) Three groups of ayu, each populated by 6 fish, were reared in three concrete ponds of same size (1×4×0.3 m). Growth of all individuals was measured on total length, and behaviour of all individuals marked by vinyl ribbon was observed at some intervals during the experiment of about 40 days in August and September, 1960. (2) Averaged growth of fish did not vary significantly among the groups. But the growth of individual whether in the size attained or its rate varied considerably, and the variation well corresponded to the social construction in which they participted. (3) Within each group, individual behaviour which was well expressed in the strength of chasing action (competition) and territory formation correlated positively to the growth of fish. The correlation as such is believed to exist not only in ayu but also in many other species of fish possessing competitive or territorial behaviour. (4) The different social structures formulated by the three groups of fish under low population density may not be adequately explained at this moment leaving further studies to be carried out. (5) It was assumed that the ayu population of low density as experimented here has not presented natural social structure of the species, thus, resulted diversity. Nonetheless, the experiment is believed to approve the writer's concept-density dependent growth of fish will be resulted through the medium of differential behaviour of associated member.