Lake size and fish diversity determine resource use and trophic position of a top predator in high‐latitude lakes

Prey preference of top predators and energy flow across habitat boundaries are of fundamental importance for structure and function of aquatic and terrestrial ecosystems, as they may have strong effects on production, species diversity, and food‐web stability. In lakes, littoral and pelagic food‐web compartments are typically coupled and controlled by generalist fish top predators. However, the extent and determinants of such coupling remains a topical area of ecological research and is largely unknown in oligotrophic high‐latitude lakes. We analyzed food‐web structure and resource use by a generalist top predator, the Arctic charr Salvelinus alpinus (L.), in 17 oligotrophic subarctic lakes covering a marked gradient in size (0.5–1084 km²) and fish species richness (2–13 species). We expected top predators to shift from littoral to pelagic energy sources with increasing lake size, as the availability of pelagic prey resources and the competition for littoral prey are both likely to be higher in large lakes with multispecies fish communities. We also expected top predators to occupy a higher trophic position in lakes with greater fish species richness due to potential substitution of intermediate consumers (prey fish) and increased piscivory by top predators. Based on stable carbon and nitrogen isotope analyses, the mean reliance of Arctic charr on littoral energy sources showed a significant negative relationship with lake surface area, whereas the mean trophic position of Arctic charr, reflecting the lake food‐chain length, increased with fish species richness. These results were supported by stomach contents data demonstrating a shift of Arctic charr from an invertebrate‐dominated diet to piscivory on pelagic fish. Our study highlights that, because they determine the main energy source (littoral vs. pelagic) and the trophic position of generalist top predators, ecosystem size and fish diversity are particularly important factors influencing function and structure of food webs in high‐latitude lakes.     

Lipid‐rich zooplankton subsidise the winter diet of benthivorous Arctic charr (Salvelinus alpinus) in a subarctic lake

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Niche segregation of coexisting Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) constrains food web coupling in subarctic lakes

1. Generalist fish species are recognised as important couplers of benthic and pelagic food‐web compartments in lakes. However, interspecific niche segregation and individual specialisation may limit the potential for generalistic feeding behaviour. 2. We studied summer habitat use, stomach contents and stable isotopic compositions of the generalist feeder Arctic charr coexisting with its common resource competitor brown trout in five subarctic lakes in northern Norway to reveal population‐level and individual‐level niche plasticity. 3. Charr and trout showed partial niche segregation in all five lakes. Charr used all habitat types and a wide variety of invertebrate prey including zooplankton, whereas trout fed mainly on insects in the littoral zone. Hence, charr showed a higher potential to promote habitat and food‐web coupling compared to littoral‐dwelling trout. 4. The level of niche segregation between charr and trout and between pelagic‐caught and littoral‐caught charr depended on the prevailing patterns of interspecific and intraspecific resource competition. The two fish species had partially overlapping trophic niches in one lake where charr numerically dominated the fish community, whereas the most segregated niches occurred in lakes where trout were more abundant. 5. In general, pelagic‐caught charr had substantially narrower dietary and isotopic niches and relied less on littoral carbon sources compared to littoral‐caught conspecifics that included generalist as well as specialised benthivorous and planktivorous individuals. Despite the partially specialised planktivorous niche and thus reduced potential of pelagic‐dwelling charr to promote benthic–pelagic coupling, the isotopic compositions of both charr subpopulations suggested a significant reliance on both littoral and pelagic carbon sources in all five study lakes. 6. Our study demonstrates that both interspecific niche segregation between and individual trophic specialisation within generalist fish species can constrain food‐web coupling and alter energy mobilisation to top consumers in subarctic lakes. Nevertheless, pelagic and littoral habitats and food‐web compartments may still be highly integrated due to the potentially plastic foraging behaviour of top consumers.     

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.     

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.     

Arctic charr in sympatry with burbot: ecological and evolutionary consequences

The trophic niche and parasite infection of Arctic charr (Salvelinus alpinus) were explored in two lakes with sympatric burbot (Lota lota) and two lakes without burbot in subarctic Norway. The CPUE of burbot and charr were similar in one lake, but burbot had a low population density in the other. Burbot were benthivorous in both lakes. Other co-occurring species like brown trout (Salmo trutta), Atlantic salmon parr (Salmo salar), grayling (Thymallus thymallus) and minnow (Phoxinus phoxinus) were also benthivores. At high densities, benthivorous burbot forced the whole Arctic charr population to utilise mainly the limnetic trophic niche. In contrast, at low burbot density or without burbot present, Arctic charr were primarily benthivorous in the littoral zone. Thus, a clear interactive segregation in diet was observed between Arctic charr and burbot at high burbot densities. There was also a high predation pressure from burbot on young Arctic charr along the benthic zones. The extensive use of zooplankton as prey caused a high parasite infection pressure of copepod transmitted Diphyllobothrium spp. larvae, with the potential for high negative impact on the Arctic charr population. As the benthivore trophic niche was occupied by burbot, the ecological opportunities for polymorphism with benthivorous ecotypes or morphs of Arctic charr were probably prevented. Therefore, the sympatry with burbot seems to have large ecological and evolutionary consequences for this Arctic charr population compared with neighbouring lakes where burbot is absent.     

Selective predation on zooplankton by pelagic Arctic charr, Salvelinus alpinus, in six subarctic lakes

Selective predation by planktivore fish appears to be an important regulatory factor of zooplankton communities, potentially causing large changes in species composition and size distributions within populations. In this study, prey preferences and size-selective predation on zooplankton by Arctic charr were examined in six subarctic lakes with Arctic charr as the dominant pelagic fish species. Most of the lakes had a zooplankton community dominated by copepods (Cyclops scutifer and Eudiaptomus graciloides), but the pelagic charr evidently selected cladoceran species (Bythotrephes longimanus, Daphnia sp. and Bosmina sp.), likely because the copepods have a higher mobility and evasiveness than the cladocerans. Furthermore, a strong size selection was also revealed for both Bosmina sp. and Daphnia sp., as individual prey from Arctic charr stomachs were exclusively larger than individuals sampled in the environment. Additionally, visibility due to size, morphology and pigmentation (egg-carrying females) was also a major factor for the selection of zooplankton prey. In conclusion, Arctic charr was found to be highly selective on zooplankton both in respect to species composition and individual size of Bosmina sp. and Daphnia sp.     

Community structure affects trophic ontogeny in a predatory fish

While most studies have focused on the timing and nature of ontogenetic niche shifts, information is scarce about the effects of community structure on trophic ontogeny of top predators. We investigated how community structure affects ontogenetic niche shifts (i.e., relationships between body length, trophic position, and individual dietary specialization) of a predatory fish, brown trout (Salmo trutta). We used stable isotope and stomach content analyses to test how functional characteristics of lake fish community compositions (competition and prey availability) modulate niche shifts in terms of (i) piscivorous behavior, (ii) trophic position, and (iii) individual dietary specialization. Northern Scandinavian freshwater fish communities were used as a study system, including nine subarctic lakes with contrasting fish community configurations: (i) trout‐only systems, (ii) two‐species systems (brown trout and Arctic charr [Salvelinus alpinus] coexisting), and (iii) three‐species systems (brown trout, Arctic charr, and three‐spined sticklebacks [Gasterosteus aculeatus] coexisting). We expected that the presence of profitable small prey (stickleback) and mixed competitor–prey fish species (charr) supports early piscivory and high individual dietary specialization among trout in multispecies communities, whereas minor ontogenetic shifts were expected in trout‐only systems. From logistic regression models, the presence of a suitable prey fish species (stickleback) emerged as the principal variable determining the size at ontogenetic niche shifts. Generalized additive mixed models indicated that fish community structure shaped ontogenetic niche shifts in trout, with the strongest positive relationships between body length, trophic position, and individual dietary specialization being observed in three‐species communities. Our findings revealed that the presence of a small‐sized prey fish species (stickleback) rather than a mixed competitor–prey fish species (charr) was an important factor affecting the ontogenetic niche‐shift processes of trout. The study demonstrates that community structure may modulate the ontogenetic diet trajectories of and individual niche specialization within a top predator.     

Habitat use and diet of sympatric Arctic charr (<Emphasis Type="Italic">Salvelinus alpinus</Emphasis>) and whitefish (<Emphasis Type="Italic">Coregonus lavaretus</Emphasis>) in five lakes in southern Norway: not only interspecific population dominance?

Stable coexistence of Arctic charr and whitefish does occur in a number of native lake fish communities in Scandinavia. Even so, whitefish introductions into Arctic charr lakes have resulted in serious decline and possibly local extinction of Arctic charr. In this article, we analyze the habitat use and diet of the two species in five Norwegian lakes differing in basin shape and environmental conditions. In two of the lakes, both species are native, and appear to live in a relatively stable coexistence. Here, whitefish mainly occupy the littoral and upper pelagic zone, while Arctic charr live in the deeper habitats. Diets are generally quite different in terms of the zooplankton species eaten. In the three other lakes, either whitefish or both species have been introduced. In the shallowest lake, habitat segregation is similar to that seen in the pristine lakes, although Arctic charr appears to be on the brink of extinction. In the remaining two lakes, however, Arctic charr dominates, and occurs in higher numbers than whitefish in all the habitats. Our observations indicate that coexistence of the two species in oligotrophic and relatively pristine lakes requires an extensive profundal zone to serve as a refugium for Arctic charr. If the littoral zone is rendered inaccessible or unprofitable for whitefish due to dominance of a third competitor or predator, or as a result of lake regulation, then Arctic charr may be the dominant species.     

Do introduced crayfish affect benthic fish in stony littoral habitats of large boreal lakes?

Invasive crayfish are spreading rapidly across Europe, where they are replacing the native crayfish species and impacting negatively on some other biota. Freshwater crayfish and many benthic fishes share similar habitat and food requirements and hence potentially compete for resources. In this study, we investigated impacts of the introduced signal crayfish (Pacifastacus leniusculus) on fish in stony littoral habitats of two large boreal lakes. We compared the littoral fish community composition and the densities of two common benthic fish species between sites with and without crayfish. To evaluate whether signal crayfish share the same food resources as benthic littoral fish or change their feeding habits, we used mixing models and trophic niche estimates based on analyses of stable isotopes of carbon and nitrogen. Both the community composition of littoral fish and the densities of benthic fish species were similar at sites with and without signal crayfish. Even though stable isotope signatures indicated strong dietary overlap between crayfish and benthic fish, the use of food sources and trophic niche widths of fish were not noticeably different between crayfish sites and non-crayfish sites. Our results suggest that, at current densities, the non-native signal crayfish does not have significant impacts on benthic fish in the stony littoral habitats of large boreal lakes.     

Polymorphic segregation in Arctic charr Salvelinus alpinus (L.) from Vatnshlidarvatn, a shallow Icelandic lake

We studied the salmonid fish Arctic charr, Salvelinus alpinus , in a small and shallow landlocked lake in NW Iceland. The lake is productive but die only fish present is Arctic charr. Despite the apparent absence of discrete benthic and limnetic habitats for fish, two forms of Arctic charr are found in the lake. They show subde differences in morphology related to swimming performance and manoeuvrability, but differences in life history such as growth, and age and size at sexual maturation are more pronounced. Both forms have benthic feeding habits with one form consuming greater number of species than the other. We suggest that the segregation of these forms is based on the evolution of a specialist from a local generalist and that this has been made possible by the absence of a common fish competitor in similar lakes, the threespined stickleback Gasterosteous aculeatus.     

Growth and Dietary Niche in Salvelinus Alpinus and Salvelinus Fontinalis as Revealed by Stable Isotope Analysis

Arctic charr, Salvelinus alpinus, and brook charr, Salvelinus fontinalis, inhabiting three lakes in the de la Trinité River and adjacent watershed, north shore, Gulf of St. Lawrence, were sampled in 1998. Arctic charr growth differed among lakes with the smallest fish coming from the largest lake. Arctic charr weight–length equation exponents were almost identical at all sample sites. Brook charr growth was also similar in all lakes. July stomach samples from Arctic charr consisted almost entirely of cladocerans in the largest lake, less so in the intermediate sized lake and were mostly aquatic insects in the smallest lake. Brook charr stomach contents were more varied and included fish. Carbon, nitrogen and sulphur stable isotope analyses (SIA) were used to provide a spatially and temporally integrated image of charr diets. SIA corroborated observed among-lake differences and similarities in species diets and suggested lake morphometry may influence measured results. The ¹⁵N signature in brook charr muscle increased with fork-length, as a result of a shift towards piscivory with size. The ¹⁵N signature in Arctic charr muscle tissue showed a significant negative correlation with fork-length in two of the studied lakes that appears related to dietary niche shifts. Results demonstrate the ability of SIA to detect dietary shifts otherwise unobservable from standard gut content analysis.     

Diet of Arctic charr (Salvelinus alpinus (L.)) and brown trout (Salmo trutta L.) in sympatry in two high altitude alpine lakes

Lake Pisses and Lake Labarre are two oligotrophic high altitude alpine lakes that have sympatric populations of Arctic charr and brown trout. These two lakes have similar morphometric, physical and chemical characteristics. The zooplanktonic and benthic fauna show little diversity. But the density of benthos (Chironomidae) and zooplankton is higher in Lake Pisses. The fish fauna of Lake Pisses is slightly more abundant than that of Lake Labarre, althought in both lakes fish density is low. A study of the diet of the two species revealed differences. In Lake Pisses, where the food supply is better, Arctic charr takes exclusively pelagic and benthic prey, whereas in Lake Labarre it also takes exogenous prey and thus comes into competition with trout. Length and body weight growth rates for Arctic charr are higher in Lake Pisses than in Lake Labarre. For trout, maximum length recorded was in Lake Pisses. The results show that the abundance of Chironomidae favours coexistence of the two species in Lake Pisses and confirm that, in the face of shortage of food, Arctic charr is better adapted than trout. This revised version was published online in July 2006 with corrections to the Cover Date.     

The biology and feeding ecology of Arctic charr in the Kerguelen Islands

Subsequent to their introduction in the 1950s, Arctic charr Salvelinus alpinus have been able to establish a self-sustaining population that has adapted to the unique conditions of the sub-Antarctic Kerguelen Islands. Here, 48 individuals (198–415 mm) were caught with gillnets and their basic biology and feeding ecology were examined using stable isotope analysis. The Lac des Fougères population split use of littoral and pelagic resources evenly, although larger fish relied more heavily on littoral production and appear to follow the size-dependent life history habitat template seen in many Scandinavian lakes where smaller sized individuals occupy the pelagic zone and larger individuals dominate the littoral habitat. In Kerguelen, Arctic charr mature at the same ages (5.6 years) as Arctic charr in both sub-Arctic and Arctic lakes. Although mortality was average in comparison to comparator sub-Arctic lakes, it was high in comparison to Arctic lakes. Maximal age (>7+) was at the lower end of the range typically seen in sub-Arctic lakes. Although they inhabit a resource-poor environment, Kerguelen Arctic charr showed no evidence of cannibalism. Thus, while Arctic charr can survive and reproduce in the relatively unproductive Kerguelen lake environments, survival and growth nevertheless appear to be traded off against survival and longevity. The uniqueness of the population location and the recency of its introduction suggest that further monitoring of the population has the potential to yield valuable insights into both the adaptability of the species and its likely responses to ongoing large-scale environmental change as represented by climate change.     

The effects of turbidity on prey consumption and selection of zooplanktivorous Gasterosteus aculeatus L.

It is well documented that reduced visibility caused by elevated turbidity can affect feeding of fish, yet the extent to which selective zooplanktivory is altered in turbid conditions remains ambiguous. In this study, we examined the influence of natural sediment-induced turbidity on the overall prey consumption and selective predation of a common brackish water littoral zooplanktivore, the particulate feeding three-spined stickleback (Gasterosteus aculeatus L.). We hypothesized that the effects of turbidity on prey consumption and prey type selection would be pronounced due to the vision-oriented feeding of this species and that these effects would differ between genders. Using aquarium experiments with three different groups of cladocerans and copepods varying in size and behavior, we studied prey consumption and selectivity of this key planktivore in varying turbidity treatments. Our results indicated significantly decreased total prey consumption in the high turbidity treatments, as well as altered selective feeding on copepods and an enhanced preference for larger cladocerans. We found gender-dependent differences in prey consumption, which are consistent with observations of other visually feeding fish with sexual size dimorphism. We conclude that high turbidity, such as that occurring in shallow coastal areas, may affect selective feeding in vision-oriented zooplanktivores and that these effects may be gender-related.     

Trophic polymorphism in brook charr revealed by diet, parasites and morphometrics

Stomach contents, parasite assemblages and morphometrics were compared in brook charr Salvelinus fontinalis from the littoral and pelagic zone of two adjacent lakes on the Canadian Shield. In lac Baie des Onze Îles, fish from the littoral zone had greater abundance of benthic prey in their stomach and were more heavily infected by parasites that use intermediate hosts associated with the littoral zone than fish captured in the pelagic zone. Littoral and pelagic brook charr from this lake also differed in regard to body shape and fin length, with each group being anatomically adapted to exploit their respective habitats. The highly significant correlation between morphometric and parasite canonical scores supports the hypothesis of functional diversification of individuals within lac Baie des Onze Îles. While fish from littoral and pelagic zones of lac Caribou did not differ in terms of diet, parasite assemblages or morphometrics, they were different to fish from lac Baie des Onze Îles in that they were less frequently infected with parasites that use gastropods as intermediate hosts, and had shorter pectoral fins. The inter-lake comparisons suggested that parasite assemblages and morphometrics of brook charr reflected the dominance of the limnetic and littoral habitats in lacs Caribou and lac Baie des Onze Îles, respectively.     

Parasites as indicators of individual feeding specialization in Arctic charr during winter in northern Norway

The relationship between infection with the food-transmitted parasites Diphyllobothrium dendriticum, D. ditremum (Cestoda) and Cystidicola farionis (Nematoda) and prey selection was studied in individual Arctic charr from Lake Takvatn, northern Norway. There was no correlation between parasites transmitted throughout prey organisms from benthic habitats (amphipods) and pelagic habitats (copepods). A strong relationship between infection with a parasite species and the corresponding intermediate host from the stomach content of individual charr, indicated an individual feeding specialization. Independent of size, charr specialized on the intermediate hosts of all three parasite species. Some charr maintained this specialization on specific prey items throughout the winter period. These parasite species are considered to be useful indicators of past prey selection.     

Effects of Partially Anadromous Arctic Charr (Salvelinus alpinus) Populations on Ecology of Coastal Arctic Lakes

Little research has been conducted on effects of iteroparous anadromous fishes on Arctic lakes. We investigated trophic ecology, fish growth, and food web structure in six lakes located in Nunavut, Canada; three lakes contained anadromous Arctic charr (Salvelinus alpinus) whereas three lakes did not contain Arctic charr. All lakes contained forage fishes and lake trout (Salvelinus namaycush; top predator). Isotope ratios (δ¹³C, δ¹⁵N) of fishes and invertebrates did not differ between lakes with and without anadromous Arctic charr; if anadromous Arctic charr deliver marine-derived nutrients and/or organic matter to freshwater lakes, these inputs could not be detected with δ¹³C and/or δ¹⁵N. Lake trout carbon (C):nitrogen (N) and condition were significantly higher in lakes with Arctic charr (C:N = 3.42, K = 1.1) than in lakes without Arctic charr (C:N = 3.17, K = 0.99), however, and ninespine stickleback (Pungitius pungitius) condition was significantly lower in lakes with Arctic charr (K = 0.58) than in lakes without Arctic charr (K = 0.64). Isotope data indicated that pre-smolt and resident Arctic charr may be prey for lake trout and compete with ninespine stickleback. Linear distance metrics applied to isotope data showed that food webs were more compact and isotopically redundant in lakes where Arctic charr were present. Despite this, lake trout populations in lakes with Arctic charr occupied a larger isotope space and showed greater inter-individual isotope differences. Anadromous Arctic charr appear to affect ecology and feeding of sympatric freshwater species, but effects are more subtle than those seen for semelparous anadromous species.     

Piscivory and cannibalism in Arctic charr

Piscivory and cannibalism in Arctic charr, Salvelinus alpinus , were studied in three lakes in northern Norway: Guolasjavri, which contains only charr, Takvatn, where Arctic charr coexist with three-spined sticklebacks, Gasterosteus aculeatus and brown trout, Salmo trutta , and Stuorajavri, where whitefish, Coregonus lavarelun dominate a fish community containing six species. The prevalence of piscivory in the Arctic charr populations generally increased with increasing predator size. In all three lakes, many charr larger than 20 cm were piscivorous, but the extent of piscivory and cannibalism varied. The greatest prevalence of cannibalism was found in Guolasjavri, where 27% of charr greater than 20 cm in length had fed upon smaller conspeciflcs. In Takvatn, 5% of charr larger than 20 cm were cannibalistic, and an additional 9% had eaten three-spined sticklebacks. In Stuorajavri, up to 74% of the charr greater than 20 cm had eaten whitefish but cannibalism was not recorded. The possible role of cannibalism in population regulation within Arctic charr populations is considered.     

Seasonal and ontogenetic variations in resource use by two sympatric Arctic charr morphs

The study compares the resource utilization of two sympatric Arctic charr morphs over an annual period in a subarctic lake. The two morphs are reproductively isolated in time and place of spawning, and are referred to as the littoral and profundal morphs (L-morph and P-morph) according to their spawning habitats. Fish were sampled monthly (ice-free season) or bimonthly (winter) using gillnets in the main lake habitats. The spatial range of the P-morph was restricted to the profundal zone throughout the whole annual period. The L-morph in contrast utilized all main habitats, exhibiting distinct seasonal and ontogenetic variations in habitat distribution. In the spring, the whole L-morph population was located along the bottom profile of the lake, in profundal and littoral habitats. During summer and autumn, habitat segregation occurred between different life-stages, juveniles mainly utilizing the profundal, pre-adults the pelagic and adult fishes the littoral zone. During winter the whole population was assembled in the littoral habitat. The L-morph also had large seasonal and ontogenetic variations in their feeding ecology, with littoral zoobenthos, zooplankton and surface insects being important prey. The P-morph had a narrower diet niche mainly consisting of chironomid larvae and other profundal zoobenthos. Hence, the two Arctic charr morphs exhibited a consistent resource differentiation during all annual seasons and throughout their life cycles, except for a dietary overlap between P-morph and juvenile L-morph charr in the profundal during summer. The findings are discussed in relation to resource polymorphism and incipient speciation.