Abundance Of Atlantic Salmon Research Articles

Validity of winter sampling for estimation of salmonid abundance by electrofishing

Electrofishing removal sampling is a common method for abundance estimation of juvenile salmonids in streams and is usually performed during autumn. European guidelines specify that sampling should be conducted at water temperatures ≥ 5 °C, but studies investigating the validity of sampling below this threshold are lacking. We conducted electrofishing experiments during autumn and winter in three Norwegian rivers, comparing abundance estimation accuracy, rate of fish emigration from electrofishing sites, and mortality for juvenile Atlantic salmon Salmo salar and brown trout Salmo trutta. Abundance estimates were significantly more accurate during autumn than during winter, less accurate for age-0 fish than for older parr, and more accurate when the number of electrofishing passes was increased. By digging through the substrate after electrofishing, we found that a higher proportion of fish were uncatchable during winter than during autumn, and that age-0 fish were more likely than older fish to be uncatchable. For salmon older than age-0, both autumn and winter sampling consistently yielded relatively accurate abundance estimates. However, the rate of fish emigration from electrofishing sites was significantly higher during autumn than during winter and if block nets are not used, emigration during autumn sampling could cause severe underestimation of Atlantic salmon abundance. Fish mortality did not differ between autumn and winter sampling and was low for both species and all age groups. We recommend the use of block nets and conducting five electrofishing passes to reduce the likelihood of severe estimation error.

Status of a stocked Atlantic salmon population in Lake Huron

In 2011, the Michigan Department of Natural Resources (MDNR) expanded stocking of Atlantic salmon (Salmo salar) in Lake Huron to enhance fishing opportunities following Chinook salmon (Oncorhynchus tshawytscha) abundance declines. Currently, little is known about the population produced from this stocking. We fit an assessment model to harvest data from Michigan jurisdictional waters to estimate Atlantic salmon population dynamics and abundance. Because of potential biases in MDNR creel survey harvest estimates, a survey was e-mailed to online purchasers of a 2019 Michigan fishing license asking about where and when Atlantic salmon were harvested to correct creel harvest estimates. Anglers were also asked about catch-and-release angling and tested on their ability to identify Lake Huron salmonids. Creel harvest estimates overlapped spatially and temporally with 42% of survey reported Atlantic salmon harvest. After correcting creel harvest estimates, total abundance of Atlantic salmon in 2019 was estimated at approximately 392,000 fish with a peak abundance of approximately 406,000 fish. Anglers released 27% of caught fish and correctly identified Atlantic salmon 28% of the time. To assess the occurrence of food resource competition, differences in condition (i.e., expected weight at length) were evaluated. Condition was higher in later years than in earlier years despite abundance increases. Our results suggested that past stocking established a population of approximately 400,000 Atlantic salmon with evidence suggesting that fish were finding sufficient food resources. Future Atlantic salmon management efforts may be improved by quantifying post-stocking survival rates and other sources (e.g., charter, Canadian) of harvest.

Direct and indirect influences of macrophyte cover on abundance and growth of juvenile Atlantic salmon

Abstract The relationships between macrophytes and the physical and biological characteristics of the environments that aquatic organisms inhabit are complex. Previous studies have shown that the macrophytes, Ranunculus (subgenus Batrachium), which are dominant in lowland chalk streams and widespread across Europe, can enhance juvenile Atlantic salmon abundance and growth to a greater degree than other physical and biological habitat characteristics. However, mechanistic understanding of how this effect might arise requires consideration of the direct and indirect relationships among habitat characteristics that are likely to be influenced by the presence of macrophyte cover. We applied structural equation modelling to data collected during a 2‐year in‐river manipulative experiment in the River Frome (southern England, U.K.) designed to quantify the magnitude and the relative importance of direct and indirect influences of Ranunculus cover and other physical and biological variables, including water velocity, water depth, prey biomass and body size, and abundance of con‐ and hetero‐specifics, on abundance and somatic growth of 0+ salmon. Results indicated a strongly positive direct influence of Ranunculus cover on salmon abundance, as well as positive influences of Ranunculus on velocity heterogeneity and water depth that are indirectly related to decreased salmon abundance. Interestingly, there was no indication of a direct influence of Ranunculus cover on salmon growth, although Ranunculus was indirectly related to increased salmon growth through its positive influence on prey biomass, an effect mediated by velocity heterogeneity and proportion of fast velocities. These findings provide novel mechanistic insights into the key role of Ranunculus in their native lowland rivers to enhance abundance and improve conditions for multiple food web components. Strategies to maintain or enhance naturally occurring Ranunculus in these rivers are therefore likely to return wide ranging ecosystem benefits, including for species of high conservation value, such as salmon. These mechanistic impacts on habitat heterogeneity and ecosystem productivity could generalise to native macrophytes in other river systems, particularly where habitat is dominated by vegetation in the absence of large substrates.

Changes in the smoltification age of Atlantic salmon (<i>Salmo salar </i>L.) of the Yokanga, Kharlovka and Vostochnaya Litsa Rivers (the Murmansk Region)

Throughout the world, in recent decades, there has been a decrease in the abundance of Atlantic salmon, the main reason for which is a decrease in its survival rate during life at sea. Its value has a direct dependence on the length and, accordingly, the age of the smolts. Foreign researchers have found that the nature of long-term changes in the age structure of smolts can vary significantly by region. At the same time, the age of Atlantic salmon in Russian rivers, which account for a significant part of the area of the range of this species, was not considered. In this regard, the analysis of long-term data characterizing the age composition of salmon smolts of some rivers of the Murmansk region has been carried out. It has found been that in the first years of the period 2000-2018, the age structure of Atlantic salmon smolts was transformed in the rivers Yokanga, Kharlovka and Vostochnaya Litsa. As a result, fish at the age of 6 ceased to occur among them, the abundance of fish at the age of 5+ significantly decreased and the abundance of fish in the age groups of 3+ or 4+ significantly increased. The most likely reason for this is the change in the temperature regime of rivers since the mid-1990s, as a result of which the life time in the rivers of juvenile salmon has been reduced. Rejuvenation of the smolt population may have ambiguous consequences for the abundance of adult salmon, which are determined by the cumulative effect of reducing natural mortality at the stage from eggs to smolts, increasing their abundance and reducing their length, and reducing the survival rate during salmon life in the sea.

TESTING OF A NEST-INCUBATOR OF CAVIAR OF SALMON SPECIES OF FISH OF THE GENUS SALMO "SALMO-3000" IN THE CONDITIONS OF THE ARCTIC RIVER INDYORA (KOLA PENINSULA)

The article presents the results of approbation of the artificial nest-incubator of eggs "Salmo-3000" for the reproduction of salmon species of fish of the genus Salmo, tests were carried out in the r. Inder (White Seabasin). An original device was used, installed at the bottom of the river's threshold section. The nest design includes: the main body - a streamlined oval flattened container formed by three layered incubation trays and a lid; remote water intake / filter; connecting branch pipe with accessories for water distribution and discharge; elements fastening to the ground and a device for driving fasteners into the ground. The nest allows you to incubate fertilized eggs of Atlantic salmon or brown trout during autumn, winter and spring (autonomy for at least 8 months) and in summer (June) to obtain viable larvae that independently settle in the river threshold or are forcibly removed from rearing devices in the basins. During the test, the following advantages were revealed (increased capacity for incubated eggs up to 3,000 salmon / brown trout eggs per device, effective water distributor, grates that limit the eggs from washing out during laying, fixing rods made of fiberglass reinforcement that do not give oxidation, water intake / filter effectively cleaning and supplying water to caviar), and the disadvantage - the complexity and duration of the installation of the device, up to 60 minutes per device. In general, the hatching efficiency of larvae was 97.2% at partial load. The output of fry into the river was 96.0%. The device can be used to restore the abundance of Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) in small rivers where hatchery reproduction is unprofitable for various reasons.

The current state of Atlantic salmon reproduction in the Keret River, White Sea basin, and how it can be increased

The Keret River is one of large salmon rivers in the White Sea basin. The estimated abundance of Atlantic salmon in the river began to decrease in 1991, reaching a maximum in 1997 due to the abruptly increased illegal fishing level and the invasion of juvenile Atlantic salmon by the monogeny of Gyrodactylus salaris. The studies conducted have shown that the salmon population in the Keret River is depressed and that the average distribution density of juveniles aged 1+ and older is 1.6 fish / 100 m2. Atlantic salmon reproduction on fish hatcheries is important for maintaining the population abundance (farm individuals make up over 50% of the population). Artificial Atlantic salmon reproduction in the Republic of Karelia’s rivers is carried out by the Karelian Branch of Glavrybvod FGBU at the Kem and Vyg fish hatcheries. The fish hatcheries could achieve better results by updating the juvenile breeding technology. An example of a new high-intensity biotechnology is the use of physical factors controlling the vital activity of fish in the various periods of ontogenesis. The study of the effect of low-intensity laser radiation in the early stages of ontogenesis has shown that red light over a certain dose range enhances the adaptability of fish by decreasing mortality and accelerating growth, thus decreasing the loss of fish products in fish hatcheries.

Response of fish assemblages to restoration of rapids habitat in a Great Lakes connecting channel

Rapids habitats are critical spawning and nursery grounds for multiple Laurentian Great Lakes fishes of ecological importance such as lake sturgeon, walleye, and salmonids. However, river modifications have destroyed important rapids habitat in connecting channels by modifying flow profiles and removing large quantities of cobble and gravel that are preferred spawning substrates of several fish species. The conversion of rapids habitat to slow moving waters has altered fish assemblages and decreased the spawning success of lithophilic species. The St. Marys River is a Great Lakes connecting channel in which the majority of rapids habitat has been lost. However, rapids habitat was restored at the Little Rapids in 2016 to recover important spawning habitat in this river. During the restoration, flow and substrate were recovered to rapids habitat. We sampled the fish community (pre- and post-restoration), focusing on age-0 fishes in order to characterize the response of the fish assemblage to the restoration, particularly for species of importance (e.g. lake whitefish, walleye, Atlantic salmon). Following restoration, we observed a 40% increase in age-0 fish catch per unit effort, increased presence of rare species, and a shift in assemblage structure of age-0 fishes (higher relative abundance of Salmonidae, Cottidae, and Gasterosteidae). We also observed a “transition” period in 2017, in which the assemblage was markedly different from the pre- and post-restoration assemblages and was dominated by Catostomidae. Responses from target species were mixed, with increased Atlantic salmon abundance, first documented presence of walleye and no presence of lake sturgeon or Coregoninae.

Spatial Heterogeneity of eDNA Transport Improves Stream Assessment of Threatened Salmon Presence, Abundance, and Location

The integration of environmental DNA (eDNA) within management strategies for lotic organisms requires translating eDNA detection and quantification data into inferences of the locations and abundances of target species. Understanding how eDNA is distributed in space and time within the complex environments of rivers and streams is a major factor in achieving this translation. Here we study bidimensional eDNA signals in streams to predict the position and abundance of Atlantic salmon (Salmo salar) juveniles. We use data from sentinel cages with a range of abundances (3–63 juveniles) that were deployed in three coastal streams in New Brunswick, Canada. We evaluate the spatial patterns of eDNA dispersal and determine the effect of discharge on the dilution rate of eDNA. Our results show that eDNA exhibits predictable plume dynamics downstream from sources, with eDNA being initially concentrated and transported in the midstream, but eventually accumulating in stream margins with time and distance. From these findings we developed a fish detection and distribution prediction model based on the eDNA ratio in midstream versus bankside sites for a variety of fish distribution scenarios. Finally, we advise that sampling midstream at every 400 m is sufficient to detect a single fish at low velocity, but sampling efforts need to be increased at higher water velocity (every 100 m in the systems surveyed in this study). Studying salmon eDNA spatio-temporal patterns in lotic environments is essential to developing strong quantitative population assessment models that successfully leverage eDNA as a tool to protect salmon populations.

Growth during the first summer at sea modulates sex-specific maturation schedule in Atlantic salmon

Recent decline in abundance of Atlantic salmon (Salmo salar) and concomitant changes in life history may result from a decline in the growth conditions during marine migration. Available literature suggests the existence of a sex-specific reaction norm linking maturation with environmental growth conditions at sea. However, the extent to which this mechanism explains variations in age at maturity remains unclear. Using a historical collection of scales (1987–2017) from the Sélune River, France, we showed that marine growth declined over the first summer and remained stable during the subsequent periods at sea among returning salmon. Results support the hypothesis of a sex-specific probabilistic reaction norm, with individual probability to return after 1 year at sea increasing when growth increases. Females may require higher growth than males to attain their maturation threshold. This mechanism is a good candidate to explain temporal variability in sea-age at return at both the individual and population level in the Sélune population and in many other southern European populations.

Assessing the effect of recovery programs for salmon (Salmo salar Linnaeus, 1758) at its Southern limit in Europe: application of segmented regression models to long-term data from the Ulla River

The abundance of Atlantic salmon in Europe has declined throughout its native range. One of the main causes is low marine survival, which has been mainly attributed to overexploitation at sea, but global warming, decreased zooplankton abundance in the North Atlantic and local changes in freshwater conditions have been also blamed. It has been also pointed out that Southern populations experience the greatest decline, but information from these areas in Europe is scarce, and also in Spain there are few long-term data series related with the salmon recovery programs carried out. In this paper, we analyse a long-term data set (1992-2018) provided by a fish-trap located in the Ulla River (Galicia, NW Spain), at the southern limit of the salmon distribution range. Salmon restocking programs have been carried out in this river from 2000, and although this management technique is widely accepted, it is necessary to document the fate of those individuals to evaluate their success. Segmented regression models (or piecewise regression) are applied here for the first time to estimate the value of the explanatory variable (the so called break-point) which can indicate the potential success of the recovery program in this river. Data analysis suggest a gradual population increase to the early 1990s, then remained relatively stable until 2007 and increased thereafter due to an increase in the number of wild salmon entering the river. An increase in the frequency of appearance of 3SW salmon since 2009 was detected, both including previous spawners and individuals at its first spawning migration after three consecutive winters at sea. Data suggest an increase in number of wild salmon entering the river. Restoring river connectivity has also helped to enhance this population. We finally consider that the increase in the number of returning salmons in the Ulla River achieved over this period is due not only to the good management and conservation practices related to salmon stocking, but also to the connectivity restoration in the river and to a higher marine survival rate.

Состояние и современные риски воспроизводства атлантического лосося Salmo salar L. реки Ура (бассейн Баренцева моря)

This paper describes variations in the abundance of Atlantic salmon in the
 Ura River from 1958 to 2019. In 1993 and 1998, a decrease in the number of
 spawners with respect to the conservation limit was observed. Positive trends in
 the abundance of parr has been observed since 2002, which indicates an unsteady
 increase in the number of spawners to the precautionary biological limit. Illegal
 fishery and spreading of salmon diseases are considered to pose threats to the
 stock status of Atlantic salmon.

Hierarchical model detects decadal changes in calibration relationships of single-pass electrofishing indices of abundance of Atlantic salmon in two large Canadian catchments

Electrofishing is a commonly used technique to assess freshwater fish population abundance, and in many programs, there has been a shift in the sampling methodologies towards less laborious techniques. These new techniques usually only provide an index of abundance and require calibration with other sampling methods such as successive removal to be used for absolute abundance estimation. Using data for juvenile Atlantic salmon (Salmo salar) collected in 400 sites sampled over 21 years in two large Canadian river catchments with a single sampling protocol, we developed a hierarchical Bayesian model to account for effort, day of sampling, area of the site, and catchment effects on the relationship between the single-pass index of abundance and the fish densities, thereby illustrating the importance of carrying out a calibration exercise on a regular basis. Our work indicates that calibration relationships can change over time even with standardized sampling protocols and that these directional changes in important components of the sampling procedure can bias the estimate of population abundance and misinform the understanding of population dynamics.

A new method to estimate habitat potential for Atlantic salmon (Salmo salar): predicting the influence of dam removal on the Sélune River (France) as a case study

Abstract Maintaining connectivity in aquatic ecosystems is important to ensure adequate ecological functioning. A large dam removal project in the Sélune River (Normandy, France) would reconnect 827 km2 of catchment area to the sea. Only the downstream section of the Selune is currently available to diadromous fish, which migrate between freshwater and the marine environment. In particular, managers focus on the future potential abundance of Atlantic salmon, Salmo salar, for conservation and fishery purposes. As in stream channel habitat drives carrying capacity of juvenile salmon, salmon abundance is usually inferred from intensive and linear habitat surveys. However, this approach is neither cost-effective for large-scale surveys nor feasible for riverbed sections with low accessibility for measurement with traditional methods, e.g. dam lakes. We used well-defined relationships between gradient, hydrology and channel habitat structure to construct a simple model to estimate potential suitable habitat for juvenile salmon. Using fine-scale habitat data from nearby rivers, we parameterized a linear mixed model to estimate the area of suitable habitat based on simple physical descriptors of river characteristics. We compared our predictions to fine-scale habitat surveys on the upper Sélune. Using only slope and width, our model was able to explain 80% of the variance in suitable habitat. Estimates indicated that dam removal on the Sélune River would generate a threefold increase in suitable habitat for juveniles. This could increase the mean number of adult salmon returning to the river by 1420.9 (s.e. = 1015.5). More generally, this model provides an alternative and cost-effective tool to help better manage salmon populations in rivers impacted by dams.

Modelling climate change effects on Atlantic salmon: Implications for mitigation in regulated rivers

Climate change is expected to alter future temperature and discharge regimes of rivers. These regimes have a strong influence on the life history of most aquatic river species, and are key variables controlling the growth and survival of Atlantic salmon. This study explores how the future abundance of Atlantic salmon may be influenced by climate-induced changes in water temperature and discharge in a regulated river, and investigates how negative impacts in the future can be mitigated by applying different regulated discharge regimes during critical periods for salmon survival. A spatially explicit individual-based model was used to predict juvenile Atlantic salmon population abundance in a regulated river under a range of future water temperature and discharge scenarios (derived from climate data predicted by the Hadley Centre's Global Climate Model (GCM) HadAm3H and the Max Plank Institute's GCM ECHAM4), which were then compared with populations predicted under control scenarios representing past conditions. Parr abundance decreased in all future scenarios compared to the control scenarios due to reduced wetted areas (with the effect depending on climate scenario, GCM, and GCM spatial domain). To examine the potential for mitigation of climate change-induced reductions in wetted area, simulations were run with specific minimum discharge regimes. An increase in abundance of both parr and smolt occurred with an increase in the limit of minimum permitted discharge for three of the four GCM/GCM spatial domains examined. This study shows that, in regulated rivers with upstream storage capacity, negative effects of climate change on Atlantic salmon populations can potentially be mitigated by release of water from reservoirs during critical periods for juvenile salmon.

Local and global climatic drivers of Atlantic salmon decline in southern Europe

The abundance of Atlantic salmon is declining throughout its geographical area. Fisheries and global warming were assumed as main drivers of the decline, and recent studies suggest that habitat changes in freshwater is a third contributor. Southern populations experience the greatest decline, and face the highest risk of extinction as global warming moves its thermal niche northwards. We analysed long-term catch data (1949–2013) from a salmon fishery in northern Spain, and examined its relationship with local and global indicators of temperature and hydrological change. CPUE data, analysed by ARIMA time-series models, exhibited a significant negative trend and a marked decrease since 1973–1974, possibly triggered by overfishing at sea and a sudden outbreak of disease. Temperature increased in the same period, particularly so since 1986–1988, being negatively correlated with CPUE. A significant change in magnitude and duration of extreme water conditions occurred from 1970s onwards. Indicators of hydrological shift were also significantly correlated with CPUE of returning salmon. The best ARIMAX models indicated however, that the decrease in salmon CPUE was mainly driven by temperature trends. This indicates that both local (temperature and flow in the river) and global (ocean temperature) factors have contributed to the decrease in salmon numbers, and that temperature has played the major role. Despite a strong reduction in fishing pressure after the 1970s widespread collapse, our study population did not recover to previous abundance levels. This suggests the operation of additional factors, being climate warming and changes in food webs of the North Atlantic the most likely reasons.

Extraction of sea temperature in the Barents Sea by a scale space multiresolution method – prospects for Atlantic salmon

ABSTRACTVariation of marine temperature at different time scales is a central environmental factor in the life cycle of marine organisms, and may have particular importance for various life stages of anadromous species, for example, Atlantic salmon. To understand the salient features of temperature variation we employ scale space multiresolution analysis, that uses differences of smooths of a time series to decompose it as a sum of scale-dependent components. The number of resolved components can be determined either automatically or by exploring a map that visualizes the structure of the time series. The statistical credibility of the features of the components is established with Bayesian inference. The method was applied to analyze a marine temperature time series measured from the Barents Sea and its correlation with the abundance of Atlantic salmon in three Barents Sea rivers. Besides the annual seasonal variation and a linear trend, the method revealed mid time-scale (∼10 years) and long time-scale (∼30 years) variation. The 10-year quasi-cyclical component of the temperature time series appears to be connected with a similar feature in Atlantic salmon abundance. These findings can provide information about the environmental factors affecting seasonal and periodic variation in survival and migrations of Atlantic salmon and other migratory fish.

Making sense of the relationships between Ne, Nb and Nc towards defining conservation thresholds in Atlantic salmon (Salmo salar).

Effective population size over a generation (Ne) or over a reproductive cycle (Nb) and the adult census size (Nc) are important parameters in both conservation and evolutionary biology. Ne provides information regarding the rate of loss of genetic diversity and can be tracked back in time to infer demographic history of populations, whereas Nb may often be more easily quantified than Nc for short-term abundance monitoring. In this study, we propose (1) an empirical context to Waples et al. (2014) who introduced a correction to bias due to overlapping generations, and (2) a mathematical relationship between Ne and Nb for direct application in Atlantic salmon populations in Québec, Canada. To achieve this, we investigate the relationships between Ne, Nb and Nc in 10 Atlantic salmon populations, Canada, for which we genotyped 100 randomly sampled young-of-the year individuals for 5 consecutive years. The results show a positive correlation between Ne, Nb and Nc, suggesting that Nb is an indicative parameter for tracking effective population size and abundance of Atlantic salmon. However, our model allows predicting Nc from Nb values at 27% that can be partly explained by high variance in Nb/Nc both among populations (37%) and among years (19%). This result illustrates the need for thorough calibration of Nb/Nc before using Nb in monitoring programs, as well as a full understanding of the limits of such an approach. Finally, we discuss the importance of these results for the management of wild populations.

Fecundity and water flow influence the dynamics of Atlantic salmon

AbstractPopulations are retained at reduced levels by resource competition and environmental stochasticity. In the Norwegian River Imsa, the relationship between fecundity of Atlantic salmon (Salmo salar) spawners and number of smolts per unit river area was investigated for cohorts spawned from 1976 to 2011. Annual number of smolts produced per unit area was best described by a multiplicative model and increased with the fecundity of the females as proxy for number of eggs deposited and the minimum water flow in August towards the end of the first growth season. Mean monthly water temperature, or water flow in any other month during the first year, had no significant effect on number of smolts produced. At sea, there was an almost linear relationship between number of emigrating smolts and returning adults, possibly because population abundance of Atlantic salmon is low relative to the carrying capacity in the ocean. Thus, both number of eggs spawned and minimum water flow in late summer influenced population abundance in the present river.

Long-term response of salmonid populations to habitat restoration in a boreal forest stream

Assessing the sustainability of restoration measures for salmonid populations and their habitat is limited due to a lack of long-term evaluations. In this paper we report on a study to assess the effect of boulder clusters, V-dams and half-log covers on stream habitat and population abundance of Atlantic salmon and brook trout two decades after installation. Structures were installed in Joe Farrell’s Brook, Newfoundland Canada in 1993 and fish population and habitat parameters were initially measured annually from 1993 to 1995. All stream sites were re-sampled in 2014. Boulder clusters or V-dams remained intact, stable and functional. By contrast, only 60% of the half-logs were in place and those remaining were in relatively poor shape with limited functionality. Boulder clusters increased the percentage area of pool habitat (p=0.05) and the availability of instream cover (p=0.04). V-dams did not significantly alter any habitat component after 20 years. Half-log covers were effective in increasing instream cover (p=0.02) and substrate coarseness (p=0.01). Density and biomass of Atlantic salmon increased rapidly after structures were installed and remained significantly higher 20 years after the pre-treatment period (1993) than the sub-basin control. By contrast brook trout did not show significant increase in density or biomass in either V-dams or half-log covers but did showed significant increases in both boulder cluster (density p=0.002; biomass p=0.01) 20 years later. This study suggests that instream structures placed in small boreal streams can function for more than two decades when properly installed and can maintain higher levels of salmonid abundance when habitat is limiting.

Change in relative abundance of Atlantic salmon and Arctic charr in Veidnes River, Northern Norway: a possible effect of climate change?

Temperature changes affect salmonids across a hierarchy of spatial and temporal scales, so that shifts in thermal river regimes may influence interspecific interactions in sympatric species. In Northern Norway, anadromous populations of Arctic charr (Salvelinus alpinus) and Atlantic salmon (Salmo salar) coexist, but in recent years landings of Arctic charr have decreased, while those of Atlantic salmon have been stable or even increased. Here we studied relative abundance, habitat use and growth rate of sympatric stream-living juveniles of both species in Veidnes River, where they are the only fish species present. In 1998/2000, juvenile Arctic charr dominated, especially in the upper and colder part of the river. In 2010, however, Atlantic salmon juveniles were now prominent in all habitat types, whereas nearly all Arctic charr were captured in slow-flowing water near the river bank. Summer air temperature has increased in the region during the last decade. Positive correlations between summer temperatures and back-calculated growth rates were documented in both species, but the growth response was significantly higher in Atlantic salmon. Accordingly, we suggest that juvenile Atlantic salmon may benefit from a warmer climate in northernmost Norway, at the expense of the more cold-water-adapted Arctic charr.