Natural mortality of adult Atlantic salmon (Salmo salar) in freshwater prior to spawning can influence spawning biomass and population productivity yet has rarely been quantified independently from mortality related to fishing. A systematic review of telemetry studies was conducted to estimate freshwater mortality rates of pre-spawning adults, excluding mortality directly attributable to fishing. From a Web of Science search (30. Sept. 2025) giving 541 records, 34 studies from 29 rivers were retained after screening, resulting in a total of 37 river- annualised survival rate specific estimates. For each record instantaneous mortality rates (Z) and corresponding mean annualised survival was calculated, accounting for uncertain fates and excluding mortalities occurring within 72 h of tagging or release. Freshwater mortality rates were generally low (mean Z = 0.43, mean annualised survival =0.80), but varied strongly between studies. A majority (n = 21) had no mortality or minimum estimates of Z that include 0. High mortality rates could in some studies be associated with high temperature or predation, with a sharp increase in mortality in studies with temperatures above 21°C. Freshwater survival was consistently higher than ocean survival of adult kelts from the same regions. Overall, freshwater mortality is typically low but can have a substantial impact on populations during thermal stress or high predation, highlighting the potential need for thermal risk informed management with continued climate warming.

Size at maturity is an important demographic characteristic due to its effect on fecundity. Synchrony in size at maturity across populations suggests similar environmental conditions, mainly through shared feeding grounds. We study such synchrony over 30 years across 187 populations using data from about 200 000 individual Atlantic salmon belonging to four geographic regions in Norway. Synchrony was high among the three southern regions, while the northernmost region had more independent fluctuations. Synchrony among closely separated populations was high, declined with distance, but was still present more than 1000 km apart. In all regions, the general pattern was that size at age was largest in the late 1980s and around 2000 and smallest between 2005 and 2010. Fulton's condition factor was consistently largest in the North region. Overall, our results are consistent with the known marine distribution, including that the northernmost populations have a reduced migration distance to feeding areas and forage more northeast than the southern populations. The observed synchrony in size at maturity is anticipated to have large-scale demographic consequences by generating synchrony in egg deposition.

Large main stem rivers typically provide a variety of mesohabitat types, but their abundance, distribution and role in providing habitat for juvenile salmonids have rarely been analysed. The quantity of meso-scale habitats and their juvenile Atlantic salmon abundance was surveyed in the large sub-arctic River Teno in northernmost Fennoscandia. Different habitat types were mapped using the Norwegian Mesohabitat Classification Method (NMCM). Juvenile salmon abundance in different mesohabitats, on various substrates, depths and lateral positions across the channel was estimated by electrofishing boat surveys. Most of the Teno main stem (67%) was dominated by fine substrate and > 50 cm s-1 flow velocity. Typical juvenile salmon-rearing habitats, such as rapids, riffles and glides, comprised 32% of the riverbed. Data from NMCM and electrofishing surveys were used in generalised linear models to describe the relationship between habitat variables and juvenile salmon (YOY and parr) abundance. Juvenile abundance varied a lot across and within mesohabitats; in general, abundance was higher in areas with depth <= 70 cm than in deeper areas, and in middle sections of the river compared to those near shorelines. Juvenile salmon were documented also in sandy areas, although in low abundances, and in relatively high abundances in areas deeper than typically considered important for juvenile salmon. These areas should be considered in assessing habitat potential for juvenile salmon production. NMCM proved to be a fast, cost-effective method for surveying large areas for habitat assessment.

Atlantic salmon Salmo salar is an anadromous species that exhibits a wide range of life history strategies. One of the least understood and documented strategies involves adult salmon ascending rivers in the autumn or early winter and remaining there without feeding for a full year before spawning. This study provides the first scientific documentation of year-early migratory salmon in Norway. About 5% of Atlantic salmon captured during tagging studies in the river Alta arrived a year prior to spawning. Tracking of 8 tagged fish in the river Alta showed that they entered the river a year prior to spawning and overwintered in the river after spawning. The longest recorded stay in freshwater was 21 mo (640 d). Scale analysis revealed that year-early migrants had smaller sizes at age during their ocean years but entered rivers with higher relative energy reserves. Genetic analysis showed that the year-early migrating fish belong to the river Alta and were not strayers from Russian populations. The factors shaping this strategy are still unclear but may involve balancing the loss of ocean growth and the costs of extended river residency against the advantage of escaping high ocean mortality rates.

A new single nucleotide polymorphism (SNP) panel for genetic stock identification in the Teno river Atlantic salmon, Salmo salar L., fishery was developed, with a view to improving on an existing microsatellite panel. Twenty-two genetically differentiated reporting units were proposed based on population genetic analyses of 1212 individuals collected at 37 locations in the river and genotyped for >33,000 genome-wide SNPs. A small subset of these SNPs was selected for genetic stock identification (GSI) using an iterative process that considered their diversity and differentiation across reporting units. A genotyping-by-sequencing assay was developed to simultaneously genotype 180 of these GSI SNPs plus a sexing marker. This new SNP panel showed comparative GSI power to the microsatellite panel, with anticipated improvements in terms of cost, speed and robustness, and transferability across laboratories and genotyping platforms. Mixed-stock analysis of the 2018 Teno river salmon catch using the new panel inferred that all 22 reporting units contributed to the fishery. Estimated catch proportions positively scaled with an independent estimate of reporting unit productivity: target spawning female biomass. This demonstrates the usefulness and efficiency of the 180 SNP panel for Atlantic salmon GSI in the Teno river system. If conducted on a regular basis, GSI can enable fine-tuning of management strategies to promote sustainable fishing.

In many populations of Atlantic salmon, individuals return to rivers months or even a year prior to spawning. This premature arrival in fresh water by far exceeds the time needed for upstream migrations to spawning areas. Leaving the ocean early means forgoing substantial growth and incurring high metabolic costs of fasting in freshwater. Differences in timing of return migration has been shown to have a genetic basis, and different sea age classes return to rivers at different times. Previously suggested explanations regarding the cause for this behaviour have included both environmental factors and life history trade-offs, but these hypotheses have not been tested. Here, we combine data on marine growth and mortality of Atlantic salmon with the calculations of energetic expenditures of fasting in freshwater to explore how timing of river entry affects the probability of survival and energy recourses available for spawning. Our results show that to maximize the product of survival until spawning and energy available at spawning, multi sea winter fish in general must enter rivers earlier than one sea winter fish. And that for a large range of sea survival rates, the optimum river entry date for multi sea winter fish is as early as possible. This strongly suggests that the phenomena of premature arrival in Atlantic salmon is adaptive behaviour.

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-quinone) is a widespread contaminant of emerging concern resulting from oxidation of 6PPD, which is an antidegradant substance added to tires. The recent identification of 6PPD-quinone as the cause of acute mortality in coho salmon has quickly motivated studies on 6PPD-quinone toxicity to other species. Subsequent findings have shown that 6PPD-quinone toxicity is highly species specific. Closely related species can differ widely in response to 6PPD-quinone from extremely sensitive to tolerant. Hence toxicity testing is currently the only way to establish whether a species exhibits 6PPD-quinone toxicity. We investigated the acute toxicity of 6PPD-quinone in pink salmon alevins (sac fry). This species has is the only Pacific salmon that so far has not been tested for 6PPD-quinone sensitivity. Fish were exposed in static water in eight treatments with initial concentrations ranging from 0.1 to 12.8 μg/L. Fish were observed for 48 h, and changes in concentrations of 6PPD-quinone were monitored throughout the experiment. No mortalities or substantial changes in behavior were recorded. Environ Toxicol Chem 2024;43:1332-1338. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Using data from 309 localities with freshwater pearl mussel, Margaritifera margaritifera, in Norway, we analyzed the effect of several landscape and environmental variables on this species’ recruitment. Median summer air temperature and, to a lesser extent, percentage wetland cover in the catchment were the strongest (negative) influences on recruitment. This is the first study to show an effect of air temperature on recruitment across a wide range of populations. The results are of importance for the conservation of the species, particularly in the context of climate change.

Hydropeaking (the release of water pulses at hydropower plants) results in temporary reductions in river channel water-covered area downstream, which may cause fish mortality through stranding. We used a mechanistic modelling approach to examine how both the form of the hydropeaking cycle and the characteristics of the affected fish control how hydropeaking may cause stranding mortality of fish. We modelled the response of Atlantic parr to hydropeaking in a regulated watercourse in central Norway (the River Nidelva) using an individual-based population model designed explicitly to examine fish behaviour and stranding mortality during hydropeaking. A response to hydropeaking, involving migration from the riverbanks towards the mid-channel on down-ramping and a return to the riverbanks on up-ramping, was based on individuals being parameterized to migrate to habitat properties that spatially changed throughout the hydropeaking cycle. We found that stranding mortality was strongly dependent on both the form of the hydropeaking cycle and on the fish response. Total stranding mortality was more dependent on the down-ramping speed than the duration of the minimum flow period. Total stranding mortality was greatest when there was a low movement speed, leading to individuals being stranded, combined with a high probability of dying per unit of time when stranded. Given the sensitivity of mortality to the fish response and the lack of detailed field studies quantifying this, we conclude that this area requires further controlled studies for parameterizing models used to predict effects of hydropeaking on fish. Simulation-based modelling, where there is a 100% control of and knowledge of fish characteristics, where a range of deterministic functions can be evaluated and in which boundary conditions can be easily controlled, may be a useful research tool to complement empirical studies.

The chemical 6PPD-quinone is highly toxic to some fish species of the Oncorhynchus and Salvelinus genera and is the oxidation product of the common car tire additive 6PPD. We present a new sample preparation method that involves liquid-liquid extraction of water samples followed by silica-based solid phase extraction prior to LC–MS/MS analysis. The new sample preparation method showed good analyte recovery from spiked water samples (78%–91%) and a low ion suppression effect, surpassing previously published methods. This new method was successfully validated, achieving a limit of quantification of 5 ng/L and estimated expanded measurement uncertainty of 18.6%. In a proof-of-concept study, the method was applied to several water samples from various sources in Southern Norway. These were runoff samples from tunnel washing, from a tunnel runoff treatment plant and downstream of the plant drain. In addition, two water samples from puddles were included: one was run-off from an artificial soccer turf field and one from a puddle on a country road. The results of the analyses revealed that the concentration of 6PPD-quinone was above the LC50 reported for coho salmon ( Oncorhynchus kisutch ) in all samples except the samples from and downstream of the treatment plant. The highest measured concentration was 258 ng/L, which is the 2.7-fold of the reported LC50 in coho salmon (95 ng/L). Our initial data emphasize the need for more comprehensive environmental monitoring of 6PPD-quinone as well as toxicological studies in aquatic organisms.

Recent identification of 6PPD-quinone as the chemical causing acute toxicity in coho salmon has led to substantial concern regarding the toxicity of this contaminant for other aquatic species. Environmental occurrence of 6PPD-quinone is probably high, because it is an oxidation product of a common tire rubber additive. Research on 6PPD-quinone toxicity in fish has revealed a rather unusual pattern, with closely related species exhibiting responses ranging from extreme sensitivity to no effect. Of 11 previously studied fish species, 6PPD-quinone was toxic to four. The species-specific toxicity of 6PPD-quinone complicates urgently needed environmental risk assessment. We investigated the acute toxicity of 6PPD-quinone in Atlantic salmon and brown trout alevins (sac fry). These species have previously not been tested for sensitivity to 6PPD-quinone. The fish were exposed in static conditions in eight treatments with initial concentrations ranging from 0.095 to 12.16 µg/L. Fish were observed for 48 h, and changes in concentrations of 6PPD-quinone were monitored throughout the experiment. No mortalities or substantial changes in behavior were recorded in either Atlantic salmon or brown trout. This provides an important first step in assessing effects of 6PPD-quinone on these economically and culturally highly important species. Environ Toxicol Chem 2022;41:3041-3045. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Although effects of acidification on salmonid fish are well studied and documented, effects of episodic high pH have rarely received attention. In the present study, we investigated effects of high-pH events on Atlantic salmon (Salmo salar) using both field and laboratory data. Effects of an episodic high-pH event on juvenile densities in a Norwegian river were studied using data from several electrofishing surveys conducted both before and after the event. Effects of high pH on survival of eggs were studied by exposing eggs to a range of high-pH treatments for different durations. Juvenile densities from the field study showed that the high-pH event had little or no effect on the cohort that had been exposed to pH 9.7-10.3 during the egg stage. This finding was in accordance with the laboratory experiment that showed no excess mortality on eggs until pH was >12. The high-pH event occurred in March during low winter flows, and densities of older juveniles in May were significantly lower in the affected area compared to controls upstream. In June and September the difference was not significant, but there was a clear spatial trend indicating that the event had a negative effect on densities of older juvenile salmon. Environ Toxicol Chem 2022;41:771-780. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Marine aquaculture of Atlantic salmon (Salmo salar) is a relatively new industry where breeding programs have led to rapid genetic change in the captive populations that were built up alongside conspecific wild individuals. Throughout its 50-years history, marine aquaculture of Atlantic salmon has been associated with escapes, and studies have shown that escapees may enter rivers, spawn successfully, and this may lead to farmed-to-wild genetic introgression and maladaptation in wild populations. Yet, an open question is what factors can best explain the variability in the proportion of farmed escapees in wild populations, and when present, which additional factors lead to introgression. Here, we combine two large-scale data sets from monitoring escaped farmed salmon and introgression in Norwegian rivers between 2006 and 2018 to model how anthropogenic, environmental, and population factors influence proportion of escapees and level of introgression. We found that increasing farming intensity and river discharge increase the expected proportions of escaped farmed salmon in rivers, whereas a larger wild salmon population size reduces the expected proportion of escapees despite increasing the expected absolute numbers of escaped farmed salmon. On a large scale, introgression is primarily a function of proportions of escaped farmed salmon, and only to a minor extent a function of local environmental factors or salmon population characteristics. This suggests that as long as salmon aquaculture is based on technologies where non-sterile fish can escape, all anadromous wild Atlantic salmon populations are at risk. Large marine protected areas without salmon aquaculture may slow down the rate of intrusion and introgression by increasing the distance between intensive aquaculture and wild populations.

Sedimentation is one of the main eco-morphological and technological challenges associated with reservoirs. Sedimentation not only reduces the functional capacity of a reservoir by filling it, but also changes downstream sediment dynamics and habitat availability for the aquatic biota. Additionally, dams hinder free bi-directional fish passage, emerging as a major threat to species of migratory fish. In the past decades, mitigation measures aimed at reducing such environmental and technological impacts have been developed. Sediment bypass tunnels (SBTs) have been shown to successfully help prevent reservoir sedimentation, whereas fish passages have been found to be potential solutions to facilitate bi-directional passage of fish. However, the construction of such structures, in particular of SBT, can be extremely costly. The development of design solutions that can function both for downstream sediment transport and up- and downstream fish passage should be considered as they can mitigate ecological deficiencies of reservoir operations while accounting for economic feasibility. Possibilities and challenges of combining SBT and fish passage were explored by bringing together a team of interdisciplinary specialists on hydraulics, sediment transport and continuity, bypassing, hydraulic structures, hydropower engineering, aquatic biology, and fish passage in a two-day workshop. Here, we present potential solutions identified during the workshop for integrating SBT and fish passage.

Otoliths are inner-ear structures of all teleost fish with functional importance for hearing and balance. The otoliths usually consist of aragonite, a polymorph of calcium carbonate, but may also take the form partly or entirely of vaterite, a different polymorph of calcium carbonate. Vateritic otoliths occur sporadically in wild fish, but with a higher frequency in hatchery-reared fish. Abnormal otoliths have direct consequences for the inner-ear functions of fish and may be a symptom of environmental stress. In this study, the authors assess the differences in the frequency of abnormal otoliths and degree of abnormality (% vaterite) for different groups of hatchery-reared Atlantic salmon (Salmo salar) smolt and adults. The groups differed in parental broodstock origin (number of generations in hatchery) and treatment temperature. Smolt from the same groups were also released to complete their ocean migration. The otoliths of the returning and recaptured adults were subsequently extracted to assess the difference in frequency and degree of abnormality between the adults and the smolt from corresponding groups. Return rate varied among groups (0.2%-2.6%). The frequency of vateritic otoliths was high (11.4%-64.4%) and differed among smolt groups. The lowest return rates corresponded with the highest frequency of abnormal otoliths for the groups, suggesting that abnormal otoliths may have negative consequences for marine survival. Furthermore, indications of an effect of fast growth on the formation of abnormal otoliths were found for only one of the experimental groups, and for none of the groups after correcting for Type 1 error. This contradicts previous reports, suggesting rapid growth as the main cause of abnormal otoliths. Adult return rates were generally low, but abnormal otoliths were common, with high coverage (% vaterite).

Traditional tagging methods for fishes can have issues relating to both animal welfare and economic costs. Biometric data such as iris patterns can be captured via digital cameras, which allows for non-invasive tagging and inexpensive and rapid analysis. The purpose of this study was to investigate if the iris of Atlantic Salmon (Salmo salar) is a suitable biometric template for long-term identification of individuals. Atlantic Salmon were individually tagged in the body cavity using PIT tags at the juvenile pre-smolt stage, and the left eye was photographed six times over a 533-day period. Changes in iris stability were assessed both qualitatively and using iris-recognition software. Identification of individual Atlantic Salmon using the iris was not successful over the entire period, as the iris pattern changed significantly with time. Over a shorter time period (four months) with frequent samplings, iris software was able to correctly identify individual fish. The results show that iris identification has potential to replace other methods for Atlantic Salmon over short timeframes.