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The survival of Atlantic salmon (Salmo salar), an increasingly rare anadromous species, has declined dramatically during its marine phase, with disproportionate impacts on the poorly understood early post‐smolt period. Logistical constraints on collecting oceanic data to inform this issue pose a formidable obstacle. To advance understanding of post‐smolt distributional ecology in the North‐east Atlantic, a comprehensive analysis of existing information was undertaken. Data were synthesized from 385 marine cruises, 10,202 individual trawls, and 9,269 captured post‐smolts, spanning three decades and ~4.75 million km2 of ocean, with 3,423 individuals genetically assigned to regional phylogeographic origin. The findings confirm major migrational post‐smolt aggregations on the continental shelf‐edge off Ireland, Scotland and Norway, and an important marine foraging area in the Norwegian Sea. Genetic analysis shows that aggregational stock composition does not simply reflect distance to natal rivers, with northern phylogeographic stock groups significantly under‐represented in sampled high‐seas aggregations. It identifies a key foraging habitat for southern European post‐smolts located in international waters immediately west of the Vøring Plateau escarpment, potentially exposing them to a high by‐catch mortality from extra‐territorial pelagic fisheries. Evidence of the differential distribution of regional stocks points to fundamental differences in their migration behaviours and may lead to inter‐stock variation in responses to environmental change and marine survival. The study shows that understanding of post‐smolt marine ecology, as regards to stock‐specific variations in habitat utilization, biological performance and exposure to mortality factors, can be significantly advanced by data integration across studies and exploiting genetic approaches. [ABSTRACT FROM AUTHOR]

The viability of wild Atlantic salmon populations is threatened by genetic introgression from escaped farmed salmon. Farmed Atlantic salmon are genetically improved for important commercial traits and a life in captivity but are poorly adapted to the natural environment. The rate of gene flow from escaped farmed to wild salmon depends on their spawning success and on offspring survival at various life stages. We here investigate relative survival of introgressed juvenile Atlantic salmon (parr) in a river in northern Norway. The studied population has experienced genetic introgression from farmed salmon for about four generations (20 years). We followed two cohorts of parr from the year of hatching (0+) to the age of 2 years (2+). Farmed genetic introgression was quantified at the individual level and on a continuous scale using diagnostic SNPs. Population‐level genetic introgression decreased from 0+ to 2+ by 64% (2011 cohort) and 37% (2013 cohort). This change was driven by a 70% (2011 cohort) and 49% (2013 cohort) lower survival from age 0+ to 2+ in introgressed parr compared to parr of wild origin. Our observations show that there is natural selection against genetic introgression with a potential cost of lower productivity. [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 endangered freshwater pearl mussel ( Margaritifera margaritifera) requires brown trout or Atlantic salmon as a host to complete its life cycle. In Norway, populations have been identified using almost exclusively either salmon or trout as hosts, even when both hosts are present. In this study we tested the hypothesis that the genetic variation in freshwater pearl mussel is host-dependent. A total of 587 specimens from 25 different localities were assayed for genetic variation at eight microsatellite loci. Within populations, the trout-mussel populations had significantly lower allelic richness and expected heterozygosity than the salmon-mussel populations. The genetic differentiation among trout-mussel populations was very large ( F = 0.332), and significantly larger than among salmon-mussel populations ( F = 0.023). Host affiliation explained more of the genetic differentiation among freshwater pearl mussel populations ( F = 0.150) than geographical location ( F = 0.013). Collectively these results suggest that there is strong reproductive isolation between freshwater pearl mussel populations. More importantly, our results indicate that trout- and salmon-mussel populations are differently genetically structured. Because of the mandatory parasitic larval stage on trout and salmon, the host-dependent genetic variation has implications for restoration and management plans for this species. [ABSTRACT FROM AUTHOR]

The river-resident Salmo salar ('småblank') has been isolated from other Atlantic salmon populations for 9,500 years in upper River Namsen, Norway. This is the only European Atlantic salmon population accomplishing its entire life cycle in a river. Hydropower development during the last six decades has introduced movement barriers and changed more than 50% of the river habitat to lentic conditions. Based on microsatellites and SNPs, genetic variation within småblank was only about 50% of that in the anadromous Atlantic salmon within the same river. The genetic differentiation ( FST) between småblank and the anadromous population was 0.24. This is similar to the differentiation between anadromous Atlantic salmon in Europe and North America. Microsatellite analyses identified three genetic subpopulations within småblank, each with an effective population size Ne of a few hundred individuals. There was no evidence of reduced heterozygosity and allelic richness in contemporary samples (2005-2008) compared with historical samples (1955-56 and 1978-79). However, there was a reduction in genetic differentiation between sampling localities over time. SNP data supported the differentiation of småblank into subpopulations and revealed downstream asymmetric gene flow between subpopulations. In spite of this, genetic variation was not higher in the lower than in the upper areas. The meta-population structure of småblank probably maintains genetic variation better than one panmictic population would do, as long as gene flow among subpopulations is maintained. Småblank is a unique endemic island population of Atlantic salmon. It is in a precarious situation due to a variety of anthropogenic impacts on its restricted habitat area. Thus, maintaining population size and avoiding further habitat fragmentation are important. [ABSTRACT FROM AUTHOR]

The potential use of the metal binding protein metallothionein (MT) as a biomarker for trace metal exposure has been evaluated in brown trout (Salmo trutta) from the Cucontaminated Rugla and the Cd/Zn-contaminated Naustebekken Norwegian rivers, as well as in hatchery control trout. Metal concentrations were measured in gills, liver and kidney as well as in ambient water, and compared with Cd/Zn MT and Cu MT levels, measured by two Cd-saturation techniques. In addition haematocrit, plasma chloride and condition factors (weight × 100/length[sup 3]) were measured, and genetic diversity determined. A negative correlation was found between the Cd/Zn MT content and the condition factor in the different trout populations, and Naustebekken trout, having the lowest condition factor, were less heterozygous than Rugla trout, indicating that Naustebekken trout have adapted to the metal in their environment. Significant positive correlations were found between accumulated levels of Cd and Cd/Zn MT in liver and kidney, suggesting that liver and kidney, but not gill, Cd/Zn MT can be used as a biomarker for chronic Cd exposure. Cu MT seems less suitable as a biomarker for Cu exposure. These data together further emphasize that MTs must be applied with caution, when assessing these proteins in biomonitoring of the natural environment. [ABSTRACT FROM AUTHOR]

Genetic interactions between farmed and wild conspecifics are of special concern in fisheries where large numbers of domesticated individuals are released into the wild. In the Atlantic salmon (Salmo salar), selective breeding since the 1970's has resulted in rapid genetic changes in commercially important traits, such as a doubling of the growth rate. Each year, farmed salmon escape from net pens, enter rivers, and interbreed with wild salmon. Field experiments demonstrate that genetic introgression may weaken the viability of recipient populations. However, due to the lack of diagnostic genetic markers, little is known about actual rates of gene flow from farmed to wild populations. Here we present a panel of 60 single nucleotide polymorphisms (SNPs) that collectively are diagnostic in identifying individual salmon as being farmed or wild, regardless of their populations of origin. These were sourced from a pool of 7000 SNPs comparing historical wild and farmed salmon populations, and were distributed on all but two of the 29 chromosomes. We suggest that the generic differences between farmed and wild salmon at these SNPs have arisen due to domestication. The identified panel of SNPs will permit quantification of gene flow from farmed to wild salmon populations, elucidating one of the most controversial potential impacts of aquaculture. With increasing global interest in aquaculture and increasing pressure on wild populations, results from our study have implications for a wide range of species., (© 2011 Blackwell Publishing Ltd.)