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.

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.

Climate change-induced extreme weather and related drought and flood conditions are heterogeneous across space and time. The variability in location, timing, and magnitude of rainfall can alter how species respond to the drought and flood disturbances. To further complicate this matter, when droughts end they are often followed by extreme flooding, which are rarely considered as a disturbance (Humphries et al. 2024), let alone assessed with its own heterogeneity. Consequently, it is difficult to quantify impacts on ecological communities across large spatiotemporal scales without considering flood-drought disturbance characteristics in sequence (Burton et al. 2020). We hypothesized that native organisms have evolved resistance to withstand repeated cycles of drought-flood disturbances, and that established non-native species have adapted to persist in novel conditions. To test this, we fit spatiotemporal models of species occurrence with local rainfall patterns as covariates in the drought and flood impacted Murray-Darling basin in Australia during the decade long Millenium Drought, and its recovery period. During these drought conditions, river-floodplain organisms in the Murray-Darling became localized in refugia that limited longitudinal and lateral connectivity (Bond et al. 2008), and following flooding the same organisms were exposed to dispersal and recruitment opportunities (Humphries et al. 2020), as well as to hypoxic blackwater events that lead to the mortality of aquatic organisms (Small et al. 2014). At the basin-scale we found that the range size of most native and non-native fishes were highly resistant to the extreme drought and post-flood conditions. At local scales, species richness, or detection, actually increased under drought conditions. Both findings highlight the resistance of species to climate change driven extreme weather, which opens new questions on community adaptations.

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

Naturförvaltning kräver god kunskap om ekosystemet, och för en god fiskförvaltning är det en grundförutsättning att man har kunskap om fisksamhällets arter och hur näringsväven det ingår i är sammansatt. Idag finns flera olika metoder för att undersöka fiskars födoval (stabila isotoper, DNA mm) men traditionella maganalyser är fortfarande en bra metod för att undersöka fiskars diet, och den ger dessutom information om fiskens trofiska placering i födoväven. Vänern har stammar av endemisk sjövandrande av lax (Salmo salar) och öring (Salmo trutta) som har ett särskilt bevarandevärde och är skyddade av EU’s habitatdirektiv. I syfte att förstå mer av födoväven dessa stammar ingår i undersökte vi dieten hos odlad vuxen lax och öring fångad i Vänern. Detta genomfördes under åren 2021-2022 och prov samlades in både vår och höst med hjälp av sportfiskare i samband med årliga sportfisketävlingar. Fisk samlades in från hela Vänern, men fördelningen uppdelat på de tio fångstzonerna var ojämn. Sammanlagt 95 laxar och 265 öringar undersöktes varav 87% av laxarna fångades på våren och 64% av öringarna. Medianlängden var 71 cm och medianvikten 4,0 kg för de analyserade laxarna och 69 cm respektive 3,9 kg för öringarna. Resultaten visar att lax och öring i det undersökta storleksspannet har likartad diet och att nors dominerar i båda arternas diet. För både lax och öring var merparten av norsen i dieten i storleksspannet 50 – 130 mm, vilket i stort speglar storleksfördelningen av nors i Vänern, men laxen åt mindre byten i genomsnitt än vad öringen gjorde. Vi fann även siklöja och storspigg och ett fåtal abborrar i dieten. Ungefär en tredjedel av maginnehållet var så långt nedbrutet att artidentifiering inte var möjlig. Under 2022 noterade vi att det fanns parasiter (bandmasken Eubotrium sp.) i 100% av laxarna och 98% av öringarna. Insamlingsmetodiken innebär att bara odlad lax och öring med ett minimått på minst 60 cm ingår i studien.

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. 

Large boreal rivers in Sweden are generally impounded by hydropower dams and a large proportion of main stem shallow flowing habitats have been lost. Tributaries often contain the last undisturbed habitats and could be important for the conservation of species diversity. In particular, tributary mouth areas could be biodiversity hot-spots, due to their vicinity to the main stem and favorable environmental conditions. In this study, we investigate whether tributary mouth areas in two impounded boreal rivers (Ume River and Lule River) could be regarded as biodiversity hot-spots for fish. Based on standardized electrofishing in 20 tributary mouths, we find that overall fish diversity is generally low. The highest species richness and diversity was found in mouth areas dominated by intermediate substrate sizes (gravel – cobble). Few, if any, species were found in areas where fine sediments (smaller than sand) dominated. The tributary mouth areas had similar species richness and diversity as areas in the tributaries located 1-km upstream of the mouth, but the fish community composition often differed between these two types of sites. Management action favoring fish diversity in the tributary mouth areas could include protection or rehabilitation of areas dominated by medium sized substrate and reduction of erosion and transport of fine sediments in the tributaries. Overall, we find no support for tributary mouths being hot-spots for fish biodiversity and while some patterns in diversity gives hints on suitable management action, it is important to further understand impacts in tributaries and their mouths and the temporal dynamics of the fish community.

Rivers networks represent hierarchical dendritic habitats within terrestrial landscapes and differences in connectivity and land use influence dispersal, and consequently biodiversity patterns. We, therefore, measured variation in water chemistry and fish abundance and related these to a number of landscape characteristics (e.g., wetland, urban, wooded, and agricultural) in the River Klaralven and its 30 permanently flowing tributaries. We hypothesized that these environmental attributes would differ between tributary and main stem habitat and that these differences would be driven by landscape attributes including land use. We found considerable intertributary variation in temperature and nutrient levels, and between the tributaries and the main stem. Generally, water temperature was lower in the tributaries, whereas nutrient levels were higher in the tributaries. The lower water temperature has implications for coldwater fishes, and we found two fishes, burbot and lamprey, associated with coldwater tributaries. We also found an inverse relationship between water quality and anthropogenic land use. Protecting tributaries with low anthropogenic impact will likely become increasingly important with ongoing global warming as they can function as thermal refugia for coldwater fishes. Hence, this study underscores the need to evaluate water courses at regional scales to identify spatial refuges and ensure connectivity.

Modeling fish community responses to dam removal is an emerging field of study as dam removals become more common, but uncertainties concerning recovery time and community stability remain. In Europe, an EU-wide biodiversity strategy plans to restore around 25,000 km of rivers to free-flowing status, which emphasizes the importance of being able to predict fish community responses after dam removal. We developed a multi-species size spectrum model for a fish community in the Mörrum River in Sweden to identify possible outcomes after a dam was removed in 2020. Electrofishing monitoring before the dam removal was used to calibrate the model. We projected multiple scenarios into the future to explore patterns of community stability, individual species responses, and recovery time while varying parameters related to dam removal mortality, base resource rate change, and maximum recruitment change. We created 30 hypothetical scenarios using an abrupt change perspective (parameters are step-based) and 30 scenarios using a gradual change perspective (parameters are smooth). In both perspectives, dam removal mortality and a decreasing resource rate reduced community biomass and delayed recovery time compared to pre-dam removal conditions. Our results demonstrate that recovery from a dam removal scenario is not necessarily a benefit for all species. In scenarios where dam removal practices or dam failures cause high mortality events and sustained impacts on base trophic level resources, recovery of pre-removal biomass may take decades, while community stability may be unstable for twice that time-period. Our study shows that size spectrum models can be applied to dam removal scenarios to explore potential recovery outcomes, particularly from a risk avoidance perspective. A benefit of using such an approach is the relatively low data requirements needed to perform projections (e.g., present species, fish growth rates, relative fish abundance). Implementing this model in other river systems, particularly at the reach scale, can help river restoration and management assess tradeoffs associated with different habitat restoration approaches prior to committing to a dam removal plan.