River fragmentation worldwide, caused by hydropower facilities and other manmade infrastructure, is disrupting the critical migrations of freshwater migratory fish species, contributing to steep population declines. Historically, research and management have placed emphasis on upstream passage, but in recent decades the importance of safe and effective downstream passage has become increasingly recognised as a necessary step to allow river connectivity for migrating fish. A key challenge for the protection of out-migrating species is that fish instinctively follow the main river flow, which at hydropower sites is often directed toward turbine intakes. Even when bypass solution is available, without sufficient flow redirected towards the bypass, it is frequently not found by the fish, unless a guidance system is implemented. Physical guidance structures, such as fine screens, have been used successfully to guide fish at small to medium power plants, yet remain difficult to scale up at larger facilities. To overcome this issue, behavioural guidance systems have been investigated as potential alternatives to physical guidance. These approaches use different behavioural stimuli to guide fish toward safe passage routes. They are often easier to implement and scale up, but so far, their fish protection performance has yielded mixed results, which raises questions about their broader applicability. Therefore, there is a pressing need for passage solutions that are both effective at guiding and protecting fish, while also being easy to build, maintain, and upscale.

This thesis contributes to the field of fish passage by growing knowledge on existing guidance barriers and by carrying out research on novel guidance methods, after reviewing studies on behavioural guidance to identify knowledge gaps. The experimental work was carried out in a large ethohydraulic flume, where various guidance barriers were studied for different diadromous species and life stages. The results showed that behavioural bubble curtains were not suitable for eel guidance when compared to a physical net solution. On the other hand, a newly developed solution, that passively moves with the flow, termed the ‘dancing rods’ barrier, proved effective for guiding salmon smolts. Furthermore, the post-spawning stage of brown trout was tested with localised flow injection, which elicited changes in swimming behaviour in the kelts, demonstrating the potential of using manipulated hydraulic jets for downstream fish protection. In addition, the thesis examines the heart rate stress response of salmon smolts during isolation from conspecifics, showing the importance of integrating social factors into future fish research and passage design.

The thesis highlights both the potential and the limitations of behavioural guidance systems. The findings illustrate that behavioural guidance can be effective, when tailored to species-specific sensory and swimming capacities and to local hydraulic conditions. The findings suggest that behavioural guidance systems may support adaptive, site-specific downstream strategies, if informed by fish behaviour and physiology, yet they are unlikely to offer a universal solution for downstream fish passage protection. By bringing together animal behavioural, applied hydraulics, and stress physiology, this thesis advances knowledge on possible fish passage solutions with the ultimate goal of safeguarding downstream migrants in fragmented rivers.

Hydropower infrastructure fragments rivers and disrupts the migrations of many freshwater fish, contributing to declines in their populations. Re-establishing river connectivity is therefore critical for both upstream and downstream migrants. During downstream migration, the fish follow the main flow, which usually leads to turbine intakes, and even when a bypass is available, the fish may miss it unless guidance is provided. Physical solutions, such as fine screens, can be effective but are often impractical in larger facilities. Behavioural guidance systems, which use sensory stimuli to lead fish away from turbines and towards safer routes, may provide an alternative approach.

This thesis reviews existing research and evaluates behavioural guidance concepts in a large-scale experimental flume, while also considering physiological stress responses of migrating fish. The results demonstrate that behavioural guidance can support downstream passage protection as part of adaptive, site-specific strategies, though it is unlikely to provide a universal solution. By integrating insights from behaviour, physiology, and ecohydraulics, this work advances the knowledge on tools aimed at safeguarding downstream migrants in fragmented rivers.

River fragmentation caused by hydropower and adjacent infrastructure disrupts diadromous fish migration, contributing to steep population declines. Although efforts for reinstating upstream passage began earlier, the importance of downstream passage is becoming recognized as integral to restoring connectivity. During downstream migration, the fish follow the main current, which at hydropower sites often leads towards the turbines. Even when bypasses exist, unless enough water flows to them, fish often miss them, and guidance is necessary. Physical guidance, such as fine screens, is often effective at small and medium-sized power plants but is harder to scale for large facilities. As an alternative, behavioural guidance systems, which use sensory cues for guidance, have been explored. These are generally easier to implement and scale, but their effectiveness varies. Therefore, there is a need for effective, practical, and scalable guidance strategies.

The thesis addresses this goal by testing fish responses to different guidance methods and reviewing behavioural solutions. The results indicated that bubble curtains had little potential for eel guidance, while the novel “dancing rods” barrier showed potential for guiding salmon smolts. For post-spawning trout kelts, local flow injections triggered avoidance, suggesting engineered currents can aid behavioural guidance. The thesis also explored heart rates of salmon smolts in a social context, showing that isolation from group increases experienced stress. Overall behavioural guidance can be a valuable part of adaptive, site-specific downstream strategies when tailored to fish biology and local hydraulic conditions, although it is unlikely to become a universal fix. By integrating ecohydraulics, fish behaviour, and stress physiology, this thesis advances knowledge on protective tools for downstream migrants in fragmented rivers.