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.     

Landlocked Arctic charr (Salvelinus alpinus) population structure and lake morphometry in Greenland – is there a connection?

Landlocked Arctic charr (Salvelinus alpinus) populations in sub-Arctic and Arctic Greenland lakes were sampled with multi-mesh-sized survey gillnets. The study covered a range of small shallow lakes (0.01 km², maximum depth <3.3 m) to large deep lakes (43 km², maximum depth >200 m). Arctic charr were found in one to three different forms in lakes with maximum depths >3 m. A dwarf form occurred in all lakes inhabited by Arctic charr and was the only form in lakes with maximum depths <8 m. In deeper lakes with maximum depths >20 m and a surface area <0.5 km², larger charr were found, although in low numbers, the length-frequency distribution being unimodal with a tail towards large sizes. In lakes with a maximum depth >20 m, large-sized charr were more abundant, and the length-frequency distribution of the population was bimodal, with a first mode around 10–12 cm and a second mode around 26–37 cm. In a single large and deep lake, a distinct medium-sized pelagic zooplankton-eating charr form occurred. Maximum size of individual charr was significantly positively correlated with lake maximum depth and volume, and the mean size of large-sized charr was significantly positively correlated with lake volume. Our study indicates that the charr population structure became more complex with increasing lake size. Moreover, the population structure seemed to be influenced by lake-water transparency and the presence or absence of three-spined stickleback (Gasterosteus aculeatus). Accepted: 31 January 2000     

Differentiation of deep-water lake charr <Emphasis Type="Italic">Salvelinus namaycush</Emphasis> in North American lakes

Deep-water morphs of lake charr, Salvelinus namaycush, are found, with one exception, in four of the largest lakes in the world: lakes Superior and Mistassini (QC) and Great Bear and Slave lakes. This paper advances a hypothesis for resource polymorphisms involving two types of deep-water morph, one of which is characteristic of the humper and the other of the siscowet charrs of Lake Superior. My hypothesis states that, first, the humper, or a humper-like morph, diverged postglacially in sympatry from the ancestral common (shallow-water) lake charr and became a feeding specialist on Mysis relicta. Second, in at least two of the four lakes the siscowet, or a siscowet-like charr, diverged as a feeding specialist on postglacially derived forms of deep-water ciscoes. In Lake Superior a successional process may have resulted in dominance of the siscowet at the expense of the humper charr. I concur with a previous inference that the one occurrence of a deep-water charr in a small lake (the above exception) represents emigration from Lake Superior. I further infer that this event involved an early humper charr, which implies that this morphotype had differentiated in Lake Superior in less than 1,900 year. I suggest that innate differences in plasticity, breeding behavior and assortive mating, and philopatry account for why Arctic charr isolate readily in small lakes whereas lake charr do not. My hypothesis assumes divergence of deep-water morphs occurred postglacially, an idea consistent with genetic and biogeographical evidence.     

Genetic differentiation of Arctic charr <Emphasis Type="Italic">Salvelinus alpinus</Emphasis> complex from Transbaikalia revealed by microsatellite markers

Variation at eight microsatellite loci was studied in Arctic charr Salvelinus alpinus complex from five Transbaikalian mountain lakes. Samples from three lakes included two sympatric charr forms (dwarf and small) differing in trophic specialization, morphology and life cycle parameters. Sympatric forms were genetically closer to each other than to charr from other lakes which evidences their independent origin in each of these lakes as the result of sympatric speciation. In each lake, gene pools of sympatric forms were segregated to a different degree (estimates of F _ST varying from 0.030–0.184 and those of ρ _ST varying from 0.119–0.359). Hierarchical analysis of allelic frequencies variance (AMOVA) in Arctic charr from Lake Baikal, the Vitim, and the Olekma basins showed that variation among and within these basins accounted for 19.5% of the interpopulational variance each. In the AMOVA design, investigating differences among sympatric forms in three lakes these differences accounted for 7.1% of the total variance.     

The impact of intra- and interspecific interactions on young-of-the-year brook charr, in temperate lakes

The abundance, growth, spatial distribution, and feeding habits of five allopatric brook charr, Salvelinus fontinalis, populations (young-of-the-year, 0+ juveniles; YOY) were compared with five other populations living sympatrically with white sucker, Catostomus commersoni. The study was made in oligotrophic lakes of the Laurentian Shield (Québec, Canada) during three sampling periods in 1989 (July, August and September). The abundance of YOY charr was significantly higher in allopatric than in sympatric populations (45·3 ± 3·8 vs 3·4 ± 3·8 fish/lake caught in 1773 m² of gillnets; P<0·005). The mean length of YOY charr did not differ among allopatric and sympatric populations at each sampling period; July: 60·2 ± 3·0 vs 60·0 ± 4·5 mm; August: 61·9 ± 4·5 vs 63·2 ± 4·1 mm; September: 77·9 ± 8·7 vs 77·3 ± 7·8 mm respectively. Horizontal distribution of allopatric YOY charr did not differ from that of sympatric charr, 65% of the fish being captured within the first 2 m depth and the rest between 2 and 7 m depth. In contrast, the vertical distribution of allopatric YOY charr from both communities was significantly different; 81% of allopatric charr were captured within 0·5 m from the substrate compared to 64% for sympatric charr (P<0·001). Differences in vertical distribution of the fish were related to differences in diet; allopatric charr fed mainly on benthic and large planktonic organisms whereas sympatric charr fed less on these organisms and more on terrestrial organisms. In the lake where YOY charr were most abundant, individuals were spatially segregated into two groups; one ‘littoral’, found in 0–2m depth, and one ‘profundal’, found in 3–6 m depth. Growth, condition, and feeding habits of charr from the two groups were different, especially during the last sampling period.     

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.     

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.     

Bird abundance and species richness on Florida lakes: influence of trophic status,lake morphology,and aquatic macrophytes

Data from 46 Florida lakes were used to examine relationships between bird abundance (numbers and biomass) and species richness, and lake trophic status, lake morphology and aquatic macrophyte abundance. Average annual bird numbers ranged from 7 to 800 birds km^–2 and bird biomass ranged from 1 to 465 kg km^–2. Total species richness ranged from 1 to 30 species per lake. Annual average bird numbers and biomass were positively correlated to lake trophic status as assessed by total phosphorus (r = 0.61), total nitrogen (r = 0.60) and chlorophyll a (r = 0.56) concentrations. Species richness was positively correlated to lake area (r = 0.86) and trophic status (r = 0.64 for total phosphorus concentrations). The percentage of the total annual phosphorus load contributed to 14 Florida lakes by bird populations was low averaging 2.4%. Bird populations using Florida lakes, therefore, do not significantly impact the trophic status of the lakes under natural situations, but lake trophic status is a major factor influencing bird abundance and species richness on lakes. Bird abundance and species richness were not significantly correlated to other lake morphology or aquatic macrophyte parameters after the effects of lake area and trophic status were accounted for using stepwise multiple regression. The lack of significant relations between annual average bird abundance and species richness and macrophyte abundance seems to be related to changes in bird species composition. Bird abundance and species richness remain relatively stable as macrophyte abundance increases, but birds that use open-water habitats (e.g., double-crested cormorant, Phalacrocorax auritus) are replaced by species that use macrophyte communities (e.g., ring-necked duck, Aythya collaris).     

The extinction of Arctodiaptomus alpinus (Copepoda) following the introduction of charr into a small alpine lake Dvojno Jezero (NW Slovenia)

In 1991 about twenty-five age 2+ specimens of the Arctic charr (Salvelinus alpinus (L.) (length between 100 and 130 mm) were introduced into the fishless small mountain lake Dvojno Jezero (= Twin Lake) (area 0.5+0.4 ha; max. depth 8 m, altitude 1670 m) in NW Slovenia. The first spawning of the Arctic charr was observed in autumn 1994 and regularly each year thereafter. Fish of different sizes were observed from 1995 onward each year but a fish census was never carried out. Autumn zooplankton samples, collected before the introduction of Arctic charr, contained adults (including ovigerous females) of two copepod species: Cyclops abyssorum tatricus and Arctodiaptomus alpinus. No planktonic Cladocera were found. Seven years after the introduction of fish standing crop of zooplankton declined 100-fold. The zooplankton contained only a few copepodites (IV and V) of C. a. tatricus. Simultaneously, water transparency declined and the concentration of chlorophyll a increased and filamentous green algae and picoplankton became abundant in the littoral zone of both lakes.     

Resource polymorphism and diversity of Arctic charr Salvelinus alpinus in a series of isolated lakes

Morphological, dietary and life‐history variation in Arctic charr Salvelinus alpinus were characterized from three geographically proximate, but isolated lakes and one large lake into which they drain in south‐western Alaska. Polymorphism was predicted to occur in the first three lakes because S. alpinus tend to become polymorphic in deep, isolated lakes with few co‐occurring species. Only one morph was evident in the large lake and two of the three isolated lakes. In the third isolated lake, Lower Tazimina Lake, small and large morphs were found, the latter including two forms differing in growth rate. The small morph additionally differed from the two large forms by having more gill rakers and a deeper body than same‐sized individuals of the large morph, consuming more limnetic and fewer benthic resources, having a greater gonado‐somatic index and maturing at a smaller size. The two large forms consumed only slightly different foods (more terrestrial insects were consumed by the medium‐growth form; more snails by the high‐growth form). Trends in consumption of resources with body shape also differed between lakes. Variability in life history of S. alpinus in these Alaskan lakes was as broad as that found elsewhere. This variability is important for understanding lake ecosystems of remote regions where this species is commonly dominant.     

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.     

Species richness of Protozoa in Japanese lakes

The protozoan fauna and species richness in the pelagic zone of 15 Japanese lakes were investigated in 1996 using polyurethane foam (PF) substrates. The most common species were flagellates, such as Cryptomonas erosa, Oikomonas termo, and Pleuromonas jaculans. Cinetochilum margaritaceum and Actinophrys sol were the most common species of the Ciliata and Sarcodina, respectively. The similarity of species occurrence was calculated from presence/absence data, but this revealed no clear trend with respect to the influence of lake properties such as trophic state, surface area, or mean depth. The occurrence pattern of Protozoa was most similar in L. Chuzenji and L. Biwa (north basin), two oligomesotrophic natural lakes. Log species richness was positively correlated with log total phosphorus (r = 0.54, P < 0.05) and negatively with log mean depth (r = −0.58, P < 0.05). The diversity index (Margalef's formula), highly correlated with the total species number (r = 0.85, P < 0.01), was negatively correlated with log lake area (r = −0.71, P < 0.01). The logarithm of Phytomastigophora number was positively correlated with log total nitrogen (r = 0.53, P < 0.05), and the logarithm of Ciliata number was negatively correlated with log lake area (r = −0.55, P < 0.05). The species richness of Protozoa on PF substrates was determined by both the nutrient status of the lake and the distance from the location of the suspended PF substrate to the lake bottom or shore. Received: September 25, 1999 / Accepted: January 6, 2000     

The land/inland-water ecotones and fish population of Lake Valley (West Mongolia)

New data on fish populations of a closed desert watershed of Mongolia were obtained in 1990 and 1991. For this region periodic droughts, with the accompanying disappearance of lakes and some parts of rivers, are typical. Two forms of a Cyprinid species Oreoleuciscus humilis (dwarf Altai osman) occur in this region during wet periods which usually last for 10-30 years. The dwarf form, is characterized by a maximum SL of 200 mm and early maturation (SL = 70 mm, four years of age). It inhabits small desert rivers in dry periods which last for 3–5 years and both rivers and the riparian zone of lakes during wet periods. The larger lake form occurs only in lakes during the wet periods. It can attain a maximum size of 450 mm and matures in six years, SL = 200 mm. These two forms of O. humilis differ in feeding habits, rates of growth, and morphology. The dwarf form feeds mainly on insect larvae and on plants. The lake form consumes the same food items until it reaches 180 mm SL and then becomes piscivorous. Populations of O. humilis in lakes are restored after a dry period, originating anew from river populations of the dwarf form.Currently there is a transition from a dry period to a wet one. Orog-Nur (one of the lakes of Lake Valley) has been filling with water since 1990. In July 1991 the depth of this lake reached 0.5–1.0 m and fish were found in the lake. The large individuals of dwarf form which came to the lake from the Tuyn-Gol River became cannibals, and their growth rate increased rapidly. The homogeneous environment and low food supply in the restored lakes are suggested to be the main causes of these phenomena.     

Feeding rate of slimy sculpin and burbot on young lake charr in laboratory reefs

Synopsis Predation and contaminants are two possible factors in the poor recruitment of young lake charr Salvelinus namaycush in the Great Lakes. We measured the feeding rate of slimy sculpins Cottus cognatus and burbot Lota lota on young lake charr (uncontaminated young from eggs of a hatchery brood stock and contaminated young from eggs of Lake Michigan lake charr) in laboratory test chambers with a cobble substrate. The median daily consumption rate of sculpins for all tests was 2 lake charr eggs (N = 22 tests; 95% confidence interval, 0–13) and 2 lake charr free embryos (N = 31 tests; 95% confidence interval, 0–10). Feeding rate did not differ between hatchery and contaminated prey. Slimy sculpins continued to feed on lake charr when another prey organism, the deepwater amphipod Pontoporeia hoyi, was present. Feeding by burbot on free embryos (4–36 d^–1) increased as the mobility of young increased, but burbot consumed about 10% of their body weight weekly in free-swimming young (140–380 d^–1). Predation on lake charr eggs by sculpins could be considerable over the 100 to 140 d incubation period, and burbot could eat large numbers of free-swimming lake charr as the young fish left the reef. Predation pressure on young lake charr may inhibit rehabilitation of self-sustaining populations of lake charr on some reefs unless a critical egg density has been reached.     

Pyloric caecal morphology of brook charr,Salvelinus fontinalis,in relation to diet

Synopsis Brook charr, Salvelinus fontinalis, shifts its diet from zoobenthos to pelagic prey when living sympatrically with white sucker, Catostomus commersoni, in lakes of the Laurentian Shield. We tested the hypothesis that this diet difference would have a significant impact on their pyloric caecal morphology in 5 lakes containing allopatric brook charr populations and 6 other lakes containing both brook charr and white sucker. We observed that the mean length of the most posterior caecum of charr was significantly greater in sympatry than in allopatry (X ± 1 SD: 9.91 ± 1.12 mm versus 8.44 ± 0.67 mm). This is equivalent to an increase of 18% of total pyloric caecal mass (dry weight) in sympatric brook charr. These results indicate that this response to differences in diet, well known in birds, also occurs in fish.     

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.     

Composition,size, and biomass of zooplankton in large productive Florida lakes

Crustacean zooplankton data were compiled from long-term observational studies at seven large shallow Florida lakes, to determine whether there are general characteristics in regard to species composition, body size, and biomass. In particular, we examined whether patterns in body size and species richness fit empirical models developed by Stanley Dodson. The lakes included range in size from 125 to 1730 km² and encompass mesotrophic to hyper-eutrophic conditions. We found that zooplankton biomass was strongly dominated by one species of calanoid copepod—Arctodiaptomus dorsalis. Large daphnids were absent, and Cladocera assemblages were dominated by small taxa such as Ceriodaphnia, Chydorus, and Eubosmina. The total number of species of pelagic cladocerans (8–12) was consistent with Dodson’s predictions based on lake area. The average size of crustacean zooplankton in Florida lakes is small in comparison with temperate communities. A. dorsalis is the smallest calanoid copepod in North America, and the mean length of Cladocera (0.6 mm) is consistent with Dodson’s results that size decreases from temperate to tropical zones. Total biomass of crustacean zooplankton was very low, ratios of zooplankton to phytoplankton biomass (0.01–0.1) are among the lowest reported in the literature, and the zooplankton displayed short-lasting early spring peaks in biomass. Cladocera were almost entirely absent in spring and summer. Factors known to occur in Florida lakes, which appear to explain these characteristics of biomass, include intense fish predation and high summer water temperature.     

Parallel divergence of sympatric genetic and body size forms of Arctic charr,Salvelinus alpinus,from two Scottish lakes

F _ST and R_ST estimates for Arctic charr from six microsatelite markers collected from two neighbouring Scottish lakes, Loch Maree and Loch Stack, confirm the presence of two distinct genetic groupings representing separate populations within each lake. In both lakes, there was also a clear body size dimorphism, with large and small body size forms that segregated according to genetic grouping. There was evidence of only subtle foraging ecology differences between morphs, with the small body size morph in both lakes being more generalist in its foraging in the summer (consuming mostly plankton but also some macrobenthos) than the large body size morph, which specialized on planktonic prey. Trophic morphology (head and mouth shape) did not differ significantly between morphs (although the small sample size for Maree makes this a preliminary finding). Cluster analysis of the microsatelite data and the presence of private alleles showed that morphologically similar forms in different lakes were not genetically similar, as would be expected under a multiple invasion hypothesis. Thus, the data do not support a hypothesis of a dual invasion of both lakes by two common ancestors but instead suggest an independent origin of the two forms in each lake. Thus parallel sympatric divergence as a result of common selection pressures in both lakes is the most parsimonious explanation of the evolutionary origin of these polymorphisms. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 748–757.     

Diversification, Sympatric Speciation, and Trophic Polymorphism of Arctic Charr, Salvelinus Alpinus Complex, in Transbaikalia

In northern Transbaikalia, independently evolving landlocked populations of Arctic charr are found in mountain lakes. To assess the diversity of charr in this region, speciation modes involved in the evolution of charr forms, and the role of trophic polymorphism in their divergence, we studied the morphology and feeding of dwarf, small, and large forms of Arctic charr from a number of Transbaikalian lakes. Meristic data on charr from five lakes support the earlier conclusion that the three forms do not represent separate lineages but have independently diverged in sympatry in each of the lakes. In 10 lakes, the dwarf form showed varying degrees of differentiation from normal (small and large) charr in meristic characters (up to morphologically distinct and presumably reproductively isolated groupings), which is viewed as various levels of sympatric divergence. Indexes of gill raker length in fish from 20 lakes vary among populations of both dwarf and normal charr, with forms having short and long rakers being sympatric in some of these lakes. However, the index can be used only for comparing charr of different forms up to about 32cm fork length (FL) because it is strongly negatively correlated with size in larger fish. The study of charr diets in 21 lakes indicates that large charr are piscivorous whereas dwarf and small charr feed on a wide range of invertebrates, partitioning these resources in different ways. Planktivores, including very specialized ones, and non-planktivores (benthic feeders, insectivores) can be identified within the small and dwarf forms. The proportion of plankton in the diets of dwarf and small charr is positively correlated with the number and length of gill rakers while the proportion of benthos is negatively correlated. Allopatric planktivorous and non-planktivorous small charr differ in body proportions; parallel emergence of such morphotypes in different parts of the range is a characteristic feature of the Salvelinus alpinus complex.     

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.