Rising seawater temperatures fundamentally reshape marine ecosystems, with the Southern Ocean exhibiting vulnerability to climatic perturbations. Antarctic teleosts have evolved under near-constant sub-zero conditions, developing unique physiological adaptations. In this study, responses of Trematomus bernacchii, an endemic Antarctic fish, to three successive marine heatwave-like events were addressed. Using implantable bio-loggers, core body temperature and heart rate were monitored over 15 days, revealing a statistically significant positive correlation between core body temperature and cardiac performance, with an approximately 25% increase per degrees C. Molecular analyses of cardiac tissue revealed a sequential antioxidant response. An early upregulation of gpx4 suggests immediate mitochondrial protection against lipid peroxidation, while the subsequent induction of gpx1, prdx3, and prdx5 indicates a broader cytosolic defence. Furthermore, the marked decoupling between elevated gpx1 transcript levels and diminished Selenium-dependent glutathione peroxidase activity highlights a hitherto unrecognised post-transcriptional regulatory mechanism under acute thermal stress. Our findings suggest that T. bernacchii activates a sequential, organ-specific stress response to short-term warming, which may allow it to overcome episodic heatwaves.

Downstream guidance of migrating fish is a major conservation challenge in fragmented riverine systems, where orientation depends on fine-scale environmental cues and physiological readiness. Flow manipulation is increasingly explored to improve behavioral guidance at hydropower facilities, particularly for salmonids during the smolt stage. We investigated the effects of laterally injected jet flow on stress physiology and behavior of Atlantic salmon (Salmo salar) smolts in a large-scale flume. Fish were exposed to three treatments: jet flow from the left (Left Jet), from the right (Right Jet), and no flow (Control). Physiological analysis revealed that the Right Jet treatment significantly elevated malondialdehyde (MDA) levels and increased the activity of the antioxidant enzymes catalase (CAT) and glutathione peroxidase (GPx) in erythrocytes, indicating the presence of oxidative stress. No significant effects were detected for cortisol (plasma), hemoglobin and lactate (whole blood), or advanced oxidation protein products (AOPP, erythrocytes). Behaviorally, smolts showed a consistent left-side preference and swam predominantly near the bottom, regardless of treatment. Nevertheless, jet flow did not significantly alter lateral or vertical swimming distribution. Together, these results demonstrate that sublethal physiological strain can occur without overt behavioral change, underscoring the value of integrative, non-lethal approaches for evaluating fish responses to hydrodynamic conditions in guidance systems.

Populations of the European eel (Anguilla anguilla), a critically endangered species, have been severely impacted by migration barriers, as losses due to turbine-induced mortality can be substantial. To prevent eels from entering turbines, effective guidance systems are needed to redirect downstream-migrating eels towards safer alternative passage routes. Although physical guidance screens may have very high guidance efficiencies, these generally come with high construction and maintenance costs and are difficult to scale up to large rivers. Behavioural guidance systems are typically less costly, but have often been ineffective. Hence, more work is needed to identify more effective behavioural solutions or physical barriers that are less costly to upscale. In this study, we assessed the performance of a physical net barrier (23 mm mesh size) and a behavioural bubble curtain guidance solution, for downstream-migrating eels and compared these with a guidance-free control at four different water velocities (0.1, 0.4, 0.7 and 1 m/s) in a large experimental flume using PIT-telemetry and video. The overall passage rate with the net barrier was 68 % higher than during the control treatment, whereas there was no significant difference between the bubble curtain and the control. We also found an effect of eel body size, where larger eels were less likely to enter the bypass than smaller eels. Velocity did not influence passage rate. Video data, in addition, revealed that b guidance along the barrier was greater, and passes through the barrier fewer, for the net barrier than for the bubble curtain and the control. The results suggest that net guidance solution for downstream guidance of eels should be explored further, whereas the bubble curtain does not appear appropriate for eel guidance.

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.

Light pollution poses significant ecological challenges for nocturnal animals reliant on natural light for migration, orientation, and circadian rhythms. The physiological effects of abrupt exposure to artificial light at night (ALAN) on migratory fish, such as the light experienced passing near illuminated infrastructures, remain poorly understood. This study investigates the physiological responses of brown trout (Salmo trutta) smolts to lowintensity (0.02 lx) and short-term (30 s) ALAN, simulating nocturnal migration light conditions near illuminated bridges. To evaluate the influence of social dynamics, trout were tested individually (solitary) or in groups of six. Using continuous cardiac monitoring with data storage tags, alongside analyses of oxidative stress markers and adenylate kinase (AK) activity in the heart, we identified distinct patterns of physiological responses. Solitary fish exhibited significant heart rate variability (HRV) increases following repeated ALAN exposure, suggesting impaired physiological regulation under repeated ALAN exposure. In contrast, trout in groups displayed consistently lower HRV over the entire 90-min experiment, implying that social dynamics likely influenced a sustained oxidative stress response, corroborated by increased AK activity. Oxidative stress markers further reflected social effects, with significant upregulation of key antioxidant enzymes (sod1, sod2, gpx1, gpx4) and elevated lipid peroxidation, identifying lipids as primary oxidative targets. The observed divergence between superoxide dismutase (SOD) activity and sod gene expression suggests adaptive post-transcriptional regulation to maintain redox balance under combined environmental and social stress. These findings reveal that social dynamics under ALAN can amplify physiological stress, potentially affecting migratory outcomes.

The challenge of managing aquatic connectivity in a changing climate is exacerbated in the presence of additional anthropogenic stressors, social factors, and economic drivers. Here we discuss these issues in the context of structural and functional connectivity for aquatic biodiversity, specifically fish, in both the freshwater and marine realms. We posit that adaptive management strategies that consider shifting baselines and the socio-ecological implications of climate change will be required to achieve management objectives. The role of renewable energy expansion, particularly hydropower, is critically examined for its impact on connectivity. We advocate for strategic spatial planning that incorporates nature-positive solutions, ensuring climate mitigation efforts are harmonized with biodiversity conservation. We underscore the urgency of integrating robust scientific modelling with stakeholder values to define clear, adaptive management objectives. Finally, we call for innovative monitoring and predictive decision-making tools to navigate the uncertainties inherent in a changing climate, with the goal of ensuring the resilience and sustainability of aquatic ecosystems. 

The lack of free-flowing rivers worldwide is one of the factors driving the decline of freshwater migratory fish populations, as dams obstruct migratory pathways. To restore river connectivity, downstream solutions are necessary, in addition to upstream efforts. Fish tend to follow bulk flow as they migrate downstream, and unless enough flow is redirected, they need to be guided towards the bypass. Guidance systems can be classified as mechanical, behavioural or hybrid. Some mechanical barriers have been proven effective for guiding out-migrants, but often there are high costs associated with scaling up these barriers for large hydropower plants. Although higher approach velocities often limit behavioural barriers, they are continuously developed and implemented, due to the lower costs associated with their operation. Furthermore, the fish are less likely to get injured or impinged with behavioural guidance, due to the lack of mechanical components. I discuss some of the currently existing behavioural guidance systems, their potential and drawbacks.

Multiple anthropogenic forces have pushed river ecosystems into undesirable states with no clear understanding of how they shouldbe best managed. The advancement of riverine fish habitat models intended to provide management insights has slowed. Investigations into theoretical and empirical gaps to define habitat more comprehensively across different scales and ecological organizationsare crucial in managing the freshwater biodiversity crisis. We introduce the concept of novel riverscapes to reconcile anthropogenicforcing, fish habitat, limitations of current fish habitat models, and opportunities for new models. We outline three priority data-drivenopportunities that incorporate the novel riverscape concept: fish movement, river behavior, and drivers of novelty that all are integratedinto a scale-based framework to guide the development of new models. Last, we present a case study showing how researchers, modeldevelopers, and practitioners can work collaboratively to implement the novel riverscape concept.