Microsatellite DNA data point to extensive but incomplete admixture in a marble and brown trout hybridisation zone

Marble trout, endemic to the Adriatic drainage basin, is severely threatened by hybridisation with non-native brown trout. In the present study, we analysed 12 microsatellite DNA loci to assess genetic population structure and differentiation between sympatric phenotypic marble and brown trout at nine sampling sites in the upper Etsch/Adige River system. F _ST and AMOVA analyses revealed significant genetic differentiation between marble and brown trout samples. Thus, admixture between brown and marble trout appears to be incomplete. However, factorial correspondence analysis depicted marble trout, Atlantic brown trout and intermediate genotypes. Bayesian-based individual assignment tests identified indigenous marble trout at five sampling sites. In four other samples no ‘pure’ marble trout were detected. Bidirectional, first-generation hybridisation, involving both sexes of both parental species was observed. In locations where ‘pure’ marble trout still exist, post-F1 hybridisation appears to be directed towards brown trout. This has likely slowed the rate of hybridisation between the two trout species and the decline of relic marble trout populations. Based on these results, restoration management actions are proposed, such as the abandonment of brown trout stocking activities, sharper angling policies, establishment of indigenous marble trout breeding strains and the elaboration of a conservation priority list.     

Effects of species invasion on population dynamics,vital rates and life histories of the native species

Invasions occurring in natural environments provide the opportunity to study how vital rates change and life histories evolve in the presence of a competing species. In this work, we estimate differences in reproductive traits, individual growth trajectories, survival, life histories and population dynamics between a native species living in allopatry and in sympatry with an invasive species of the same taxonomic Family. We used as a model system marble trout Salmo marmoratus (native species) and rainbow trout Oncorhynchus mykiss (non-native) living in the Idrijca River (Slovenia). An impassable waterfall separates the stream into two sectors only a few 100 meters apart: a downstream sector in which marble trout live in sympatry with rainbow trout and an upstream sector in which marble trout live in allopatry. We used an overarching modelling approach that uses tag-recapture and genetic data (>2,500 unique marble and rainbow trout were sampled and genotyped) to reconstruct pedigrees, test for synchrony of population dynamics and model survival and growth, while accounting for individual heterogeneity. The population dynamics of the two marble trout populations and of rainbow trout were synchronous. We found higher prevalence of younger parents, higher mortality and lower population density in marble trout living in sympatry with rainbow trout than in marble trout living in allopatry. There were no differences in the average individual growth trajectories between the two marble trout populations. Faster life histories of marble trout living in sympatry with rainbow trout are consistent with predictions of life history theory.     

Assessing conditions influencing the longitudinal distribution of exotic brown trout (<Emphasis Type="Italic">Salmo trutta</Emphasis>) in a mountain stream: a spatially-explicit modeling approach

Trout species often segregate along elevational gradients, yet the mechanisms driving this pattern are not fully understood. On the Logan River, Utah, USA, exotic brown trout (Salmo trutta) dominate at low elevations but are near-absent from high elevations with native Bonneville cutthroat trout (Onchorhynchus clarkii utah). We used a spatially-explicit Bayesian modeling approach to evaluate how abiotic conditions (describing mechanisms related to temperature and physical habitat) as well as propagule pressure explained the distribution of brown trout in this system. Many covariates strongly explained redd abundance based on model performance and coefficient strength, including average annual temperature, average summer temperature, gravel availability, distance from a concentrated stocking area, and anchor ice-impeded distance from a concentrated stocking area. In contrast, covariates that exhibited low performance in models and/or a weak relationship to redd abundance included reach-average water depth, stocking intensity to the reach, average winter temperature, and number of days with anchor ice. Even if climate change creates more suitable summer temperature conditions for brown trout at high elevations, our findings suggest their success may be limited by other conditions. The potential role of anchor ice in limiting movement upstream is compelling considering evidence suggesting anchor ice prevalence on the Logan River has decreased significantly over the last several decades, likely in response to climatic changes. Further experimental and field research is needed to explore the role of anchor ice, spawning gravel availability, and locations of historical stocking in structuring brown trout distributions on the Logan River and elsewhere.     

Habitat degradation and trout stocking can reinforce the impact of flash floods on headwater specialist Alpine bullhead Cottus poecilopus – A case study from the Carpathian Mountains

In freshwater streams, flooding is a typical source of natural disturbance that plays a key role in the dynamics of animal populations and communities. However, habitat degradation and fish stocking might increase the severity of its impact. We tested the effects of a flash flood on the abundance of three size classes of headwater dwelling Alpine bullhead, Cottus poecilopus, in the streams of the Carpathian Mountains in the Czech Republic, that are stocked with hatchery‐reared brown trout, Salmo trutta. We showed that the overall abundance of Alpine bullhead was highest at the sites with the least degraded habitat (i.e., natural habitat) and we caught almost no Alpine bullhead at the sites with the most degraded habitat. The flash flood had a strong negative effect on the abundance of the largest individuals of Alpine bullhead. Abundance of small and medium size Alpine bullhead was negatively affected by the abundance of adult stocked brown trout before as well as after the flash flood. However, negative effect of adult brown trout abundance on abundance of large Alpine bullhead was not significant before the flash flood, and it became significant after the flash flood. This could indicate an accumulation of negative impacts of trout stocking and flash flood on this size class. Overall, our results suggest that stocking of hatchery trout and habitat degradation can reinforce the impact of flash floods on the population of Alpine bullhead in the streams of the Carpathian Mountains.     

Trout (<Emphasis Type="Italic">Salmo trutta</Emphasis>) mitochondrial DNA polymorphism in the centre of the marble trout distribution area

The marble trout, a lineage of the Salmo trutta complex, is endemic to the Southern Alpine region. Although it is endangered throughout its entire distribution range, population genetic data were lacking for the central area, including the upper Etsch/Adige River system (South Tyrol, Northern Italy). A total of 672 Salmo trutta specimens, comprising phenotypic marble trout and phenotypic brown trout, from 20 sampling sites throughout South Tyrol were analysed by sequencing the complete mitochondrial DNA control region. Thirteen distinct haplotypes were identified, which clustered within three major genetic lineages: the Marmoratus (MA), the Atlantic (AT) and the Danubian (DA) lineage. 41.7% of the investigated individuals carried haplotypes of the MA lineage, 47.9% of the AT lineage and 10.4% of the DA lineage. It is noticeable that AT haplotypes were present at all sampling sites and no “pure” marble trout population with exclusively MA haplotypes was found. This points to a considerable impact of stocking with allochthonous brown trout, given that there is no evidence for natural colonisation by individuals of the AT lineage. However, our data indicate, for at least four localities, a limited gene flow between the native marble trout and hatchery-reared strains. Future conservation and rehabilitation measures will thus have to concentrate on the identification of remnant pure marble trout individuals from such mixed populations. Handling editor: C. Sturmbauer     

Comments on the morphology of Nicsmirnovius eximius (Kiser, 1948) (Aloninae,Anomopoda, Cladocera) in Thailand,with description of its male

The density, diet and habitat use of brown trout (Salmo trutta) and Siberian sculpin (Cottus poecilopus) were studied in the subalpine River Atna in southeastern Norway in the autumn during a six year period (1986–1991). There was an inverse relationship between the density of brown trout and Siberian sculpin. Diet overlap, as indicated by the Schoener index, was high between the two species, ranging between 0.48 and 0.86. Chironomid larvae and other aquatic insects were the most common food items for both species. Brown trout also consumed substantive amounts of surface insects. Siberian sculpin typically occupied sites with finer substrates and greater water depths than brown trout, even though there was considerable overlap in habitat use between the two species. Because the two species shared similar habitats, we suggest that the potential for species interactions exists, particularly at sites where density of sculpin is high.     

The selection of the ‘wild’: A combined molecular approach for the identification of pure indigenous fish from hybridised populations

The identification of pure indigenous fish from hybridised populations represents a key issue in fisheries management and conservation biology. In the present study an approach for selection of purebred marble trout (Salmo trutta marmoratus C.) individuals out of admixed populations was set up and assessed. In a first step, baseline data sets of pure marble trout and pure brown trout specimens based on twelve microsatellite loci were used to simulate five consecutive generations of admixture. The baseline and the resulting simulation data sets were then combined with data of a ‘real’ hybridised marble trout population to perform a single individual assignment test as implemented in STRUCTURE. By this procedure the assignment approach was calibrated and it was possible to compare admixture coefficients obtained for individuals from different populations. The ranking of individual admixture coefficients on a plot and comparison with simulated data revealed that the test population was composed of pure marble trout individuals, first generation hybrids between marble trout and brown trout, and hybrid backcross specimens between both groups. However, by defining a critical q-value of 0.1 and additionally integrating individual sequence data of the mtDNA control region, it was possible to indicate individuals, which could be selected for the establishment of a pure marble trout strain.     

The impact of an introduced predator on a threatened galaxiid fish is reduced by the availability of complex habitats

1. The availability of complex habitats such as macrophytes may be vital in determining the outcomes of interactions between introduced predators and native prey. Introduced brown trout (Salmo trutta) have impacted numerous small native freshwater fishes in the southern hemisphere, but the potential role of complex habitats in determining the direct outcomes of brown trout – native fish interactions has not been experimentally evaluated. 2. An in‐lake enclosure experiment was used to evaluate the importance of structurally complex habitats in affecting the direct impacts of brown trout on a threatened galaxiid fish. Five Galaxias auratus and a single brown trout were added to enclosures containing one of three different habitat types (artificial macrophytes, rocks and bare silt substrate). The experiment also had control enclosures without brown trout. Habitat‐dependence of predation risk was assessed by analysis of G. auratus losses to predation, and stomach contents of remaining fish were analysed to determine if brown trout directly affect the feeding of G. auratus and whether this is also habitat‐dependent. 3. Predation risk of G. auratus differed significantly between habitat types, with the highest mortality in enclosures with only bare silt substrate and the lowest in enclosures containing artificial macrophytes. This result highlights the importance of availability of complex habitats for trout – native fish interactions and suggests that increasing habitat degradation and loss in fresh waters may exacerbate the direct impacts of introduced predators. 4. Stomach contents analyses were restricted to fish in enclosures with artificial macrophytes and rocks, as most fish were consumed in enclosures with brown trout and only bare silt substrate. These analyses suggest that brown trout do not directly affect the feeding of G. auratus in complex habitats, but it is still unknown whether its feeding is reduced if complex habitats are unavailable.     

Life history and genetic characterisation of sea trout Salmo trutta in the Adriatic Sea

1. The brown trout Salmo trutta is characterised by both anadromous (sea trout) and resident populations, naturally occurring in Atlantic and Ponto-Caspian rivers. Sea trout are currently considered absent from rivers of the Mediterranean area, probably because of the non-optimal chemical–physical characteristics of the Mediterranean Sea. However, the occasional bycatch of smoltified S. trutta in the Adriatic Sea is well known among fishermen and the biological explanation of this phenomenon is still controversial. The aim of this study was to compare the genetic diversity of freshwater and marine brown trout to try to understand the factors underlying the presence of putative anadromous brown trout in the Adriatic Sea.
2. In this study, we analysed the genetic diversity of: (1) wild brown trout collected from the Esino River (central Italy); (2) a domestic strain of brown trout used for stocking the study area; and (3) a sample of Adriatic sea trout collected near the outlet of the Esino River. Together with genetic analysis, we carried out scale analysis in order to track the freshwater/marine stages of the life cycle in the sea trout samples. The genetic characterisation was carried out by polymerase chain reaction–restriction fragment length polymorphism analysis of the mtDNA fragment ND-5/6 and the nuclear locus LDH-C1* and by genotyping 15 microsatellite loci. The genetic polymorphism obtained was used to investigate intra- and inter-population genetic diversity, rates of genetic introgression between wild and domestic samples and the origin of sea trout specimens by using assignment tests.
3. Our genetic analyses demonstrated that the sea trout analysed in this study are from the domestic strain of Atlantic origin used in central Italy for stocking activities. The level of genetic introgression between native and domestic samples is high in the Esino River. The populations more resilient to introgressive hybridisation appeared to be those living in the portion of the river network dominated by carbonate rocks. Assignment tests (GeneClass) suggest the existence of a link between stocking efforts and the freshwater origin of the sea trout. In addition, data obtained from the analysis of scales, size measurement, and sex determination showed a pattern of smolt age, size, and sex ratio very similar to those observed in other anadromous populations.
4. In conclusion, the present study highlighted that sea trout from the central Adriatic Sea originated from brown trout of Atlantic origin inhabiting the Esino River. Their seaward migratory behaviour could represent a consequence of an active migration instead of a passive displacement by water flow. Our results also showed that traditional stocking practices represent a negative activity for the conservation of the last Mediterranean native S. trutta populations.

     

Winter ecology of Arctic charr (<Emphasis Type="Italic">Salvelinus alpinus</Emphasis>) and brown trout (<Emphasis Type="Italic">Salmo trutta</Emphasis>) in a subarctic lake,Norway

We studied habitat choice, diet, food consumption and somatic growth of Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) during the ice-covered winter period of a subarctic lake in northern Norway. Both Arctic charr and brown trout predominantly used the littoral zone during winter time. Despite very cold winter conditions (water temperature <1°C) and poor light conditions, both fish species fed continuously during the ice-covered period, although at a much lower rate than during the summer season. No somatic growth could be detected during the ice-covered winter period and the condition factor of both species significantly declined, suggesting that the winter feeding rates were similar to or below the maintenance requirements. Also, the species richness and diversity of ingested prey largely decreased from summer to winter for both fish species. The winter diet of Arctic charr <20 cm was dominated by benthic insect larvae, chironomids in particular, and Gammarus lacustris, but zooplankton was also important in December. G. lacustris was the dominant prey of charr >20 cm. The winter diet of brown trout <20 cm was dominated by insect larvae, whereas large-sized trout mainly was piscivorous, feeding on juvenile Arctic charr. Piscivorous feeding behaviour of trout was in contrast rarely seen during the summer months when their encounter with potential fish prey was rare as the small-sized charr mainly inhabited the profundal. The study demonstrated large differences in the ecology and interactions of Arctic charr and brown trout between the winter and summer seasons.     

Global warming threatens the persistence of Mediterranean brown trout

Current climate change exacerbates the environmental restrictions on temperate species inhabiting low latitude edges of their geographical ranges. We examined how temperature variations due to current and future climate change are likely to affect populations’ persistence of stream‐dwelling brown trout Salmo trutta at the vulnerable southern periphery of its range. Analysis of 33 years of air temperature data (1975–2007) by time‐series models indicated a significant upward trend and a pronounced shift in air temperature around 1986‐1987. This warming is associated with an ongoing population decline of brown trout, most likely caused by a loss of suitable thermal habitat in lower latitudes since the 1980s. Population decrease may not be attributed to physical habitat modification or angler pressure, as carrying capacity remained stable and populations were not overexploited. We developed regional temperature models, which predicted that unsuitable thermal habitat for brown trout increased by 93% when comparing climate conditions between 1975–1986 and 1993–2004. Predictions from climate envelope models showed that current climate change may be rendering unsuitable 12% of suitable thermal habitat each decade, resulting in an overall population decrease in the lower reaches of around 6% per year. Furthermore, brown trout catches markedly decreased 20% per year. Projections of thermal habitat loss under the ecologically friendly B2 SRES scenario showed that brown trout may lose half of their current suitable habitat within the study area by 2040 and become almost extinct by 2100. In parallel to the upstream movement of brown trout thermal habitat, warm water species are increasing their relative abundance in salmonid waters. Empirical evidence was provided of how current climate change threatens some of the most healthy native brown trout populations in Southern Europe and how forthcoming climate change is expected to further decrease the conservation status of the species.     

Phylogeography,genetic structure,and conservation of the endangered Caspian brown trout, <Emphasis Type="Italic">Salmo trutta caspius</Emphasis> (Kessler, 1877), from Iran

The Caspian Sea, the largest inland closed water body in the world, has numerous endemic species. The Caspian brown trout (Salmo trutta caspius) is considered as endangered according to IUCN criteria. Information on phylogeography and genetic structure is crucial for appropriate management of genetic resources. In spite of the huge number of studies carried out in the Salmo trutta species complex across its distribution range, very few data are available on these issues for S. trutta within the Caspian Sea. Mitochondrial (mtDNA control region) and nuclear (major ribosomal DNA internal transcribed spacer 1, ITS-1, and ten microsatellite loci) molecular markers were used to study the phylogeography, genetic structure, and current captive breeding strategies for reinforcement of Caspian trout in North Iranian rivers. Our results confirmed the presence of Salmo trutta caspius in this region. Phylogenetic analysis demonstrated its membership to the brown trout Danubian (DA) lineage. Genetic diversity of Caspian brown trout in Iranian Rivers is comparable to the levels usually observed in sustainable anadromous European brown trout populations. Microsatellite data suggested two main clusters connected by gene flow among river basins likely by anadromous fish. No genetic differences were detected between the hatchery sample and the remaining wild populations. While the current hatchery program has not produced detectable genetic changes in the wild populations, conservation strategies prioritizing habitat improvement and recovering natural spawning areas for enhancing wild populations are emphasized.     

Effects of Fish-farm Activity on the Limnetic Community Structure of Brown Trout, Salmo trutta, and Arctic Charr, Salvelinus alpinus

Establishment of four fish-farms during the period 1971 to 1994 in the oligotrophic lake Skogseidvatnet affected Arctic charr, Salvelinus alpinus, but not brown trout, Salmo trutta. From 1971 to 1987, an increase in mean individual size of Arctic charr was recorded, while the mean individual size of brown trout remained stable. Arctic charr were found to use deeper benthic areas than brown trout. Approximately 8% of the Arctic charr population (>26cm), were found to switch to waste food from fish-farms, resulting in a novel feeding habitat for the species. They were, however, found in gillnets distant from the fish farm cages, indicating high mobility. The habitat segregation between the two species can most likely be explained by selective differences and asymmetric competition with brown trout as the dominant species. Based on the present results, changes in the Arctic charr population may be due to increased food availability and due to a new habitat use as a waste food feeder. The reason for the brown trout population to have remained stable with respect to mean size, growth pattern and habitat use, may be due to a different diet choice than Arctic charr in this lake. Brown trout were found to feed mainly on terrestrial insects, while Arctic charr fed mainly on zooplankton and on waste food.     

Spatial patterns of hybridization between bull trout, <Emphasis Type="Italic">Salvelinus confluentus</Emphasis>, and brook trout, <Emphasis Type="Italic">Salvelinus fontinalis</Emphasis> in an Oregon stream network

Hybridization with introduced species represents a serious threat to the persistence of many native fish populations. Brook trout (Salvelinus fontinalis) have been introduced extensively throughout the native range of bull trout (S. confluentus) and hybridization has been documented in several systems where they co-exist and is seen as a significant threat to the persistence of bull trout populations. We identified a group of diagnostic microsatellite loci to differentiate bull trout and brook trout and then used these loci to examine the spatial distribution of hybrids in the Malheur River basin, Oregon USA. In random samples of approximately 100 fish from each of three creeks we identified 181 brook trout, 112 bull trout and 14 hybrids. Although bull trout, brook trout and hybrids were found in all three creeks, they were not evenly distributed; brook trout were primarily found in the lower sections of the creeks, bull trout further upstream, and hybrids in the areas of the greatest overlap. One creek with a population of brook trout in a headwater lake provided an exception to this pattern; brook trout were found distributed throughout the creek downstream of the lake. Several post-F1 hybrids were identified suggesting that hybrids are reproducing in the Malher River Basin. Mitochondrial DNA analysis indicated that both female bull trout and brook trout are involved in hybridization events. Analysis of population structure suggested that brook trout have established multiple spawning populations within the Malheur system. Data presented in this study suggest that relative abundance of brook trout and habitat quality are important factors to consider when evaluating the threat of hybridization to bull trout populations.     

Dynamic micro-geographic and temporal genetic diversity in vertebrates: the case of lake-spawning populations of brown trout (Salmo trutta)

Conservation of species should be based on knowledge of effective population sizes and understanding of how breeding tactics and selection of recruitment habitats lead to genetic structuring. In the stream‐spawning and genetically diverse brown trout, spawning and rearing areas may be restricted source habitats. Spatio–temporal genetic variability patterns were studied in brown trout occupying three lakes characterized by restricted stream habitat but high recruitment levels. This suggested non‐typical lake‐spawning, potentially representing additional spatio–temporal genetic variation in continuous habitats. Three years of sampling documented presence of young‐of‐the‐year cohorts in littoral lake areas with groundwater inflow, confirming lake‐spawning trout in all three lakes. Nine microsatellite markers assayed across 901 young‐of‐the‐year individuals indicated overall substantial genetic differentiation in space and time. Nested gene diversity analyses revealed highly significant (≤P = 0.002) differentiation on all hierarchical levels, represented by regional lakes (F_LT = 0.281), stream vs. lake habitat within regional lakes (F_HL = 0.045), sample site within habitats (F_SH = 0.010), and cohorts within sample sites (F_CS = 0.016). Genetic structuring was, however, different among lakes. It was more pronounced in a natural lake, which exhibited temporally stable structuring both between two lake‐spawning populations and between lake‐ and stream spawners. Hence, it is demonstrated that lake‐spawning brown trout form genetically distinct populations and may significantly contribute to genetic diversity. In another lake, differentiation was substantial between stream‐ and lake‐spawning populations but not within habitat. In the third lake, there was less apparent spatial or temporal genetic structuring. Calculation of effective population sizes suggested small spawning populations in general, both within streams and lakes, and indicates that the presence of lake‐spawning populations tended to reduce genetic drift in the total (meta‐) population of the lake.     

Food borne parasites as indicators of trophic segregation between Arctic charr and brown trout

The habitat and diet choice and the infection (prevalence and abundance) of trophically transmitted parasites were compared in Arctic charr and brown trout living sympatrically in two lakes in northern Norway. Arctic charr were found in all main lake habitats, whereas the brown trout were almost exclusively found in the littoral zone. In both lakes the parasite fauna reflected the niche segregation between trout and charr. Surface insects were most common in the diet of trout, but transmit few parasites, and accordingly the brown trout had a relatively low diversity and abundance of parasites. Parasites transmitted by benthic prey such as Gammarus and insect larva, were common in both salmonid host species. Copepod transmitted parasites were much more common in Arctic charr, as brown trout did not include zooplankton in their diets. Parasite species that may use small fish as transport hosts, were far more abundant in piscivorous fish, especially brown trout. The seasonal dynamics in parasite infection were also consistent with the developments in the diet throughout the year. The study demonstrates that the structure of parasite communities of charr and the trout is highly dependent on shifts in habitat and diet of their hosts both on an annual base and through the ontogeny, in addition to the observed niche segregation between the two salmonid species.     

Effects of prey abundance,distribution, visual contrast and morphology on selection by a pelagic piscivore

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Invasion of north European streams by brook trout: hostile takeover or pre-adapted habitat niche segregation?

We combine evidence from small-scale experiments with a large-scale field survey to clarify the roles of biotic resistance and pre-adapted habitat niche segregation to the invasion success of the North American brook trout (Salvelinus fontinalis) in North European streams previously dominated by brown trout (Salmo trutta). Interspecific aggressions among the two species were negligible, yet there was distinct habitat niche segregation between them: brook trout occupied mainly pool habitats while brown trout tended to reside in fast-flowing riffles. Habitat niche segregation among brook trout and brown trout prevailed across a wide array of scales from experimental flumes to entire drainage systems, although the segregation pattern was weaker in the field. Habitat differentiation among the two species reflected their differential habitat requirements, suggesting that a match between a species’ niche requirements in its native range and habitat availability in the new environment is a prerequisite for understanding invasion success.     

Habitat selection,reproduction and predation of wintering lemmings in the Arctic

Snow cover has dramatic effects on the structure and functioning of Arctic ecosystems in winter. In the tundra, the subnivean space is the primary habitat of wintering small mammals and may be critical for their survival and reproduction. We have investigated the effects of snow cover and habitat features on the distributions of collared lemming (Dicrostonyx groenlandicus) and brown lemming (Lemmus trimucronatus) winter nests, as well as on their probabilities of reproduction and predation by stoats (Mustela erminea) and arctic foxes (Vulpes lagopus). We sampled 193 lemming winter nests and measured habitat features at all of these nests and at random sites at two spatial scales. We also monitored overwinter ground temperature at a subsample of nest and random sites. Our results demonstrate that nests were primarily located in areas with high micro-topography heterogeneity, steep slopes, deep snow cover providing thermal protection (reduced daily temperature fluctuations) and a high abundance of mosses. The probability of reproduction increased in collared lemming nests at low elevation and in brown lemming nests with high availability of some graminoid species. The probability of predation by stoats was density dependent and was higher in nests used by collared lemmings. Snow cover did not affect the probability of predation of lemming nests by stoats, but deep snow cover limited predation attempts by arctic foxes. We conclude that snow cover plays a key role in the spatial structure of wintering lemming populations and potentially in their population dynamics in the Arctic.