Bubbles guide migrating smolts around hydropower plants

Abstract: 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.