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Evolution of phenotypic plasticity requires genotype–environment interaction. The discovery of two large‐effect loci in the vgll3 and six6 genomic regions associated with the number of years the Atlantic salmon spend feeding at sea before maturation (sea age), provides a unique opportunity to study evolutionary potential of phenotypic plasticity. Using data on 1246 Atlantic salmon caught in the River Surna in Norway, we show that variation in mean sea age among years (smolt cohorts 2013–2018) is influenced by genotype frequencies as well as interaction effects between genotype and year. Genotype–year interactions suggest that genotypes may differ in their response to environmental variation across years, implying genetic variation in phenotypic plasticity. Our results also imply that plasticity in sea age will evolve as an indirect response to selection on mean sea age due to a shared genetic basis. Furthermore, we demonstrate differences between years in the additive and dominance functional genetic effects of vgll3 and six6 on sea age, suggesting that evolutionary responses will vary across environments. Considering the importance of age at maturity for survival and reproduction, genotype–environment interactions likely play an important role in local adaptation and population demography in Atlantic salmon. [ABSTRACT FROM AUTHOR]

The state of sea trout in 1251 Norwegian watercourses was assessed based on a scoring system for human pressures, abundance data, and local knowledge.Over 16,000 km of rivers and lakes were available to sea trout in these watercourses, spanning from the temperate to Arctic regions.Sea trout were classified to be in a good or very good state in fewer than 25% of the watercourses and in a poor or very poor state in almost 40%. Twenty‐nine watercourses had lost their sea trout populations.Salmon lice from aquaculture salmon farms had by far the largest adverse effect on sea trout among the human impact factors, both in the number of watercourses (83%) and river area affected (60%), and the total effect on sea trout abundance.Agriculture and hydropower production also had strong adverse impacts (35% and 19% of watercourses), but substantially lower than that caused by salmon lice. Culverts related to road crossings and other habitat alterations also had impacts on sea trout in many watercourses (27%).Exploitation of sea trout has been reduced in Norway in recent years, both in the marine and freshwater fisheries. Yet, the exploitation pressure was moderate or high in almost 14% of the watercourses where the state of sea trout was poor or very poor, suggesting a high potential for overexploitation in these.The state of sea trout was best in the northern sparsely populated areas. However, distribution of watercourses with sea trout in a poor or very poor state was more linked to aquaculture, agriculture, and hydropower production than human population density.The developed approach for large‐scale mapping of state and pressures, which is vital for prioritizing management measures, may inspire other nations in their conservation effort for this important species. [ABSTRACT FROM AUTHOR]

Given the limited information on prey use during the marine residency period for Atlantic salmon, scales were collected from salmon at return to the River Namsen (Norway) for spawning after 1 year at sea, and scale material from the first and second summer marine feeding periods was analysed using stable isotope methods to understand dynamics of their trophic ecology. As the salmon increased in size from the first to second summer, they reduced their feeding niche and specialised more (narrowed the δ13C range) and increased their dependency on higher tropic level (δ15N) prey, likely fish. Changes in δ13C indicated a consistent pattern of movement towards the north and west between summer feeding periods. Hence, salmon during their first year at sea may have a migration route roughly resembling that of previous spawners, as inferred from earlier tagging studies. Feeding conditions and nutrient composition during the last summer at sea, i.e. in the months before returning to the river for spawning, impacted final body size and within‐season timing of return. Fish undergoing the largest trophic niche shift (δ13C and δ15N combined) between summer feeding periods, returned earliest. The earliest returning fish had the fastest specific growth rates at sea. Hence, salmon encountering abundant high‐quality fish food during the marine migration, particularly during the last months, may reach a size and energetic state whereby it is better to return early to a safer environment in freshwater than risk being eaten by a big predator at sea. Both trophic status (δ15N), resource use (δ13C) and growth rates were significantly correlated between feeding periods. Nutrient composition during the first summer at sea did not impact the fish body length after the following winter, but growth conditions during the first summer evidenced carry‐over effects from the first to the second summer of feeding. [ABSTRACT FROM AUTHOR]

The timing of seaward migration is a key life‐history trait for many anadromous fish species, with growth and survival at sea depending on a match/mismatch scenario between the timing of the sea entry and optimal conditions. Based on a 25‐year study with 15,226 individually tagged brown trout (Salmo trutta) in a Norwegian river, we analysed how the within‐season timing of sea migration impacted growth and survival. In both first‐time and veteran migrants, marine growth was highest for early migrating individuals, large individuals, and those with a low condition factor when entering the sea. Survival was highest for individuals entering the sea early in the season. In first‐time migrants, survival increased with body length. Survival also increased with the number of other smolts migrating simultaneously. As the early smolts were the most successful, it may seem strange that many smolts migrate later in the season. We suggest that late‐migrating smolts may not be of a size and/or physiological state suitable for early marine conditions, and may make the best of a bad situation. [ABSTRACT FROM AUTHOR]

Wild Atlantic salmon populations have declined in many regions and are affected by diverse natural and anthropogenic factors. To facilitate management guidelines, precise knowledge of mechanisms driving population changes in demographics and life history traits is needed.Our analyses were conducted on (a) age and growth data from scales of salmon caught by angling in the river Etneelva, Norway, covering smolt year classes from 1980 to 2018, (b) extensive sampling of the whole spawning run in the fish trap from 2013 onwards, and (c) time series of sea surface temperature, zooplankton biomass, and salmon lice infestation intensity.Marine growth during the first year at sea displayed a distinct stepwise decline across the four decades. Simultaneously, the population shifted from predominantly 1SW to 2SW salmon, and the proportion of repeat spawners increased from 3 to 7%. The latter observation is most evident in females and likely due to decreased marine exploitation. Female repeat spawners tended to be less catchable than males by anglers.Depending on the time period analyzed, marine growth rate during the first year at sea was both positively and negatively associated with sea surface temperature. Zooplankton biomass was positively associated with growth, while salmon lice infestation intensity was negatively associated with growth.Collectively, these results are likely to be linked with both changes in oceanic conditions and harvest regimes. Our conflicting results regarding the influence of sea surface temperature on marine growth are likely to be caused by long‐term increases in temperature, which may have triggered (or coincided with) ecosystem shifts creating generally poorer growth conditions over time, but within shorter datasets warmer years gave generally higher growth. We encourage management authorities to expand the use of permanently monitored reference rivers with complete trapping facilities, like the river Etneelva, generating valuable long‐term data for future analyses. [ABSTRACT FROM AUTHOR]

Invasive pink salmon (Oncorhynchus gorbuscha) has been present in variable, but low, numbers in Norwegian waters since c. 1960, but beginning in 2017 their numbers have exploded in rivers in northern Norway, with considerable numbers also recorded in rivers in southern Norway and other countries bordering the North Atlantic. Analysis of pink salmon scales from two rivers draining to the western Barents Sea showed declining growth during the first weeks after entering the sea, and some individuals even showed a pronounced growth arrest, based on detailed scale circulus analyses. This was followed by a period of growth increase and stability during late summer and autumn, which may reflect a transition to better food sources, as the fish migrate from coastal waters to the open ocean, and as they grow larger and can eat larger and more energy efficient food items. Growth declined to a minimum during winter. Fish body size at spawning was positively correlated with the distance from scale focus to the last winter circulus, as well as with the number of circuli. When dividing scale growth into three periods, better growth during the first period at sea was related to increased fish body length at spawning, but this early growth explained only a minor part (6%) of the variation in final body length. The reason for this may be large individual variation in growth combined with large mortality during the first weeks at sea. If mortality is selective, removing fish with poor growth may reduce a correlation between early growth and body size at spawning. Scale growth during late summer and early autumn explained more of the variation in fish length at spawning (27%). Hence, late summer and early autumn was likely an important period for marine growth and survival in the invasive pink salmon. [ABSTRACT FROM AUTHOR]

Genetic methods for the estimation of population size can be powerful alternatives to conventional methods. Close‐kin mark–recapture (CKMR) is based on the principles of conventional mark–recapture, but instead of being physically marked, individuals are marked through their close kin. The aim of this study was to evaluate the potential of CKMR for the estimation of spawner abundance in Atlantic salmon and how age, sex, spatial, and temporal sampling bias may affect CKMR estimates. Spawner abundance in a wild population was estimated from genetic samples of adults returning in 2018 and of their potential offspring collected in 2019. Adult samples were obtained in two ways. First, adults were sampled and released alive in the breeding habitat during spawning surveys. Second, genetic samples were collected from out‐migrating smolts PIT‐tagged in 2017 and registered when returning as adults in 2018. CKMR estimates based on adult samples collected during spawning surveys were somewhat higher than conventional counts. Uncertainty was small (CV < 0.15), due to the detection of a high number of parent–offspring pairs. Sampling of adults was age‐ and size‐biased and correction for those biases resulted in moderate changes in the CKMR estimate. Juvenile dispersal was limited, but spatially balanced sampling of adults rendered CKMR estimates robust to spatially biased sampling of juveniles. CKMR estimates based on returning PIT‐tagged adults were approximately twice as high as estimates based on samples collected during spawning surveys. We suggest that estimates based on PIT‐tagged fish reflect the total abundance of adults entering the river, while estimates based on samples collected during spawning surveys reflect the abundance of adults present in the breeding habitat at the time of spawning. Our study showed that CKMR can be used to estimate spawner abundance in Atlantic salmon, with a moderate sampling effort, but a carefully designed sampling regime is required. [ABSTRACT FROM AUTHOR]

Consistent individual differences in behavior have been demonstrated for many animals, but there are few studies of consequences of such repeated behavior in the wild. We tested consistency in migration timing to and from the sea among anadromous Arctic char (Salvelinus alpinus) and brown trout (Salmo trutta), using data from a study period of about 25 years, including more than 27,000 uniquely Carlin‐tagged individuals that migrated to sea for feeding in the spring and returned to the river in late summer for up to 13 successive years. Consistency was found between individuals across time in timing of the seaward migration. Individuals migrating early during their first migration tended to migrate early the following years, and late migrants tended to migrate late. The same pattern was found also at ascent to freshwater. Hence, this study demonstrated that individual fish in nature can differ in behavior related to migration timing and that these differences can be consistent during their lifetime. Early migrants increased their mass more than late migrants and had a higher specific growth rate. Early migrating Arctic char, but not brown trout, experienced a longer life after the first migration to sea than late migrants. In both species, maturity occurred earlier in individuals that migrated early. For brown trout, but not for Arctic char, fecundity was significantly correlated to the timing of smolt migration. Hence, the repeatable individual variation in migration timing seemed to have ecological and fitness consequences in terms of growth, longevity, timing of maturity, and lifetime fecundity. [ABSTRACT FROM AUTHOR]

Escapes of domesticated fish from aquaculture, followed by interbreeding with wild conspecifics, represent a threat to the genetic integrity and evolutionary trajectory of natural populations. Approximately fifty years of Atlantic salmon production has left an unprecedented legacy of widespread introgression of domesticated escapees in wild Norwegian populations. A major question, however, is whether current aquaculture practice will lead to additional introgression in the near future. As part of the updated Norwegian risk assessment of fish farming, we conducted a risk assessment for further introgression of domesticated escapees in wild populations in Norway. Extensive data of reported numbers of escapees, observed proportions of escapees in rivers, removal of escapees pre‐spawning, and the resilience of wild populations through demographic and genetic status informed the risk assessment. The analysis revealed that rivers in 10 of the 13 aquaculture production zones covering Norway display a moderate or high risk of further introgression of domesticated escapees. This comes in addition to widespread introgression that is already documented. We therefore conclude that so long as aquaculture production continues at its present level and form, there is a moderate‐to‐high risk of further introgression of domesticated salmon in many native populations throughout much of Norway. [ABSTRACT FROM AUTHOR]

The proportion of angled Atlantic salmon Salmo salar L. being caught and released has increased. If individuals are repeatedly captured, this may have fish welfare consequences. Of 995 Atlantic salmon tagged during catch and release in eight Norwegian rivers, 10% were captured twice, while 3% were captured three times within the same fishing season. The probability that released salmon were captured again decreased with decreasing time left of the fishing season, decreased for larger‐sized fish and varied among rivers/years. Increased exploitation rates within the river, indicating an increased fishing pressure, strongly increased the probability that fish would be recaptured. However, the proportion of salmon caught a second time was much lower than the total exploitation rates in the same rivers (which was on average 46%). For fish tagged in the sea, the likelihood of being angled decreased with time since entering the river, which may explain why the recapture rates of caught and released fish were lower than the total exploitation rates. [ABSTRACT FROM AUTHOR]

We assembled the great snipe blood transcriptome using data from fourteen lekking males, in order to de novo identify candidate genes related to sexual selection, and determined the expression profiles in relation to mating success. The three most highly transcribed genes were encoding different haemoglobin subunits. All tended to be overexpressed in males with high mating success. We also called single nucleotide polymorphisms ( SNPs) from the transcriptome data and found considerable genetic variation for many genes expressed during lekking. Among these, we identified 14 polymorphic candidate SNPs that had a significant genotypic association with mating success (number of females mated with) and/or mating status (mated or not). Four of the candidate SNPs were found in HBAA (encoding the haemoglobin α-chain). Heterozygotes for one of these and one SNP in the gene PABPC1 appeared to enjoy higher mating success compared to males homozygous for either of the alleles. In a larger data set of individuals, we genotyped 38 of the identified SNPs but found low support for consistent selection as only one of the zygosities of previously identified candidate SNPs and none of their genotypes were associated with mating status. However, candidate SNPs generally showed lower levels of spatial genetic structure compared to noncandidate markers. We also scored the prevalence of avian malaria in a subsample of birds. Males infected with avian malaria parasites had lower mating success in the year of sampling than noninfected males. Parasite infection and its interaction with specific genes may thus affect performance on the lek. [ABSTRACT FROM AUTHOR]

1. During the last centuries, the breeding range of the great snipe Gallinago media has declined dramatically in the western part of its distribution. To examine present population dynamics in the Scandinavian mountains, we collected and analysed a 19-year time series of counts of great snipe males at leks in central Norway, 1987–2005. 2. The population showed large annual fluctuations in the number of males displaying at lek sites (range 45–90 males at the peak of the mating season), but no overall trend. 3. We detected presence of direct density-dependent mechanisms regulating this population. Inclusion of the density-dependent term in a Ricker-type model significantly improved the fit with observed data (evaluated with Parametric Bootstrap Likelihood Ratio tests and Akaike's Information Criterion for small sample size). 4. An analysis of (a number of a priori likely) environmental covariates suggests that the population dynamics were affected by conditions influencing reproduction and survival of offspring during the summer, but not by conditions influencing survival at the wintering grounds in Africa. This is in contrast to many altricial birds breeding in the northern hemisphere, and supports the idea that population dynamics of migratory nidifugous birds are more influenced by conditions during reproduction. 5. Inclusion of these external factors into our model improved the detectability of density dependence. This illustrates that allowing for external effects may increase statistical power of density dependence tests and thus be of particular importance in relatively short time series. 6. In our best model of the population dynamics, two likely density-independent offspring survival covariates explained 47·3% of the variance in great snipe numbers (predation pressure estimated by willow grouse reproductive success and food availability estimated by the amount of precipitation in June), whereas density dependence explained 35·5%. Demographic stochasticity and unidentified environmental stochasticity may account for the remaining 17·2%. [ABSTRACT FROM AUTHOR]

The genes of the major histocompatibility complex (MHC) code for proteins involved in antigen recognition and triggering of the adaptive immune response, and are therefore likely to be under selection from parasites. These selection regimes may vary in space and time. Here we report a strong geographical structure in MHC class II B genes of a migrating bird, the great snipe ( Gallinago media). Genetic differentiation in the MHC between two ecologically distinct distributional regions (Scandinavian mountain populations vs. East European lowland populations) was still present after statistically controlling for the effect of selectively neutral variation (microsatellites) using partial Mantel tests. This suggests a role for selection in generating this spatial structure and that it represents local adaptation to different environments. Differentiation between populations within the two regions was negligible. Overall, we found a high number of MHC alleles (50, from 175 individuals). This, together with a tendency for a higher rate of nonsynonymous than synonymous substitutions in the peptide binding sites, and high Tajima's D in certain regions of the gene, suggests a history of balancing selection. MHC variation is often thought to be maintained by some form of balancing selection, but the nature of this selection remains unclear. Our results support the hypothesis that spatial variation in selection regimes contributes to the high polymorphism. [ABSTRACT FROM AUTHOR]