Behavior and survival of fish migrating downstream in regulated rivers

Dams present obstacles to fish migrating between freshwater and marine habitats. This thesis evaluated downstream migrations of fish in five rivers in Sweden and North America, four of which were regulated (i.e., dammed). It focused on species from the subfamily Salmoninae and addressed the following questions: What is the survival of downstream migrating fishes passing turbines at dams? Estimated survival of Atlantic salmon (Salmo salar) and sea trout (S. trutta) kelts (0.547-0.748) was significantly lower than smolts (0.903-0.947) at the Stornorrfors and Sikfors power stations in northern Sweden. Estimated survival of juvenile Chinook salmon (Oncorhynchus tshawytscha) ranged from 0.814 to 0.946 at McNary Dam on the Columbia River, USA. Does mortality associated with dam passage affect fish population productivity? Yes. Salmon productivity was impacted by fish passing a single dam in two Swedish rivers, but the populations responded differently to strategies to mitigate dam passage mortality. Relative increases in female salmon escaping annually after 20 years were greater in the Piteälven River (68%) than the Vindelälven River (46%), when both smolts and kelts were protected and were four times greater (38 vs. 10%) when only kelts were protected. What are the potential mechanisms of mortality associated with fish passing turbines at dams? While results of some model studies suggested turbine blade strike was a major mechanism, assigning mortality to a specific mechanism or power station component was difficult. Studies at McNary Dam indicated potential impacts to fish sensory systems during turbine passage increased fish vulnerability to predation in the river below the dam, and this “delayed mortality” comprised from 46 to 70% of total mortality. A research program to identify fish injury thresholds to support the design of new turbines that could improve fish survival past dams is presented. What are the potential strategies for mitigating mortality associated with fish passing turbines at dams? Changing turbine operations alone did not significantly improve fish survival. Fish passage through a 3.9-km long system that bypassed fish around turbines at Bonneville Dam, USA, resulted in high fish survival (0.946), no injuries, mild stress responses, and fish passage times that were similar to water flow. Bypass design criteria for Pacific salmon are described. Water temperature and day length were key proximate factors controlling Atlantic salmon smolt migration timing; migrations in two study rivers were initiated around 8o C. A model based on temperature predicted that the daily rate of smolts leaving rearing areas peaked near 12.6o C. This information will allow timing past dams to be estimated and aid development of fish passage mitigation strategies in regulated rivers