Studies on the life history of land-locked Ayu Plecoglossus altivelis in Lake Ikeda - I. Differences in the height of dorsal fin and body color between territorial and non-territorial Ayu Plecoglossus altivelis in Lake Ikeda.

Observations on the life history of land-locked Ayu Plecoglossus altivelis were made in Lake Ikeda, in Kagoshima Pref., from 1981 to 1985. In this study, two types of Ayu were recognized in its social behavior of immature adult fish in the Lake. One is territorial type and the other is non-territorial type. Some differences were found in the shape of dorsal fin and the color pattern of body between two social types. The territorial fish have longer posterior dorsal fin rays and darker fin membranes than those of non-territorial ones. The former have three clear yellow oval marks on their gill covers and anterior body sides, however, the latter have only one mark and its coloration is pale or faint. Thesevariations are not related with body size and sexuality but with the social type. It is suggested that large and clear yellow marks are signal for recognition of their status among owners of territory and for showing the domination of owner to non-territorial fish. Thus, these differences between two social types of Ayu show a possibility of being applied to the evaluation of fishing ground of this species.