ビワマス幼魚の降河と銀毛化

抄録

Downstream migration and body silvering of landlocked biwa salmon Oncorhynchus rhodurus indigenous to Lake Biwa were observed in Lake Biwa and the rivers which flow to the lake in central Japan from 1983 to 1986. Underyearling biwa salmon inhabited the gently flowing shallows of the river shore in April, and moved into the riffle in May, when they began to migrate downstream into the lake. Downstream migration of the juvenile fish (5 to 7cm in standard length) was mainly observed from the middle of May to the end of June. The downstream migration seems to be related with rainfall, as the number of the fish moved into the lake increased on the day and/or the next day of rain, Although the sex ratio of the juvenile fish in the river in May was equal, the ratio of male became greater from June to July. Juvenile fish were scarcely observed in the river after July; in autumn only a small number of residual precocious mature males inhabited the river. After descending into the lake, they inhabited the offshore zone of the lake deeper than 20m. There was a significant difference in the slopes in the growth lines between the juvenile fish in the river and in the lake; the fish in the river grew faster than those in the lake. Body silvering of the juvenile fish began from the end of May in the river, and parr marks were scarcely observed in some of the fishes just after descending into the lake. Although parr marks completely disappeared in the fish greater than 10cm in standard length after September, even those fish did not show in-tense darkening at outer extremities of dorsal fin. The size and age of biwa salmon migrating downstream were similar to those of chum and pink salmon, and different from those of closely related amago and masu salmon. The process of body silvering in biwa salmon seems to be related to the downstream migratory behavior, since they are known not to acquire seawater adapt-ability as underyearlings