The development of hydroelectric power production and the damming of water courses that ensues causes a threat to the migration patterns of Atlantic salmon (Salmo salar), by altering their habitat and compromising river connectivity. Because the species’ survival heavily depends on the completion of their migration between rivers and oceans, the design of guidance structures that promote successful passages around dams is a critical goal. Physical structures have been used to steer downwards-migrating smolts through safe fishways but they have not always proven effective, both from an ecological and economical perspective. In this study, the potential of bubbles in guiding salmon trajectories around a hydropower dam was tested as an alternative to existing guiding systems. Here the hypotheses were: i) bubbles guide downstream-migrating smolts around dams; ii) the reaction of smolts to a bubble barrier is linked to their individual boldness; iii) bubbles have varying effects on different school sizes. The first and third hypotheses were tested in the field, by recording sonar footage at the entrance of a fishway, downstream of a bubble barrier. The second hypothesis was evaluated by conducting a scototaxis experiment and an assessment of the reaction of young salmon to bubbles in the laboratory. My analysis of the fish abundance data collected in the river confirmed the first and third hypotheses. In the field, the presence of a predator (Esox lucius) was revealed to be a more influential factor than bubbles in regulating the number of smolts steered towards the fishway, indicating that predators generated a larger anxiety-like response in smolts than bubbles. In the controlled laboratory setting, there was no significant correlation between anxiety-like behaviour and reaction to bubbles and the second hypothesis did not appear to be valid. Therefore, the cause of avoidance of the barrier remains unclear, but bubble barriers seem to be an efficient and cost-effective structure for guiding downstream-migrating salmon smolts.

Fragmentation of rivers due to the rapid expansion of hydropower plants is one of the major factors responsible for the decline in the Atlantic salmon (Salmo salar L.) in the Baltic Sea. Current methods to aid downstream migrating salmon smolt still struggle with effectively stopping the smolt from going through the hydropower turbines, by guiding them towards the available salmon ladders. A promising guiding method is using a barrier of bubbles which may serve as a non-intrusive alternative to conventional guiding structures. This study evaluated the risk of artificial selection caused using bubble barriers and compared the results with an earlier study.  The three hypotheses tested were: i) the salmon display a scototaxis behaviour related to their boldness, ii) temperature, and size of arena affect the scototaxis of the salmon, and iii) the salmons reaction to a bubble barrier is correlated to their boldness. The hypotheses were tested in a controlled laboratory setting by conducting three scototaxis tests and an experiment with several bubble barriers, where the reaction of salmon to bubbles could be assessed. There was significant positive correlation (r = 0,62 & p = 0,012) between the boldness of an individual and the time it took to swim through the bubbles. Even though a significant difference in scototaxis as a result of lowered temperature were discovered, the difference was too small to have a noticeable effect on the overall results. A bubble barrier may therefore put artificial selection pressure on the salmon population.

Loss of connectivity in riverine systems due to construction of hydropower dams has resulted in a worldwide decline of anadromous salmonid species such as Atlantic salmon (Salmo salar L.) and sea trout (Salmo trutta L.). The future of these species depend on the presence of available spawning habitat in freshwater river systems. Modern research and mitigation efforts mainly focus on ensuring a successful upstream passage past dams e.g. fish ladders. Atlantic salmon and sea trout are iteroparous, and are thus able to spawn repeatedly during their lifetime. Individuals surviving upstream migration and spawning generally face a hazardous journey back to their marine feeding grounds. In this large scale natural field study I evaluate the possibility of using a bubble barrier as a non-physical structure to guide downstream migrating kelt past the turbines at a large hydropower station in northern Sweden. Results from this study clearly show that kelt effectively can be diverted using a bubble barrier in daylight conditions with a mean water velocity of 1.1 m s-1 (p=0,01). From a fishway managers perspective, increasing survival of salmonid kelt is a substantial step towards achieving a viable population with increased numbers of repeat spawners and large individuals. This study presents new results in a sparsely explored subject; the diversion of post-spawn salmonid migrants using non-physical barriers.