Morphological adaptation to avoid downstream displacement in juvenile landlocked salmon

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.

1. 对于栖息于高拦沙坝（check dam）上游的溪流鱼类而言，洪水引发的下游漂移问题尤为严峻——这类高坝会阻断鱼类的上游洄游路径。尽管下游漂移未必会直接致死，却可能导致坝上游区域的本土鱼类种群规模大幅下降。
2. 诸多陆封型鲑鳟鱼类（salmonid）种群可在这类坝上游生境中存续，这表明它们已在此处繁衍多代，或许已经适应了坝上游生境的独特环境条件。
3. 本研究针对如下假说展开验证：栖息于高坝上游的鲑鳟鱼类（salmonid）Oncorhynchus masou ishikawae幼体，具备可抵御洪水引发的下游漂移的功能形态特征。研究于本州（Honshu）纪伊半岛（Kii Peninsula）最南端的高濑川（Koza river）流域开展野外调查与实验，该区域是日本年降水量最高的区域之一，年降水量可达4000毫米。
4. 野外调查显示，相较于开阔溪流及瀑布上游种群，采自坝上游生境的幼体拥有更深的尾柄与体高。野外实验表明，尾柄与体高更深的个体在洪水期间更不易发生下游漂移。此外，将采自不同生境的幼体进行人工授精并在统一人工培育环境下培育后发现，坝上游种群的幼体仍比其他种群拥有更深的尾柄与体高。
5. 上述研究结果均验证了该假说：栖息于坝上游生境的幼体形态特征，已针对洪水扰动产生了适应性演化。