Spatial and temporal genetic structure of a river‐resident Atlantic salmon (Salmo salar) after millennia of isolation (original) (raw)
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Conservation Genetics, 2013
Although the tendency of Atlantic salmon Salmo salar to form differentiated populations among rivers and among tributaries within large river systems ([100 km-long) is well documented, much less is known about population structure within small river systems (\30 km-long). In the present study, we investigated the genetic effects of straying of hatchery-reared salmon on population structure and genetic composition within the Ellidaár river system, a small system (21 km total length) in SW Iceland. We analyzed spatial and temporal variation of wild and domesticated samples (farmed and ranched; n = 931) using seven microsatellite loci. Estimates of population differentiation [F ST , genetic tree (D A )] and Bayesian cluster analysis (STRUCTURE) revealed a significant population structure as well as relative long-term temporal stability of the genetic composition in the main river from 1948 to 2005. However, the genetic composition of the tributary populations was unstable and genetically homogenized in recent years. Wild-hatchery hybrids were detected during the influx of strays as well as few years after, suggesting that introgression has changed the genetic composition of the wild populations. More investigations are needed in Iceland and elsewhere on possible fine-scale population differentiation and factors leading to it. Finescale population differentiation as observed in the present study has implications for the resolution with which harvest and habitat management of salmon should be conducted. In addition, farming and ranching operations should be located to minimize potential negative effects of strays on wild fish.
Life-history and habitat features influence the within-river genetic structure of Atlantic salmon
Molecular Ecology, 2007
Defining populations and identifying ecological and life-history characteristics affecting genetic structure is important for understanding species biology and hence, for managing threatened or endangered species or populations. In this study, populations of the world's largest indigenous Atlantic salmon ( Salmo salar ) stock were first inferred using model-based clustering methods, following which life-history and habitat variables best predicting the genetic diversity of populations were identified. This study revealed that natal homing of Atlantic salmon within the Teno River system is accurate at least to the tributary level. Generally, defining populations by main tributaries was observed to be a reasonable approach in this large river system, whereas in the mainstem of the river, the number of inferred populations was fewer than the number of distinct sampling sites. Mainstem and headwater populations were genetically more diverse and less diverged, while each tributary fostered a distinct population with high genetic differentiation and lower genetic diversity. Population structure and variation in genetic diversity among populations were poorly explained by geographical distance. In contrast, age-structure, as estimated by the proportion of multisea-winter spawners, was the most predictive variable in explaining the variation in the genetic diversity of the populations. This observation, being in agreement with theoretical predictions, emphasizes the essence of large multisea-winter females in maintaining the genetic diversity of populations. In addition, the unique genetic diversity of populations, as estimated by private allele richness, was affected by the ease of accessibility of a site, with more difficult to access sites having lower unique genetic diversity. Our results show that despite this species' high capacity for migration, tributaries foster relatively closed populations with little gene flow which will be important to consider when developing management strategies for the system.
Molecular ecology, 2011
Disentangling the effects of natural environmental features and anthropogenic factors on the genetic structure of endangered populations is an important challenge for conservation biology. Here, we investigated the combined influences of major environmental features and stocking with non-native fish on the genetic structure and local adaptation of Atlantic salmon (Salmo salar) populations. We used 17 microsatellite loci to genotype 975 individuals originating from 34 French rivers. Bayesian analyses revealed a hierarchical genetic structure into five geographically distinct clusters. Coastal distance, geological substrate and river length were strong predictors of population structure. Gene flow was higher among rivers with similar geologies, suggesting local adaptation to geological substrate. The effect of river length was mainly owing to one highly differentiated population that has the farthest spawning grounds off the river mouth (up to 900 km) and the largest fish, suggesting local adaptation to river length. We detected high levels of admixture in stocked populations but also in neighbouring ones, implying large-scale impacts of stocking through dispersal of non-native individuals. However, we found relatively few admixed individuals suggesting a lower fitness of stocked fish and/or some reproductive isolation between wild and stocked individuals. When excluding stocked populations, genetic structure increased as did its correlation with environmental factors. This study overall indicates that geological substrate and river length are major environmental factors influencing gene flow and potential local adaptation among Atlantic salmon populations but that stocking with non-native individuals may ultimately disrupt these natural patterns of gene flow among locally adapted populations.
Freshwater Biology, 2009
1. An important goal of conservation biology is to preserve the evolutionary potential of a species by maintaining natural levels of genetic diversity. Here, we assess the population differentiation in the Atlantic salmon, Salmo salar, listed in Annex II of the European Habitats Directive, to provide valuable information for its conservation in Normandy (France).2. Samples collected from 10 natural sites revealed that 13 of 14 microsatellite loci were polymorphic. Significant differentiation among populations was detected (FST = 0.054, P < 0.001), and all FST pairwise comparisons except one were significant. A genetic split was observed between populations inhabiting streams with limestone geology compared to those inhabiting streams with siliceous geology, which could reflect adaptative differences.3. Hatchery stocks used for the restocking of two rivers were genetically distinct from native stocks.4. Analysis of three stream habitats restored in 1995 showed that all were recolonized naturally by wild salmon from geographically close populations and no founder effects were detected. Allelic richness was similar between recolonized and wild populations.5. From a management perspective, our study revealed that restoration of habitat is very effective to recreate new populations in rivers from which salmon have disappeared and that natural recolonization can be fast and effective in terms of genetic diversity.
Fisheries Management and Ecology, 2011
Population genetic studies can be useful for informing conservation and management. In Atlantic salmon, Salmo salar L., population structuring frequently occurs between river systems, but contrasting patterns occur within rivers, highlighting the need for catchment-specific studies to inform management. Here, population structure of Atlantic salmon was examined in the River Tamar, United Kingdom, using 12 microsatellite loci. Gene diversity and allelic richness ranged from 0.80 to 0.84 and from 8.96 to 10.24, respectively. Some evidence of genetic structure was found, including significant genetic differentiation between samples in different subcatchments (pairwise h and tests of genic differentiation), results from assignment tests and a pattern of isolation by distance. Conversely, STRUCTURE revealed only one population cluster, and an analysis of molecular variance showed no significant variation between subcatchments. Evidence of population bottlenecks depended on the mutation model assumed and is discussed with reference to catchment-specific studies of stock abundance. Implications for implementing management actions are considered.
Molecular Ecology, 2001
Atlantic salmon (n = 1682) from 27 anadromous river populations and two nonanadromous strains ranging from south-central Maine, USA to northern Spain were genotyped at 12 microsatellite DNA loci. This suite of moderate to highly polymorphic loci revealed 266 alleles (5 -37/locus) range-wide. Statistically significant allelic and genotypic heterogeneity was observed across loci between all but one pairwise comparison. Significant isolation by distance was found within and between North American and European populations, indicating reduced gene flow at all geographical scales examined. North American Atlantic salmon populations had fewer alleles, fewer unique alleles (though at a higher frequency) and a shallower phylogenetic structure than European Atlantic salmon populations. We believe these characteristics result from the differing glacial histories of the two continents, as the North American range of Atlantic salmon was glaciated more recently and more uniformly than the European range. Genotypic assignment tests based on maximum-likelihood provided 100% correct classification to continent of origin and averaged nearly 83% correct classification to province of origin across continents. This multilocus method, which may be enhanced with additional polymorphic loci, provides fishery managers the highest degree of correct assignment to management unit of any technique currently available.
A genetic test of metapopulation structure in Atlantic salmon (Salmo salar) using microsatellites
Canadian Journal of Fisheries and Aquatic Sciences, 1997
The principal objective of this study was to describe the pattern of genetic exchange and isolation of Atlantic salmon (Salmo salar) populations among geographical regions of the province of Quebec, Canada. Seven riverine populations, associated with three putative regional metapopulations (North Shore, Gaspé Peninsula, and Ungava), were analyzed using microsatellites. Our results did not support the putative metapopulation structure. Significant heterogeneity in allelic frequency was observed among most rivers independently of their location or group subdivision. Interpopulation genetic variance (φ ST) indicates less heterogeneity among rivers than χ 2 analysis and was mainly associated with the geographical distance of the most isolated rivers, the Natashquan and the Koksoak. Even with low genetic variance among populations, the overall significant allelic heterogeneity among rivers strongly suggests that each population, whether separated by thousands or tens of kilometres, should be considered and managed as a specific stock. Résumé : L'objectif principal de cette étude était de décrire le degré d'échange génétique entre des populations de saumon atlantique (Salmo salar) appartenant à différentes régions géographiques de la province de Québec. Sept rivières, associées à trois metapopulations régionales hypothétiques (Côte-Nord, péninsule de Gaspé et l'Ungava), ont été analysées à l'aide de microsatellites. Nos résultats ne soutiennent pas une structure composée de metapopulations. Une hétérogénéité significative des fréquences allèliques entre la majorité des rivières indépendamment de leur localisation ou de leur subdivision par région. La variance génétique interpopulation (φ ST) suggère une hétérogénéité entre les rivières mais à un degré moindre qu'indiqués par les χ 2 et était associée essentiellement avec la distance géographique des rivières plus isolées, telles que la Natashquan et la Koksoak. Même si on observe de faibles variances génétiques ou structure de population, l'hétérogénéité allèlique significative et générale entre les rivières suggère fortement que les populations, voisines ou séparées par des milliers de kilomètres, soient considérées et gérées comme des stocks spécifiques.
Conservation Genetics, 2008
In wild populations, defining the spatial scale at which management and conservation practices should focus remains challenging. In Atlantic salmon, compelling evidence suggests that genetic structure within rivers occurs, casting doubt on the underlying premise of the river-based management approach for this species. However, no comparisons of within-river genetic structure across different systems have been performed yet to assess the generality of this pattern. We compared the within-river genetic structure of four important salmon rivers in North America and evaluated the extent of genetic differentiation among their main tributaries. We found a hierarchical genetic structure at the river and tributary levels in most water systems, except in the Miramichi where panmixia could not be rejected. In the other cases, genetic differentiation between most tributaries was significant and could be as high as that found between rivers of the same geographical region. More importantly, the extent of genetic differentiation between tributaries varied greatly among water systems, from well differentiated (h ST = 0.035) to undifferentiated (h ST =-0.0003), underlying the difficulty in generalizing the ubiquity of within-river genetic structure in Atlantic salmon. Thus, this study underlines the importance of evaluating the genetic structure of Atlantic salmon in large water systems on a case by case basis in order to define the most appropriate spatial scale and focal unit for efficient management and conservation actions. The potential consequences of management at an inappropriate spatial scale are discussed.
Small populations may be expected to harbour less genetic variation than large populations, but the relation between census size (N), effective population size (Ne), and genetic diversity is not well understood. We compared microsatellite variation in four small peripheral Atlantic salmon populations from the Iberian peninsula and three larger populations from Scotland to test whether genetic diversity was related to population size. We also examined the historical decline of one Iberian population over a 50-year period using archival scales in order to test whether a marked reduction in abundance was accompanied by a decrease in genetic diversity. Estimates of effective population size (Ne) calculated by three temporal methods were consistently low in Iberian populations, ranging from 12 to 31 individuals per generation considering migration, and from 38 to 175 individuals per generation if they were regarded as closed populations. Corresponding Ne/N ratios varied from 0.02 to 0.04 assuming migration (mean = 0.03) and from 0.04 to 0.18 (mean = 0.10) assuming closed populations. Population bottlenecks, inferred from the excess of heterozygosity in relation to allelic diversity, were detected in all four Iberian populations, particularly in those year classes derived from a smaller number of returning adults. However, despite their small size and declining status, Iberian populations continue to display relatively high levels of heterozygosity and allelic richness, similar to those found in larger Scottish populations. Furthermore, in the R. Asón no evidence was found for a historical loss of genetic diversity despite a marked decline in abundance during the last five decades. Thus, our results point to two familiar paradigms in salmonid conservation: (1) endangered populations are capable of maintaining relatively high levels of genetic variation despite their small size, and (2) marked population declines may not necessarily result in a significant loss of genetic diversity. Although there are several explanations for such results, microsatellite data and physical tagging suggest that high levels of dispersal and asymmetric gene flow have probably helped to maintain genetic diversity in these peripheral populations, and thus to avoid the negative consequences of inbreeding. Keywords: allelic richness, Atlantic salmon, bottlenecks, effective population size, asymmetric gene flow, heterozygosity, microsatellites