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.

Hydropeaking causes unnatural flow and water level fluctuations in the riparian zone. Biological processes in the riparian zone,such as nutrient cycling, are affected by changes in temperature, oxygen, and moisture, which are all to some extent influencedby the water level. Here, we studied the relationship between hydropeaking and decomposition of organic matter in the riparianzone. Specifically, we measured the decomposition rate (k) and decomposition endpoint (the stabilization factor, S), and exam-ined associations between k, S, hydropeaking, and different environmental variables. Decomposition was estimated using theTea Bag Index at 33 sites in Swedish rivers and streams with different degrees of hydropeaking. Rivers and streams were sampledin the summer of 2020 (N = 26) and autumn of 2023 (N = 16), with nine sites sampled on both occasions. In summer 2020, k wasnegatively related to high hydropeaking intensity, but there was no effect of hydropeaking on S. In autumn 2023, there was noeffect of hydropeaking on k or S. Of the other environmental variables tested (substrate grain size, temperature, and length ofgrowing season), the growing season had a positive relationship with S in autumn 2023. Our findings indicate that reducing hy-dropeaking during summer, when biological activity is high, may be beneficial for maintaining riparian function.

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.

Clear-cutting of forests with little or no regard for riparian buffers alters the local abiotic habitat of streams within and downstream of clear-cuts by increasing temperature, incident light, suspended sediments and resource inputs such as carbon and inorganic nutrients. It is also well documented that streams with narrow or non-existent riparian buffers affect local stream ecosystem processes. Here, we ask whether ecosystem processes can also be affected downstream of clear-cuts. We tested this in nine headwater streams that run through recently harvested clear-cuts (1-6 years ago) with varying buffer widths (<10 and >= 15 m) in northern Sweden. We compared biofilm (periphytic algal and bacterial mats) and whole stream metabolic rates in stream reaches situated upstream of the clear-cuts, in the clear-cuts and downstream of the clear-cuts. We found that biofilm gross primary productivity (GPP) in streams with thin buffers (<10 m) increased, on average, by 54 % downstream of clear-cuts in July, but that the net effect on the whole ecosystem was still a decrease in ecosystem productivity due to high respiration rates. In September, the situation was different as there was a 50 % decrease in biofilm GPP downstream of clear-cuts, and the net effect was again a decrease in ecosystem productivity. Wide buffer zones (>15 m) could mitigate these longitudinal changes for both biofilm and whole stream metabolism, except in one stream that was dominated by fine sediments. Importantly, the magnitude of downstream propagation in biofilm GPP was related to the magnitude of responses in the clear-cut, which in turn was driven by nutrient concentrations. To upscale the potential magnitude of clear-cutting in Sweden, we estimated that nearly 6 % (or 57,400 km) of the total forested stream length is situated within and 100 m downstream of clear-cuts that were harvested 1-6 years ago. Based on this study, we conclude that clear-cut effects on stream ecosystem processes are not only local, but can also be propagated to downstream recipient waters if riparian buffer width in the clear-cut is less than 15 m.

Epigenetic modifications, particularly DNA methylation, are increasingly recognized as mechanisms underlying phenotypic plasticity and potential mediators of transgenerational responses to environmental change. We investigated the persistence of early life temperature-induced DNA methylation changes and the role of parental life history in shaping methylation patterns in juvenile brown trout (Salmo trutta). Fertilized eggs from crosses of anadromous and resident trout were incubated under natural or elevated temperatures (by +3 degrees C) until first feeding, after which all fish were reared under common conditions. Whole-genome bisulfite pooled sequencing was conducted on juveniles 10.5 months post-fertilization. We found weak and inconsistent evidence for persistent temperature-induced methylation changes, with little overlap among different parental cross types. In contrast, parental life history, particularly maternal origin, significantly influenced offspring methylation patterns. Maternally derived differences were more extensive than paternal effects and were enriched for genes related to metabolism, nervous system function, and digestion, suggesting potential adaptive relevance. These findings suggest a limited long-term impact of early-life thermal conditions on methylation and emphasize a stronger role of transgenerational epigenetic effects in brown trout. Given that climate change is expected to alter thermal regimes in future aquatic ecosystems, our results, along with other recent publications, suggest that parental environmental history may be a more significant driver of epigenetic variability than the temperature experienced during early life. Understanding such mechanisms is critical for predicting how populations may respond to ongoing and future climate change.

By analyzing otolith microchemistry, we examined the use of freshwater and marine environments by brown trout Salmo trutta L. that spawn in the Swedish River Emån and migrate to the Baltic Sea. We estimated the time juveniles spent in freshwater and the number of times the fish returned to freshwater, presumably to spawn. Twenty-six percent of the fish migrated to sea by 1 year of age. However, 13% spent less than one year in the river. Most brown trout (48%) migrated to the sea between 1 and 2 years of age. On average, brown trout, which averaged 4.4 years in age (range 3–6 years), returned to freshwater 2.3 times, and there was an inverse relationship between time spent in freshwater after hatching and the number of visits to freshwater. Our results do not support the classical life history pattern, where brown trout spend one or more years in freshwater before migrating to the sea. Here, we found evidence that part of the population leaves freshwater during their first year. While the cause for precocial migration in the River Emån is not known, our results from this permanently flowing river do not support the idea proposed for other Baltic Sea populations, where the risk of drought has been suggested to be the cause. 

Conditions early in ontogeny can have considerable effects later on in life. Many salmonids spawn during the autumn, and temperature during subsequent embryogenesis may have far-reaching effects on life-history traits, especially when considering ongoing climate change. Even biotic conditions during embryogenesis, such as predation threat, may affect later life stages. Here, we examined how predator odours and increased temperatures affect embryonic growth and development of a fish (brown trout Salmo trutta). We found that embryos had lower body mass and greater yolk volume close to hatching when subjected to predator odours. Trout embryos incubated at temperatures representing natural winter conditions were larger than embryos incubated at higher temperatures, although the latter hatched earlier. Fry sizes at emergence did not differ between treatments, perhaps because of compensatory growth during spring. This study shows that predator presence can have similar effects on embryonic growth of salmonids as warming winters, with possible impact later in ontogeny. 

In many rivers, downstream-migrating salmonid smolts must pass multiple dams often with high losses as a result. Fish experience mortality both in dam and reservoir passage, and spilling water might allow fish to avoid turbine passage and hence increase migration survival. In River Klaralven, Sweden landlocked Atlantic salmon smolts migrate along a 180 km long reach passing eight dams. A previous telemetry study estimated an accumulated migration success of 16% under conditions with no or very little spill. Here we repeat this study, under a planned spill regime at a subset of hydropower dams. Overall passage success through the eight dams was 32%, which is greater than the 16% reported from the same river section in a year without spill. Most of this increase, however, was attributable to the situation at one dam, where spill constituted a large proportion of total discharge. In addition, we found that loss rates km-1 were similar over dammed reaches and the lentic habitats, but greater than in the free-flowing reference reaches. Results for migration speed paralleled this result with the highest speeds observed in the free-flowing reaches.

Variation in thermal conditions during embryogenesis can have far-reaching impact throughout ontogeny and may give rise to behavioural variation. Many animals, such as salmonids, exhibit behavioural trade-offs related to foraging and predator avoidance. How embryonic temperature affects these behaviours has remained unexplored. Not only abiotic conditions during embryogenesis but also biotic factors such as predator conditioning may affect fish behaviour, especially anti-predator responses. We examined how elevated temperatures and predator odours throughout embryogenesis affect the behaviour of 28-37 mm young-of-the-year brown trout (Salmo trutta) in encounters with predators, namely Atlantic salmon (Salmo salar; 20 cm) and burbot (Lota lota; 40 cm). Juvenile brown trout were more active and aggressive if they were incubated in warmer water as eggs than if they were incubated in colder water, and trout remained inactive longer when encountering predators if they were cold incubated. Brown trout were less active and aggressive when an Atlantic salmon was present than when a burbot or no predator was present. Behavioural responses did not differ between trout that had been subjected to water with versus without predator odours during embryogenesis, possibly because brown trout were not subjected to conspecific alarm cues during egg incubation. This study shows that thermal conditions during embryogenesis can influence fish behaviour early in life and thus contribute to behavioural variation, with potential effects on life history. Considering the rapid warming of northern regions, elevated embryonic temperatures may contribute substantially to variation in salmonid behaviour in the near future. Variation in environmental conditions during embryogenesis of salmonids can have far-reaching impact throughout ontogeny and may give rise to variation in anti-predator behaviour. In a laboratory experiment, we showed that elevated temperatures throughout embryogenesis increased the activity and aggression of 28-37 mm brown trout fry and reduced the time to first movement in encounters with predators (burbot and Atlantic salmon). Predator odour during embryogenesis did not affect brown trout fry behaviour.image