Hydropeaking may result in behavioral responses in riverine organisms that experience rapid flow fluctuations. In stream fish, the balance between energy spent on swimming and foraging vs. food intake affects fitness, a balance strongly influenced by flow conditions. This study investigates the effects of rapid flow fluctuations on the foraging success, group cohesion (nearest neighbor distance) and microhabitat segregation of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) parr (45-55 mm) in both sympatric and allopatric groups in a controlled laboratory stream environment. Our results indicate that fluctuating flow does not significantly influence the foraging success in either species, regardless of group composition (sympatry or allopatry). We found that group composition, rather than flow fluctuation, significantly influenced group cohesion. Salmon exhibited shorter nearest neighbor distances than trout did in allopatry, but not in sympatry. In terms of microhabitat selection, salmon consistently held position closer to the substrate than trout, regardless of group composition, suggesting selective segregation as the main mechanism driving microhabitat selection. Trout, on the other hand, held position higher above the substrate in allopatry but used a wider range of positions when in sympatry with salmon, highlighting interactive segregation as a possible mechanism driving microhabitat use, which underscores the importance of fish community in shaping patterns of habitat use. Our findings suggest that hydropeaking may not strongly impair the foraging ability and sociability of salmon and trout parr. Additionally, we shed light on the mechanisms influencing microhabitat selection of salmon and trout parr in rivers with a hydropeaking flow regime. One possible application of our results is as inputs for individual-based models currently being developed for understanding the effects of hydropeaking on stream salmonid populations.

Hydropower dams come with high ecological and social costs, not least concerning longitudinal connectivity in rivers, which causes declines and sometimes local extinctions of fish species. Fishways are widely used to allow fish to pass dams, but their efficiency is highly variable between species and sites. Many species, and at places entire fish communities, remain understudied, likely hindering the implementation of effective remedial measures. Here, we studied fish passage behaviour in a vertical slot fishway in the Po River, Italy. Almost 1000 individual fish of nine species, representing the local fish community, were tagged and released within and downstream of the fishway. The only species passing the fishway at relatively high numbers were potamodromous barbel and Italian chub, and for these species passage success was positively related to fish size. Passage was more likely to occur at night than during the day for barbel, but not for chub. In relation to the dispersal of invasive species, it is noteworthy that a few individuals of wels catfish and common carp passed the fishway.

Knowledge about the biology and ecology of species is fundamental for their management and conservation. Despite this, many fish species and life stages are understudied, and there is a great need for research efforts to understand their ecology. Italian riffle dace (Telestes muticellus; order Cypriniformes) is a small-sized (< 15 cm) stream fish native to the Italian peninsula. There is a scarcity of research on its ecology and behaviour. In this study, we explored the movement ecology of Telestes muticellus in a Mediterranean mountain stream in Northern Italy using passive integrated transponder (PIT) telemetry. Over 15 months, 267 fish were located 2-27 times. Most fish remained stationary, while a few roamed over several hundreds of meters. Fish covered a larger linear range during spring and autumn than during other seasons. T. muticellus showed a strong preference for pools, and this preference was strongest during summer. We observed no differences in habitat use and movements between day and night, and fish size had no major effect. Within pools, many T. muticellus displayed remarkably small home ranges (median 8 m(2)). Fish increased their linear ranges over a period of flood events, indicating that high-flow events may be important for the downstream and upstream dispersal of T. muticellus in small mountain streams.

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. 

The European eel is critically endangered due to heavy impact of anthropogenic factors, such as habitat fragmentation, overexploitation and climate change. During downstream migration, silver eels may encounter hydropower plants, which often result in delay or mortality from impingement on trash-racks or turbine passage. These problems can be mitigated with downstream passage solutions, such as angled racks that guide downstream-migrating eels to safe passage routes. The importance of bar spacing and phenotypic diversity for passage performance is, however, largely unknown. In this study, we investigated how morphological parameters (body mass, eye and fin indices) and behavioral score (open field test) influenced passage rate at an experimental intake equipped with a bypass and angled racks with either 15 or 30 mm bar spacing. Both racks were efficient in guiding eels into a bypass. There was a strong positive effect of body mass and a weak positive effect of open field test score on passage rate. Other factors such as eye and fin indices played a minor role. These results demonstrate the performance of angled racks with bypasses and form a useful starting point for further research regarding the relationships between individual variation in behavior, morphology and passage solutions for silver eels.

We use the case study of coho salmon and steelhead trout to discuss the coexistence of two similar stream salmonid species. The question of coexistence between these two species dates back to the early 1960s, making it a good example of the application of ecological theory to explain patterns observed in nature. Explanations for habitat selection and segregation between coho salmon and steelhead trout have included the mechanisms proposed by Nilsson (The biological basis of freshwater fish production. John Wiley and Sons, New York, 1967), interactive segregation and selective segregation. Through examples of laboratory and field studies of coho salmon and steelhead trout in Southeast Alaska, we suggest that habitat selection appears to be largely selective, but interactive segregation may become important during times of resource limitation. Despite a half-century of scientific inquiry into the coho-steelhead coexistence question, there is still much to be learned about habitat selection of these species; given the precarious status of many populations of coho salmon and steelhead trout within their native range, efforts to understand their ecological requirements are warranted.

Fragmentation of rivers by manmade barriers has impeded the ability of riverine fish to move freely. Barriers can be improved by fishways that can partially mitigate the negative impacts by acting as aquatic corridors. Effective fishways require knowledge about the physiological and spatial demands of fish species, but the existing knowledge largely derived from laboratory settings. Evaluating fishway performance is needed for optimisation of their hydraulic design and positioning. Qualitative methods include trapping, electrofishing, and camera observations to estimate the number of individuals passing (effectiveness). For quantitative assessment, the study of individual fish behaviour can identify fishway sections in need of improvement and estimate associated efficiencies. This can be accomplished by telemetric techniques such as PIT tagging, radio, and hydroacoustic telemetry.

River flow intermittence is a natural phenomenon intensified by human activities, such as water abstraction and the effects of climate change. A growing number of rivers are predicted to experience intermittent flows, which may impact the diversity and abundance of freshwater species. Dry riverbeds directly diminish the availability of habitats for freshwater organisms, and suitable environments can turn into ecological traps with reduced survival rates, posing a significant threat to population persistence. Even though fish movements can enable drought-affected populations to persist, little is known about individual fish movement between intermittent and perennial reaches. Here, we study the movement of individual PIT-tagged Italian riffle dace (Telestes muticellus) in an intermittent and perennial river reach before, during and after two severe drying events. A high proportion of fish from the intermittent reach survived the drying riverbed through directed upstream migration. This was manifested in fish living in the intermittent reach of the river displaying significantly higher linear ranges, and net travelled distances during the monitoring period than fish in the perennial reach, which remained resident with limited linear range and net distances travelled. This finding underscores the importance of conserving longitudinal river connectivity in the face of increased water scarcity and intermittent flow patterns.

Intermittent flow is a natural phenomenon in many stream systems worldwide. At the same time, droughts are an increasing threat to ecosystems as a consequence of climate change and water diversion. Severe droughts can change once suitable habitats into ecological traps that cannot support fish communities. Although individual fish movements can allow populations affected by drought to persist, the knowledge about individual fish movement between intermittent and perennial stretches remains limited. Here we present Italian riffle dace (Telestes muticellus) movement patterns (n = 17) in an intermittent and a neighbouring perennial stream stretch before and after a severe summer drought. Fish initially resident in the intermittent section had similar summer survival as fish from the perennial section. The majority of fish from the intermittent river stretch survived the drought by upstream movements to perennially watered reaches (87.5%). Fish from the intermittent stretch showed an average upstream movement of about 100 m, whereas fish from the perennial stretch remained relatively stationary within the stream. Our result highlights the ability of Italian riffle dace to cope with drought by a directed migration to river reaches with the perennial flow. It also underscores the need to preserve longitudinal river connectivity in the face of increasing water scarcity and associated intermittent flows.

To combat climate change, societal pressure to develop fossil-free hydroelectricity is growing. There is a great need, however, for environmental assessment tools that can predict the effects of streamflow regulation on biodiversity in hydropower-regulated rivers. Ecological modelling lets practitioners: 1) set broad bounds on population-level responses of key species and 2) identify knowledge gaps and prioritize research needs. Individual-based models (IBMs) are powerful tools for assessing relative benefits of alternative management actions, and therefore help to develop more sustainable hydropower solutions. We applied the inSALMO 7.3-SD IBM for populations of brown trout (Salmo trutta) and Atlantic salmon (S. salar) in the lower Gullspång River, Sweden. We simulated the effects of various minimum hydropeaking flow releases (from 9 to 21 m3/s) on outmigration production. We found that the number of age-1 outmigrants of both species decreased with increasing minimum flow release of the hydropeaking scenarios. The number of age-2 trout outmigrants did not change considerably with increasing the minimum release, but decreased sharply at the highest flow. The most age-2 salmon outmigrants were produced by flow scenarios with minimum releases of 15 and 18 m3/s. The model predicts, therefore, varying species- and life stage-specific effects of flow regulation. Moreover, increased flow caused juveniles to stay in the river longer and outmigrate at larger size, which exposes them to simulated predation longer but could increase post-outmigration survival. By providing insights into mechanisms driving population dynamics, IBMs can help promote the sustainability of high-conservation-value fish species.