Temporal variability in marine feeding of sympatric Arctic charr and sea trout

The marine feeding pattern of anadromous brown trout (sea trout) Salmo trutta and Arctic charr Salvelinus alpinus was studied during June to August in 1992–1993 and 2000–2004 in a fjord in northern Norway. In general sea trout fed proportionally more on fishes than on crustaceans and insects (81, 1 and 18% by mass, respectively) by comparison with Arctic charr (52, 25 and 22% by mass, respectively). Herring Clupea harengus dominated the total fish diet of both species, but the Arctic charr also fed significantly on gadoids and sandlance Ammodytes spp. While sea trout became virtually all piscivorous at fork lengths ( L _F) ≥250 mm, the Arctic charr was ≥400 mm L _F before shifting totally to a fish diet. Despite annual variation in diet and forage ratios, there was a clear shift in diet from 1992–1993 to 2000–2004. Sandlance and different crustaceans constituted most of the diet during the initial period with a shift towards gadoids and especially herring during the latter period. This shift seemed to be associated with a high abundance of herring larvae during the latter sampling period, indicating a preferential selection on herring when present, particularly by sea trout. Furthermore, an index indicated dietary overlap in years with intensive feeding on herring of both species, and usually differences in the trophic ecology during years feeding mostly on other prey species. In combination, it was hypothesized that the two species reflect the type of marine prey present within a fjord system over time, and therefore provide an index of variation in the production and biological diversity of their potential prey within fjords.     

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.     

Extensive genetic differentiation between recently evolved sympatric Arctic charr morphs

The availability of diverse ecological niches can promote adaptation of trophic specializations and related traits, as has been repeatedly observed in evolutionary radiations of freshwater fish. The role of genetics, environment, and history in ecologically driven divergence and adaptation, can be studied on adaptive radiations or populations showing ecological polymorphism. Salmonids, especially the Salvelinus genus, are renowned for both phenotypic diversity and polymorphism. Arctic charr (Salvelinus alpinus) invaded Icelandic streams during the glacial retreat (about 10,000 years ago) and exhibits many instances of sympatric polymorphism. Particularly, well studied are the four morphs in Lake Þingvallavatn in Iceland. The small benthic (SB), large benthic (LB), planktivorous (PL), and piscivorous (PI) charr differ in many regards, including size, form, and life history traits. To investigate relatedness and genomic differentiation between morphs, we identified variable sites from RNA‐sequencing data from three of those morphs and verified 22 variants in population samples. The data reveal genetic differences between the morphs, with the two benthic morphs being more similar and the PL‐charr more genetically different. The markers with high differentiation map to all linkage groups, suggesting ancient and pervasive genetic separation of these three morphs. Furthermore, GO analyses suggest differences in collagen metabolism, odontogenesis, and sensory systems between PL‐charr and the benthic morphs. Genotyping in population samples from all four morphs confirms the genetic separation and indicates that the PI‐charr are less genetically distinct than the other three morphs. The genetic separation of the other three morphs indicates certain degree of reproductive isolation. The extent of gene flow between the morphs and the nature of reproductive barriers between them remain to be elucidated.     

Variation in scale shape among alternative sympatric phenotypes of Arctic charr Salvelinus alpinus from two lakes in Scotland

Landmark‐based geometric morphometric analysis was used to detect differences in scale shape between ecologically distinct phenotypes of Arctic charr Salvelinus alpinus coexisting in the same lake. Relative warp analysis and standard multivariate analyses of the partial warps, obtained after a Procrustes superimposition, showed that scale landmarks were efficient in discriminating among two closely related alternative phenotypes within each of the two lakes. In Loch Tay, S. alpinus exhibited a bimodal body size‐frequency distribution among sexually mature fish, whereas in Loch Awe, S. alpinus are unimodal in body size but segregated into two distinct spawning phenotypes. In both lakes, alternative phenotypes showed significant differences in foraging ecology, habitat use and life history. It is probable that differences in scale shape reflect differences in ecology of these forms.     

Contrasting life history strategies of sympatric Arctic charr morphs, Salvelinus alpinus

In polymorphic populations morphs usually diverge in morphology, ecology and life history, which is most likely driven by adaptations to different environments or resources. Sympatric morphs may develop differences in several life history traits to be able to maximize fitness in alternative niches and habitats. Here, the contrasting life history traits of three sympatric Arctic charr (Salvelinus alpinus (L.)) morphs in a deep and oligotrophic lake in sub-arctic Norway are addressed. The charr morphs differ in spawning habitat and trophic niche. One is a littoral spawning morph that feeds on benthic invertebrates and zooplankton in the littoral and pelagic zones (referred to as the LO-morph), and two other are profundal spawning morphs that either utilize profundal soft bottom benthos as food resource (the PB-morph) or are piscivorous (the PP-morph). The LO-morph typically had intermediate life-history traits relative to the two profundal morphs that had highly contrasting life history traits, especially in growth and age and size of maturity. The PB-morph matured at a young age (～3 years) and at a small body size (～8.5 cm), thereby increasing their fitness by investing in reproduction early in life, which results in a short generation time and decreased probability of being predated before first reproduction. The PP-morph on the other hand, matured at an old age (～9.2 years) and a large body size (～26 cm), thereby increasing their fitness by investing in somatic growth to enhance initial fecundity, and also to reach a large body size profitable for piscivory. The different trade-off regime between the PP- and PB-morphs seems to be caused by adaptation to alternative trophic niches, and appears to be an important factor for the co-occurrence of the two sister-morphs in the profundal zone.     

Marine trophic niche use and life history diversity among Arctic charr Salvelinus alpinus in southwestern Greenland

Life history strategies and potential marine niche use of Arctic charr Salvelinus alpinus (n = 237, 84–652 mm, total body length, L_T) were determined during the ice-free season (2012) at three different watercourses in south-western Greenland. All Arctic charr were collected from freshwater habitats. Based on stable isotopes of δ³⁴S, the Arctic charr were categorized as either marine- or freshwater-dependent feeders. The use of time-integrated trophic tracers (stable isotopes of δ¹³C, δ¹⁵N, δ³⁴S) suggested that several trophic groups of Arctic charr operate alongside within each fjord system. The groups suggested were one group that specialized in the marine habitat, in addition to two freshwater resident morphs (small-sized resident and/or large-growing cannibalistic individuals). Stomach contents consisted entirely of freshwater and terrestrial prey (i.e., insects), indicating that marine-dependent feeders also fed in freshwater habitats after return from their marine migration. Growth and maturity patterns further supported variable life history strategies within each watercourse. The life history strategy patterns and marine trophic niche use were consistent across the watercourses along several hundred kilometres of coastline. This study represents the first ecological baseline for partially anadromous populations of Greenland Arctic charr.     

Genetic differences between two sympatric morphs of Arctic charr confirmed by microsatellite DNA

Significant genetic differences ( F _ST = 0·032) were found between littoral and profundal morphs of Arctic charr Salvelinus alpinus from Fjellfrøsvatn, northern Norway, using microsatellite DNA analysis. The morphs had strong reproductive isolation in time and space; the segregation of a separate profundal morph is rare in postglacial lakes.     

Evidence for genetic differences in the offspring of two sympatric morphs of Arctic charr

The sub-arctic Lake Fjellfrøsvatn, northern Norway, has two morphs of Arctic charr that are reproductively isolated because they spawn 5 months apart. The smaller morph (≤14 cm L_F ) is confined to the profundal zone of the lake and the larger morph is mainly littoral. Three hypotheses were tested: (i) the offspring of the profundal Arctic charr grow slower than the offspring of the littoral Arctic charr under identical conditions, thus indicating a genetic basis for the slow growth of the profundal Arctic charr in the wild; (ii) the wild phenotypes of the two morphs are morphometrically different and the differences are persistent in the offspring; (iii) the offspring of the two morphs have different behaviour traits under similar treatments. The first hypothesis was rejected; offspring of the profundal morph grew slightly better than offspring of the littoral morph at 10° C in the laboratory. The second and third hypotheses were supported by the data. Wild-caught fish of the two morphs differed in several morphometric characters and most of the differences persisted in the offspring. In the laboratory, offspring of the littoral morph were more active, more aggressive and more pelagic than offspring of the profundal morph and naive offspring of the profundal morph were more effective in eating live chironomid larvae than were offspring of the littoral morph. The data for morphometry and behaviour, but not growth, provide evidence for genetic differences between the two Arctic charr morphs of Fjellfrøsvatn.     

Reproductive strategies in two sympatric morphotypes of Arctic charr in Kalandsvatnet, west Norway

Differences in gonadal development and time of spawning were found in two coexisting morphotypes of Arctic charr Salvelinus alpinus in Kalandsvatnet, Norway. Spawning depth and some main life‐history variables also differed. These observations suggest that the two morphotypes do not interbreed freely.     

Introduction of Mysis relicta (Mysida) reduces niche segregation between deep-water Arctic charr morphs

Hydrobiologia - Understanding the concordance between aquatic assemblages in ecological assessments and their responses to human-induced disturbances are fundamental steps toward achieving...     

Variation in parasite resistance of Arctic charr,Salvelinus alpinus,between and within sympatric morphs

Genetic variation in resistance against parasite infections is a predominant feature in host–parasite systems. However, mechanisms maintaining genetic polymorphism in resistance in natural host populations are generally poorly known. We explored whether differences in natural infection pressure between resource‐based morphs of Arctic charr (Salvelinus alpinus) have resulted in differentiation in resistance profiles. We experimentally exposed offspring of two morphs from Lake Þingvallavatn (Iceland), the pelagic planktivorous charr (“murta”) and the large benthivorous charr (“kuðungableikja”), to their common parasite, eye fluke Diplostomum baeri, infecting the eye humor. We found that there were no differences in resistance between the morphs, but clear differences among families within each morph. Moreover, we found suggestive evidence of resistance of offspring within families being positively correlated with the parasite load of the father, but not with that of the mother. Our results suggest that the inherited basis of parasite resistance in this system is likely to be related to variation among host individuals within each morph rather than ecological factors driving divergent resistance profiles at morph level. Overall, this may have implications for evolution of resistance through processes such as sexual selection.     

Genetic analysis of four sympatric morphs of Arctic charr,Salvelinus alpinus,from Thingvallavatn,Iceland

Synopsis The degree of genetic differentiation among four morphs of Arctic charr (small benthivorous, large benthivorous, piscivorous, and planktivorous) from Thingvallavatn, Iceland, was determined electrophoretically. Five of 36 enzyme loci were found to be polymorphic (Est2, Gpi3, Ldh4, Mdh4, 5 and Pgm2). However, only Est2 and Mdh4,5 showed enough variability to permit statistical analysis of divergence among morphs. All four morphs are very closely related; the values of Nei's (D) range from 0.00004 to 0.00126. These morphs are conspecific and do not represent different evolutionary lineages. There is significant genetic differentiation between the small benthivorous charr and the other three morphs. The relative relatedness of morphs based on gene frequency data is only partially concordant with that based on morphology and ecological specialization. The biological significance of this result is unclear because of the limited number of polymorphic loci upon which the genetic analysis is based and the high degree of relatedness among morphs.     

The functional significance of inherited differences in feeding morphology in a sympatric polymorphic population of Arctic charr

Artificially fertilised eggs from wild-caught Arctic charr parents of two sympatric morphs (benthivorous and planktivorous) from Loch Rannoch, Scotland were reared in the laboratory under identical conditions. During the subsequent 2 years, aspects of their trophic anatomy and feeding behaviour were compared. As previously described for wild-caught fish, charr derived from the benthivorous morph had an increasingly wider mouth gape for a given body length than those derived from the planktivorous morph. The functional significance of these differences in gape was tested by comparing the maximum size of prey that could be handled by each of the two morphs. In both forms, a larger gape enabled larger food particles to be eaten, but the elevation of the regression of maximum prey size on gape was higher in the benthivorous form, indicating the existence of additional morphological and/or behavioural differences influencing the size of prey consumed. When offered a choice between a typical benthic prey item and a typical pelagic food item, charr of benthivorous origin were more likely to feed on the former, whereas those of planktivorous origin were more likely to feed on the latter. Thus inherited differences in gape place constraints on foraging ability and are associated with inherited differences in dietary preference. We conclude that the functional significance of the foraging specialisations indicate a strong selection pressure for the evolution of the divergence and propose that heterochronic growth is the mechanism resulting in the divergence of tropic anatomy.     

Exploring trophic niches and parasite communities of sympatric Arctic charr and brown trout populations of southern Norway

Hydrobiologia - Catchment-scale variation between lake habitats has the potential to simultaneously influence the trophic niche and parasite community of fish hosts. In this study, we investigated...     

Contrasting patterns in trophic niche evolution of polymorphic Arctic charr populations in two subarctic Norwegian lakes

Hydrobiologia - Lipid biomarkers in sediments, which are indicative of biological production, provide important information regarding the environmental conditions in and around lakes, and can be...     

Incipient speciation through niche expansion: an example from the Arctic charr in a subarctic lake

Two reproductive isolated morphs of Arctic charr (Salvelinus alpinus), termed profundal and littoral charr according to their different spawning habitats, co-occur in the postglacial lake Fjellfr?svatn in North Norway. All profundal charr live in deep water their entire life and have a maximum size of 14cm, while the littoral charr grow to 40cm. Some small and young littoral charr move to the profundal zone in an ontogenetic habitat shift in the ice-free season and the rest of the population remains in epilimnic waters. The two morphs had different diet niches in the profundal zone: the profundal charr ate typical soft-bottom prey (chironomid larvae, pea mussels and benthic copepods), while the young littoral charr mainly consumed crustacean zooplankton. In four other lakes without a profundal morph (i.e. monomorphic populations), young charr also performed ontogenetic habitat shifts to the profundal zone and fed on zooplankton. The profundal morph of Fjellfr?svatn therefore utilize a food resource niche that neither the littoral morph nor comparable monomorphic populations exploit. This suggests that intraspecific resource competition has driven incipient ecological speciation of the profundal charr of Fjellfr?svatn. The exploitation of the soft-bottom resources by the profundal charr supports earlier experimental findings that the profundal morph is genetically different in trophic behaviour and morphology. The sympatric ecological divergence within the profundal habitat is possible because unexploited food resources (soft-bottom profundal prey) are available. Apparently, this represents a case of incipient segregation by expansion to new resource types (niche invasion), and not by subdivision of one broad ancestral niche.     

The genetics of sympatric Arctic charr [Salvelinus alpinus (L.)] populations from Loch Rannoch, Scotland

Two morphologically and ecologically distinct forms of Arctic charr, Salvelinus alpinus , are found in Loch Rannoch, Scotland. The differences in morphology are adaptations to different modes of life, one being pelagic, the other benthic. Both forms have been the subjects of extensive genetic studies including cytogenetics, nuclear and mitochondrial DNA analysis, and protein electrophoresis. Significant differences between the two forms are revealed by some techniques but not others and provide evidence for the reproductive isolation of these two morphs. The findings are discussed in relation to the derivation of sympatry and the phylogenetics of Arctic charr.     

Environmental factors associated with genetic and phenotypic divergence among sympatric populations of Arctic charr (Salvelinus alpinus)

The mechanisms by which phenotypic and genetic divergence may occur among sympatric, conspecific populations have been widely discussed but are still not well understood. Possible mechanisms include assortative mating based on morphology or variation in the reproductive behaviour of phenotypes, and both have been suggested to be relevant to the differentiation of salmonid populations in post-glacial lakes. Here, we studied Arctic charr (Salvelinus alpinus) in Windermere, where putative populations are defined by spatial and temporal variation in spawning. Genetic differentiation was assessed based on nine microsatellite loci, and phenotypic variation was assessed from morphometric characters. We test hypotheses about the relative role of morphology, spawning season and spawning habitat in the evolution of genetic divergence among these populations. Distinct from other lake systems, we find that both morphological and genetic differentiation are restricted primarily to one of two interconnecting basins, that genetic and morphological differentiation are decoupled in this lake and that both phenotype and environment have changed over the last 20 years. The implication is that breeding habitat plays a primary role in isolating populations that differentiate by drift and that phenotypically plastic changes, potentially related to foraging specializations, have either become secondarily decoupled from the genetically defined populations or were never fundamental in driving the evolution of genetic diversity in the Windermere system.     

Vaccination influences growth of Arctic charr

There is limited knowledge about the effects of oil-based vaccines on the growth of Arctic charr Salvelinus alpinus, in particular at different rearing temperatures. One-year-old Arctic charr were immunized intraperitoneally at 2.9 degrees C with a metabolizable oil-adjuvanted, bivalent vaccine containing killed typical and atypical Aeromonas salmonicida bacteria. After vaccination the non-vaccinated (controls) and vaccinated individually marked fish were held for 20 d at 10.0 degrees C and then for 7 wk at 10.3, 14.1 or 18.1 degrees C. During the first 20 d at 10.0 degrees C the growth rate (G) was higher for non-vaccinated than vaccinated fish. Thereafter vaccinated charr had higher G than control fish at 10.3 and 14.1 degrees C. In contrast, at 18.1 degrees C there was no difference in G and therefore no compensation of earlier growth suppression in vaccinated fish was observed at that temperature. The study indicates that vaccination has no ultimate negative effects on the growth of Arctic charr at temperatures ranging from 10.3 to 14.1 degrees C.