Behavior and distribution of upstream-migrating juvenilerhinogobius sp. (The orange form)

Upstream-migration of juvenileRhinogobius sp. OR was studied along the Ado River, a tributary of Lake Biwa. Juvenile gobies, mainly 18–21 mm in standard length, migrated from the lake in a belt of about 3 m wide in lower reaches and about 10–20 cm wide in middler reaches. Their upstream-migration was restricted to places shallower than 20 cm with bottom current velocities less than 20 cm/sec. Individual migration speed varied between 4.2–8.9 m/min at places with bottom current velocity of 8.8–20.0 cm/sec, slowing down with increasing current velocity. Most active migration occurred between 14:00 and 16:00, number of upstream-migrating juveniles decreasing with distance from the river mouth. Riffles in the mid-and upper reaches, having few slow current areas, probably present barriers to upstream-migration of the juvenile gobies, although both adults and subadults of the species are distributed in more upper reaches.