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.

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.

When fish migrate downstream, they follow bulk flow and unless enough flow is redirected towards a bypass, they need guidance to pass the dam. Guidance relying on behavioural responses by fish are generally less effective but less expensive than physical guidance structures, and hence there is an interest in finding more effective behavioural guidance systems. Here we test a newly developed behavioural guidance system referred to as the ’dancing rods’ guidance barrier. The system consists of a series of evenly-spaced parallel floating polyethylene rods that are anchored to the river bottom, vacillating with the flow, thereby presenting the fish will a “permeable wall” whose purpose is to lead the fish away from the turbines. A single pilot trial with 106 out-migrating Atlantic salmon smolts carried out in a large experimental flume showed that the barrier was effective in deterring fish from passing through it. Only 5.7% of the fish crossed the rods barrier downstream, while the rest of the fish remained upstream 51.9%, stayed around the start box or followed the barrier downstream until they reached the end of a bypass ramp 39.6%. Further testing is required to establish the potential of the ‘dancing rods’ as a guiding structure.