1. Downstream displacement by flood is serious for stream fishes inhabiting above tall check dams that block the ability of fishes to migrate upstream. Although downstream displacement may not be lethal, it can cause a large decline of the local population above the tall dam.

2. Many landlocked salmonid populations persist in such above-dam habitats, indicating that they have succeeded for many generations there. They may adapt to the unique situation in the above-dam habitats.

3. This study examined the hypothesis that juveniles of the salmonid fish, Oncorhynchus masou ishikawae, inhabiting above tall dams have a functional morphology for resisting downstream displacement due to floods. Field survey and experiments were conducted in the Koza river system, the southernmost region of the Kii Peninsula in Honshu, a region with one of the highest rainfalls in Japan (annual precipitation is up to 4000 mm).

4. Juveniles collected from above-dam habitats showed a deeper caudal peduncle depth and body depth than open-stream and above-waterfall populations in the field. Individuals with a deeper caudal peduncle depth and body depth were more likely to resist downstream displacement during flooding in a field experiment. Juveniles originating from above-dam habitats also showed a deeper caudal peduncle depth and body depth than the other populations when they were fertilized and reared in an artificial common environment.

5. These results consistently support the hypothesis that morphology in juveniles inhabiting above-dam habitats has adapted against flood disturbance.