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Arctic charr (Salvelinus alpinus): a case study ofthe importance of understanding biodiversity andtaxonomic issues in northern fishesJames D. Reist a , Michael Power b & J. Brian Dempson ca Fisheries & Oceans Canada, Central & Arctic Region, Winnipeg MB, R3T 2N6, Canadab Department of Biology, University of Waterloo, Waterloo ON, N2L 3G1, Canadac Fisheries and Oceans Canada, St. John's, Newfoundland and Labrador, A1C 5X1, CanadaVersion of record first published: 10 Oct 2012.

To cite this article: James D. Reist , Michael Power & J. Brian Dempson (2013): Arctic charr (Salvelinus alpinus): a case studyof the importance of understanding biodiversity and taxonomic issues in northern fishes, Biodiversity, 14:1, 45-56

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B I O D I V E R S I T Y

Arctic charr (Salvelinus alpinus): a case study of the importance of understanding biodiversity

and taxonomic issues in northern fishes

James D. Reista*, Michael Powerb and J. Brian Dempsonc

aFisheries & Oceans Canada, Central & Arctic Region, Winnipeg MB, R3T 2N6, Canada; bDepartment of Biology,University of Waterloo, Waterloo ON, N2L 3G1, Canada; cFisheries and Oceans Canada, St. John’s,

Newfoundland and Labrador, A1C 5X1, Canada

(Received 20 June 2012; final version received 27 August 2012)

Charrs, fishes of the genus Salvelinus (Salmonidae), comprise about 7–20þ species important in the ecosystems andfisheries of Arctic and sub-Arctic waters. Charrs vary by life-history type (anadromous vs. lacustrine), are noted forvariability in morphology, colouration and feeding and habitat use across their range, and occur from temperateregions to the northernmost areas of land. Charr diversity is manifested as an often confusing array of taxonomicand functional diversity. Components of taxonomic diversity include unresolved species’ complexes, subspecies,glacial lineages and genetic stocks. Features associated with functional diversity include semi-distinct life historyvariants and differential habitat and feeding tactics resulting in distinctive growth forms. Complex periglacialhistories, allopatric and sympatric diversification processes, founder effects, hybridisation and introgression havecontributed to the observed variability and complexity. The categorisation of taxonomic and functional diversityand understanding their roles, drivers and vulnerabilities underpins conservation and management efforts for taxaand their ecosystems. In that context, Arctic charr is an excellent example of diversity and the many levels ofdifficulties associated with determining the significance of diversity. Despite recently increased scientific efforts,resolving the nature and importance of charr diversity remains both a challenge and a priority because rapidanthropogenically driven change, especially climate change, is altering ecosystems and biodiversity faster thandiversity can be inventoried, its importance understood and protective measures developed.

Keywords: allopatry; sympatry; morphotype; ecological differentiation; climate change

Introduction

Approximately five North American and about

22 Eurasian species of charrs (salmonid fishes of the

genus Salvelinus) are recognised using standard taxon-

omy (Behnke 1980, 1984, 2002; Maitland 1995;

Chereshnev 2003). As a group, charrs exhibit a wide

range of variation in morphology, life history, ecology

and habitat usage. Accordingly, some authors recog-

nise many forms as distinct, particularly in Europe (e.g.

Kottelat and Freyhof 2007), and assign species-level

status to forms demonstrating any level of morpho-

logical distinctness and/or limited spatial distribution

(e.g. 32 taxa recognised for Europe by Kottelat and

Freyhof, 2007). In contrast, other taxonomists group

charrs into what is known by biologists as a ‘complex’,

a result of the observed variation in morphological

characteristics and life-history traits (e.g. Maitland

1995). Within the broader classification of charrs, the

Arctic charr (S. alpinus) group is particularly noted for

the range of taxonomic and functional diversity (e.g.

Jonsson and Jonsson 2001; Klemetsen 2010), present-

ing a sometimes bewildering array of phenotypic and

ecological diversity at both local and regional scales

throughout its distributional range (Figure 1). The

different forms may sometimes exhibit differences in

feeding ecology (Snorrason et al. 1994) and, when co-

occurring in the same lake, the forms may be repro-

ductively isolated (e.g. Frost 1965; Hesthagen, Hindar,

and Jonsson 1995). The range of observed variation

raises the question as to whether the different forms

represent true species in the context of the biological

species concept (Mayr 1963) or transient forms of

unresolved systematic status. Although overtly aca-

demic, the question of what represents a species is

particularly germane to any discussion of diversity

because of the implication the answer holds for

standard measurements of diversity (e.g. alpha, beta

and gamma). Accordingly, Arctic charr serve as a

useful example for discussions concerning biodiversity

in the context of rapidly changing northern environ-

ments because they present both definitional and

management challenges.To better understand diversity in Arctic charr it is

necessary to understand both the nature and the

*Corresponding author. Email: jim.reist@dfo-mpo.gc.ca

� 201 Crown

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, 2 0 1 3Vol. 14, No. 1, 45–56,

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determinants of the differences among and withinArctic charr populations. To that end we describe thegeneralist nature of Arctic charr as it occurs through-out its distributional range and the importance ofecology for determining the observed differenceswithin and among Arctic charr populations. Finally,we end with a discussion of when a variant becomes aspecies and discuss the challenges the answer presentsfor understanding diversity in the context of climatechange.

Arctic charr: the ecological generalist

Arctic charr are distributed throughout the Holarcticin fresh water (mostly lakes) and near-shore marinewaters ranging from the southern fringes of temperateareas in eastern North America and the Alps inEurope, northwards to the northernmost extent ofland in Eurasia and North America (�84�N) (Johnson1980). Southerly limits in fresh water are extended bycold waters (e.g. profundal zones of deep lakes)particularly in alpine areas of central and westernEurope (Figure 2), likely as glacial relicts.

Arctic charr is generally considered a habitatgeneralist and occurs in a wide variety of aquatichabitats (lakes, streams, rivers and the sea) across itsdistributional range. Arctic charr can be anadromousand reach large sizes as a result of repeated summermarine feeding forays. Alternatively, they may

complete the entire life-cycle within fresh water wherethey have been documented to vary significantly in sizeand body morphology (Power, Reist, and Dempson2008). Within lakes, Arctic charr will use all habitattypes (e.g. pelagic, littoral, profundal), with usagegenerally dependent on co-occurring species in the lakeand season of the year. Flexibility in habitat usageextends to include flexibility in spawning site require-ments. Thus, while Arctic charr probably evolved as alake spawner, the species uses both streams and riversand has relied on its reproductive flexibility andopportunism to spread more quickly and widely inthe post-glacial period than competitor species(Power 2002).

Morphological variation within and among Arcticcharr populations has been described extensively in theliterature, generally from European lakes (e.g.Henricson and Nyman 1976; Klemetsen and Grotnes1980; Hindar and Jonsson 1982; Snorrason et al. 1994;Pavlov 1997; Jonsson and Jonsson 2001; Klemetsenet al. 2003; Klemetsen 2010) but increasingly fromCanadian lakes (e.g. Reist et al. 1995; O’Connell andDempson 2002; Power et al. 2009). Frequently, mor-phological variation is associated with a functionaldifference in foraging ecology (e.g. Fraser, Adams, andHuntingford 1998; Guiguer et al. 2002; Klemetsenet al. 2006). For example, there is often a small,epibenthic form that feeds on zoobenthos and alarge, pelagic form that feeds on zooplankton(e.g. Lake Vangsvatnet, Hindar and Jonsson 1982)

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Figure 1. Examples of diversity within Arctic charr in North America: (a) Anderson Lake, central Canadian Arctic, anadromousmale, 730mm FL (fork length); (b) North Lake, Cornwallis Island, central Canadian Arctic, landlocked male cannibal, 590mmFL; (c) 12 Mile Lake, Cornwallis Island, landlocked dwarf, 350mm FL; (d) North Baffin Island, resident, 248mm FL.Illustrations by P. Vescei.

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(see Figures 1 and 3). A larger piscivorous form mayalso exist that predates on the other forms (Jonssonand Jonsson 2001). Varying degrees of reproductiveseparation exist among sympatric morphotypes, rang-ing from a high degree of interbreeding to completeisolation.

Genetic studies have indicated that in most casesthe forms are repeatedly derived from a single originaltype in the lake, with form occurrence likely dependingon ecological conditions such as the number ofavailable niches (Klemetsen et al. 2003). However,examples from Gander Lake, Canada (Gomez-Uchidaet al. 2008) and Lake Aigneau (Power et al. 2009),suggest repeated invasions of allopatrically derivedcolonisers may be responsible for documented mor-phological and habitat usage differences in some lakes.Thus, both sympatric and allopatric processes ofdiversification intermix in some situations to add

further complications to delineating causation.Morphological diversity in Arctic charr is thought tobe adaptive (Skulason and Smith 1995). Where dis-tinctive forms exist there is often evidence of anecological basis for its occurrence, with morphologi-cally specialised forms appearing to feed more effec-tively than intermediate forms (e.g. Guiguer et al. 2002;Adams et al. 2003). Studies that point to a genetic basisfor such differences are generally lacking, however,recent evidence suggests variable gene expression mayunderlie the capacity for growth variation in someinstances (MacQueen et al. 2011).

Although diversity is often discussed and demon-strated in the context of lacustrine populations,diversity is also evident within and among anadromouspopulations, most notably in the variation in individ-ual life-history tactics. Within some populations, aportion of the population will remain as lake residents

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Figure 2. General distribution of Arctic charr in the Holarctic (redrawn and corrected from Reist and Sawatzky 2010). Dottedmargins indicate uncertain distributional limits.

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(Johnson 1980) with consequent effects on size, age-at-maturity, reproductive abilities and body form follow-ing as a result of differential access to prey resources(Power, Reist, and Dempson 2008). The resultingmorphological and meristic variation among popula-tions in this instance is often less obvious as a result oflake-resident and anadromous reproductive mixing.Nevertheless, differing propensities for anadromyamong life-history variants result in significant impli-cations for overall population vulnerability where freshand marine habitats are differentially affected byenvironmental stressors such as climate change.

Diversity within Arctic charr populations is notconfined to morphological variation. Many lacustrineArctic charr populations exhibit size polymorphisms inthe form of bimodal length-frequency distributions(Parker and Johnson 1991; Johnson 1995) where only a

few individuals will grow to a very large size. The sizedifferences are typically driven by the occurrence ofpiscivory, notably cannibalism that is thought to playan important role in population regulation (Amundsen1994; Griffiths 1994; Svenning and Borgstrom 1995;Hammar 2000). For example, where cannibalism existsArctic charr have a top-down structuring effect onpopulation size-structure, with most fish reaching onlymoderate sizes due to limited food availability andincreased intra-specific competition.

Evidence of biological origin suggests Arctic charr,and charr in general, evolved in a cold periglacialenvironment and have been selected for life in cold,nutrient impoverished and highly variable environ-ments. The lack of a fixed reproductive cycle, variablewithin and between season patterns of somatic growth,longevity, diet flexibility, lack of fixed migratory

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Figure 3. Arctic charr ecophenotypes from Lake Hazen, northern Ellesmere Island, Canadian Arctic. (a) Largeform (presumptive piscivore) – 650mm FL; (b) Small form – 419mm FL; (c) possible dwarf form – 250mm FL. Illustrationsby P. Vescei.

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patterns either within or between habitat types, and thedevelopment of exploratory behaviour patterns asso-ciated with regular movements between differingnesting, overwintering, nursery and feeding habitats(Power 2002) have all facilitated survival and propa-gation. Arctic charr have thus been exposed to agreater range of natural selection forces (e.g. temper-ature and nutrient gradients) than most other northernfish species. Accordingly, the diversity observed withinand among Arctic charr populations reflects both theadaptive ability of the species and the range ofconditions to which Arctic charr have successfullyadapted over time. Geologically young, recently degla-ciated habitats (i.e. 510,000 years) or species-poorecosystems are generally thought to facilitate poly-morphic development in Arctic charr and a growingnumber of studies have suggested that relaxation ofinter-specific competition and open niche availabilityare essential prerequisites (Skulason and Smith 1995).Deep lakes with separate littoral, pelagic and profun-dal zones offering unique foraging opportunities arecharacteristic of systems where Arctic charr morpho-types have been confirmed (Jonsson and Jonsson 2001;Kristjansson et al. 2011).

Contemporary scale diversity: the importance of

ecology

As generalists, Arctic charr are known to display aconsiderable amount of dietary variability, much of itassociated with morphological variation. Adaptationsto utilise a specific food resource linked to distinctivemorphologies within a population are referred to astrophic polymorphisms and for Arctic charr have beendemonstrated extensively in the literature (e.g.Klemetsen and Grotnes 1980; Hindar and Jonsson1982; Walker, Greer, and Gardner 1988; Snorrasonet al. 1994; Pavlov 1997; Jonsson and Skulason 2000;Adams et al. 2003; Klemetsen et al. 2003), includingNorth America (Guiguer et al. 2002; Power,O’Connell, and Dempson 2005; Power et al. 2009).Where distinctive feeding forms exist, there is often asmall, epibenthic form that feeds on zoobenthos and alarge, pelagic form that feeds on zooplankton. Suchmorphological diversity is thought to be adaptive(Skulason and Smith 1995), either facilitating moreefficient use of scarce prey resources or exploitation oflimited habitat. The polymorphisms demonstrated todate in Arctic charr show considerable diversity,having been associated with varying combinations ofmorphological, behavioural and life-history trait dif-ferences (Adams et al. 2003) and may occur indepen-dently of lake morphometry or history. For example,documented Canadian Arctic charr polymorphisms

such as Lake Hazen, Ellesmere Island (Figure 3) andGander Lake, Newfoundland (Guiguer et al. 2002;Power, O’Connell, and Dempson 2005) occur in lakessituated at opposite ends of the latitudinal range thatvary markedly in climate, environment, glacial historyand the complexity of resident fish communities.Polymorphisms in Gander Lake and Lake Aigneau(Power, O’Connell, and Dempson 2005; Power et al.2009) occur in lakes with similar community complex-ity but vastly different lake morphologies (large anddeep versus small and shallow).

Relatively young or species-poor ecosystems aregenerally thought to facilitate polymorphic develop-ment and a growing number of studies have suggestedthat relaxation of inter-specific competition and openniche availability are essential prerequisites (Skulasonand Smith 1995). Thus deep lakes with multiple basinsor separate littoral, pelagic and profundal habitatzones offering unique foraging opportunities are char-acteristic of many of the ecosystems where morpho-types have been confirmed (Jonsson and Jonsson2001). Nevertheless, other environmentally basedhypotheses exist to explain variations in morphology(typically size) associated with distinctive dietary hab-itats. Allopatric and sympatric speciation, geneticinertia, piscivory, inter- and intra-specific competitionand alternative life histories have all been implicated asplausible causative mechanisms (Griffiths 1994). Thepiscivory hypothesis postulates that individuals switch-ing to a mainly fish diet will experience acceleratedgrowth rates as a result of prey metaphoetesis (Kerr1979) and some empirical support exists in the litera-ture. With respect to Arctic charr, Johnson (1976)suggested increased inter-specific competition forscarce food resources would yield a population size-structure where large, slow-growing fish limitedrecruitment to the upper modal size-class from thepool of small fish through feedback mechanisms thatmight include differential use of more productivefeeding habitats and cannibalism. Thus cannibalism,as noted above, has been reported as a key driver ofsize-linked trophic polymorphisms in a number ofstudies (e.g. Sparholt 1985; Riget, Nygaard, andChristensen 1986; Svenning and Borgstrøm 1995),including Lake Hazen (Guiguer et al. 2002).

Reductions in inter-specific competition character-istic of simple fish communities may also broaden therange of habitat and food types available for use byArctic charr in many northern lakes. The availabilityof increased forage opportunities has been suggested asa prerequisite to the development of trophic polymor-phisms (Skulason and Smith 1995). Hindar andJonsson (1982, 1993), and Parker and Johnson(1991), have also suggested the number of availablehabitat types and variations in growth conditions

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between habitats are critical to the development oftrophic polymorphisms. Most studies report differen-tial habitat use (profundal, pelagic or littoral) and ashift in diet with habitat type when describing trophicpolymorphisms. As the sole fish occupant of manylakes especially north of about 75�N latitude, Arcticcharr are able to occupy different habitat types thatinter-specific competition would normally preclude,and thus to develop the morphological characteristicsnecessary to optimise habitat resource use as a result ofrepeated selection.

The development of individuals into a specificmorphotype is probably a conditional strategy (Hindarand Jonsson 1993) whereby individual tactics aredetermined by the size or dominance rank of theindividual at specific developmental stages influencedby a combination of ecological, environmental and/orgenetic factors. Determinants of development are notunderstood but may include: inherited traits, randomdifferences in food intake (thus growth), and differ-ences in initial habitat selection and habitat heteroge-neity (Lømnicki 1988). Genetic evidence for differencesbetween identifiable morphotypes varies. In Canada,good evidence of genetic differences associated withtrophic polymorphisms and morphological divergenceshas been provided for Gander Lake (Gomez-Uchidaet al. 2008) and Lake Aigneau (Power et al. 2009). Incontrast, Arbour, Hardie, and Hutchings (2011) notethat two morphotypes in Lake Hazen do not appear torepresent genetically distinct populations and suggestthat the morphological differences may have arisenfrom phenotypic plasticity. Reviews of Europeanpopulations show similarly varying degrees of geneticdifferentiation from a certain degree of interbreedingto complete reproductive isolation (Gislason et al.1999; Westgaard, Klemetsen, and Knudsen 2004;Wilson et al. 2004). In cases where genetic differencesare small (e.g. Lake Hazen) local differentiation of asingle ancestral lineage is implied. Where geneticdifferences are larger (e.g. Lake Aigneau) colonisationby multiple lineages appears likely. Thus, both allo-patric diversification (i.e. colonisation by two lineagesof Arctic charr) and peripatric differentiation of oneinto two (or more) forms apparently contribute tocharr diversity in some cases (e.g. three forms in LochRannoch, Scotland, Verspoor et al. 2010; two incipientspecies in Fjellfrøsvatn, Norway, Knudsen et al. 2006).

Evolutionary scale diversity: when does a variant

become a species?

The described variation within and among Arctic charrpopulations belies simple classification, complicatesunderstanding of causative mechanisms and poses

obvious problems for determining where and when

identifiable differences are significant enough to be

recognised and preserved by management action.

Within the context of the diversity in form and

function described above several means of classifying

biodiversity in Arctic charr can be recognised, includ-

ing: (1) taxonomic – species- and/or subspecies-level

groups; (2) races – distinct evolutionary lineages

primarily derived from periglacial events; (3) life

history modes – migratory forms (including both

potadromous and anadromous types) and non-migra-

tory forms (including facultative non-migrants in

systems connected to the sea as well as populations

land-locked by migratory barriers and/or distance);

(4) ecophenotypes – forms within locations differenti-

ated from each other through ecological, temporal, diet

and/or morphological specialisations; (5) growth forms

– consistent patterns of growth including slow-growing

fish and fast-growing fish with the latter most often

exhibiting cannibalism; and (6) genetic stocks or stock

complexes – populations or groups that are mostly

reproductively distinct from each other and identifiable

as such using existing genetic techniques.Although roughly hierarchical, the classification

categories are neither mutually exclusive nor comple-

tely nested. Variation associated with categories 1 and2 above typically results from evolutionary-scale

differentiation driven by major geomorphic events

(e.g. glaciations and colonisations) with taxa being

separated for long time periods and evolving in

isolation. Taxonomic classification, therefore, most

closely conforms to the biological species concept

(Mayr 1970) as a means of defining diversity and thus

recognises the distinctions between Arctic and Dolly

Varden (S. malma) charr as species-specific (Reist,

Johnson, and Carmichael 1997). By contrast, variation

associated with categories 3–6 appears to result

primarily from contemporary-scale differentiation.

That is, local habitat diversity, ecological opportunity

and/or restricted gene flow combined with diet and

morphological specialisation that results in trophic

polymorphisms and habitat specialisations (i.e. eco-

phenotypes and growth forms categories 4 and 5) and

may be inducible within relatively short periods of time

as translocation experiments from Maine have dem-

onstrated (Michaud, Power, and Kinnison 2008).

Aspects of diversity in all categories appear to be

under both genetic and non-genetic (i.e. facultative)

controls to varying degrees (e.g. Wilson et al. 2004;

MacQueen et al. 2011). Association of Arctic charr

with specific locations, restricted movements and/or

high-fidelity homing to natal systems combined with

limited gene flow results in category 6 differentiation

that is easily measurable for management purposes.

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Two additional points are relevant. First, allclassification categories are influenced by genetics,however, genetic evidence is typically poorly docu-mented for categories 3–5, either because of thelimitations of available tools to detect differences orbecause of a clear lack of accepted thresholds fordetermining when measurable genetic differences arebiologically significant. Secondly, non-genetic varia-tion underlies all categories. Thus exhibited diversitycan be facultative, determined by local (or individual)circumstances. These non-genetic contributions com-plicate interpretation of the relevance of variation inmanagement because of the speed with which variationcan arise and fade in populations as environmentalcircumstances change. Although theory and evidencehave favoured the notion that contemporary-scaledifferentiation in Arctic charr occurs in species-and resource-poor (oligotrophic) northern habitats,evidence from recent Canadian studies (GanderLake: O’Connell, Dempson, and Power 2005, LakeAigneau: Power et al. 2009) suggest differentiationcan occur in more southerly locations dominatedby more complex fish communities. Accordingly,accrual of a larger number of case studies will berequired to properly test the species-, resource-poorhypothesis.

The characteristics that often form the basis fordifferentiation of coordinate groups for taxonomicclassifications (e.g. morphological characters such asgill raker counts, mouth position and form), are alsotypically used as the basis for determining differentforms within the other classification categories becauseof their relevance for ecological function (e.g. gill rakernumber is diagnostic of prey use). Thus, use of thesame characteristics for classification across severalcategories and the complex mix of genetic and non-genetic variation tend to confuse and confound inter-pretations of the significance of that variation.Nevertheless, to appropriately understand Arcticcharr diversity the phenomenon must be consideredfrom both the typical taxonomic as well as functionalperspectives because the two aspects of diversity areintimately intertwined.

Together the modes of differentiation and theoperational constraints for classification noted aboveprovide the basis for the ‘charr problem’ (Nordeng1983; Klemetsen 2010.) That is, should diverse formswithin Arctic charr be each recognised as distinctspecies or is it more appropriate to simply refer to thecollection of forms as a single taxonomic entity bestdescribed as a complex? The question has been thesubject of much discussion (see, e.g., McPhail 1961;Nyman 1972; Nyman, Hammar, and Gydemo 1981).Resolution of this issue is fundamental for assessingand conserving biodiversity and also will affect

assessments of extant taxonomic and functionaldiversity (see below). A further factor complicatinginterpretation of Arctic charr diversity is hybridisationand introgression among species, races and divergingforms. This is a relatively frequent occurrence in charrs(e.g. Hammar, Dempson, and Verspoor 1991; Wilsonand Bernatchez 1998). The resulting genetic exchangeover both evolutionary and contemporary time scalesserves to increase diversity within lineages (e.g.,introduction of novel genotypes) or to homogeniselineages if it occurs to a significant degree. Confusinginterpretations of genotypic diversity result(Bernatchez et al. 1995) and ultimately furthercontribute to ‘the charr problem’.

Contemporary-scale and evolutionary-scale diver-sification in charrs are clearly linked, particularlythrough sympatric, peripatric and parapatric diversifi-cation which can occur over relatively short timeframes. Although intuitively attractive, relatively fewclear examples of distinct speciation involving thesemodes exist (i.e. exceptions include Knudsen et al.2006; Verspoor et al. 2010). Thus, individual examplesof polymorphism appear to represent different stagesalong a continuum of diversification in a sympatriccontext overlain in some cases by allopatric processes.Disturbance regimes (primarily glaciations in recentevolutionary time) have apparently heavily modifiedoutcomes; however, in relatively undisturbed situationsspeciation processes become complete. Perhaps thebest example of the latter is Lake El’gygytgyn, a threemillion year old meteorite impact crater lake in centralChukotka which has never been glaciated. Threedistinct charrs are present, two of which are endemicto the lake (longfin charr, Salvethymus svetovidovi;small-mouth charr, Salvelinus elgyticus), and onefound elsewhere only in central Taimyr (Boganid’scharr, Salvelinus boganidae) (Chereshnev and Skopets1990; Skopets and Chereshnev 1991). Classification ofthe longfin charr into a separate genus beliescharacteristics of this species that are basal to thegroup and the relatively old age of the taxon, believedto be near to that of the lake. Evolution of the formwithin the lake environs is implied. Differentiation ofthe small-mouth charr was likely through sympatricprocesses, whereas wider distribution of Boganid’scharr suggests an allopatric origin followed bycolonisation of the lake (Viktorovsky et al. 1981).Thus, whereas both allopatric and sympatric processesoccur, the particular mixture for each situation differs.Accordingly, continued documentation and categor-isation of the origins of charr diversity in individualsituations is required to fully appreciate itssignificance.

The mixing of processes by which diversity arises,differing degrees of diversification along continua, and

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inadequate objective definitions of species for charrsfurther affects how that diversity is interpreted andmanaged. The resolution of ‘the charr problem’ thushas two aspects: (1) accumulation and in-depth anal-ysis of underlying causation of additional examples ofwithin- and among-system diversity, and (2) develop-ment of an acceptable and adequate taxonomic hier-archy with objective definitions of the levels ofdiversity. The former is a relatively straight forwardendeavour; the latter, however, has a philosophicalbasis (i.e. ‘when is a variant sufficiently differentiatedto be a species’) thus is more difficult to resolve.Regardless, increased knowledge of charr diversityand the contexts within which it arises and ismaintained are fundamental to future efforts.Moreover, efforts to link functional withtaxonomic diversity, particularly in the context ofmanagement and conservation mandates, areincreasingly needed.

The implications of climate change for charr diversity

In stable environments the discussion about therelevance of diversity in Arctic charr might remainlittle more than an academic debate. The environmentscolonised by Arctic charr in the post-glacial period andthe cold, resource poor environments to which they areadapted, however, are rapidly changing. These envi-ronments are being increasingly perturbed by humaninterventions associated with northern developmentthat have resulted in stressors with direct consequencesfor the continued maintenance of Arctic charr diver-sity, e.g. polluted environments, barriers to migration,over-fishing, climate warming (Maitland 1995).Although Arctic charr have been successful in thepast and now represent one of the most diverse fishtaxa in northern aquatic ecosystems (as describedabove), the physiological and life-history traits thathave facilitated past adaptation may not be suited tothe predicted rapid changes likely to be induced innorthern ecosystems by climate change or development(e.g. Reist et al. 2006).

Theoretical studies of climate change have sug-gested that warming will increase thermal habitats formany northern fish species as a result of longerseasonal periods or larger volumes of water of optimalor near-optimal growth temperatures (Reist et al.2006). As a consequence the geographic ranges ofmany southern fishes are likely to expand north,increasing the inter-specific competition faced byArctic charr and forcing contraction in the associateddistribution of the species within and among lakes. Forexample, Arctic charr are known to be a cold-adaptedspecies and have low resistance to high temperatures

(Baroudy and Elliott 1994). Thus increased water

temperatures are likely to limit habitat choice as Arctic

charr move to avoid warmer littoral zone temperatures

(Langeland and L’Abee-Lund 1998). Arctic charr will

thus be restricted to lake bottom habitats in deeper

lakes (430m) and eliminated from shallower lakes,

where summer temperatures routinely exceed temper-

atures known to induce significant thermal stress, as

they currently are at lower latitudes (Power, Reist, and

Dempson 2008).In lakes with distinctive morphological forms, the

resulting displacement to the profundal zone will

increase intra-specific competition between forms and

is likely to result in the elimination of one form and an

overall loss of Arctic charr diversity. As noted by

Power, O’Connell, and Dempson (2012) habitat sepa-

ration of forms has played a key role in the reduction

of resource competition among morphologically dis-

tinct forms and facilitated the maintenance of within-

lake diversity among Gander Lake Arctic charr.Associated changes in the relative abundance of

other fish species are also likely to have top-down

effects on the composition and abundance of prey

species at lower trophic levels. Arctic charr appear to

be vulnerable to such changes and may be locally

displaced in many localities (Reist et al. 2006). Arctic

charr may also be exposed to new pathogens as

competitor fish species shift their geographic

distributions northward. The arrival of both new

species and pathogens is likely to result in reduced

numbers of lake-dwelling Arctic charr and may, in

some instances, eliminate populations altogether

(Lehtonen 1996) with a consequent impact on regional

Arctic charr diversity.Climate-induced changes in life-history tactics,

particularly migration, may also have consequent

impacts on diversity. For example, the reduced

propensity to migrate in southern populations

(Doucett et al. 1999) suggests regional reductions in

the co-occurrence of lake-resident and anadromous

forms. Studies of linkages with levels of lake primary

production suggest reduced occurrence of Arctic charr

marine migrations from higher productivity systems,

with climate-driven increases in terrestrial primary

production expected to increase primary production in

lakes and, therefore, ultimately reduce the prevalence

of anadromy in Arctic charr populations (Finstad and

Hein 2012). Nevertheless, more information about the

relative influences of temperature and other ecological

variables in determining the occurrence and relative

success of anadromous and lacustrine populations is

required before detailed predictions about the possible

effects of climate change on diversity within Arctic

charr populations can be made.

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Relevance of charr diversity

The diversity among Arctic charr across all thecategories described here is important for a variety ofreasons. Genetically based diversity provides evolu-tionary options in the context of the variable andchanging environments inhabited by the species. Bothgenetic and facultative aspects of diversity may alsofacilitate the structure and functioning of aquaticecosystems by increasing the number and nature oftrophic pathways present (e.g. Folke et al. 2004). ForArctic systems that are typically relatively depauperateat the species’ level, diversity below the species’ levelmay, therefore, fulfil a stabilising role in aquaticecosystems similar to that provided by communitycomplexity in lower latitude systems. Diversity helpsensure the stability and resistance of ecosystem struc-ture to perturbations, particularly the increasinglyfrequent anthropogenically induced perturbationsaffecting northern aquatic ecosystems (Reist et al.2006). Diversity may also provide functional templatesallowing Arctic charr to more readily adapt to rapidenvironmental change. Accordingly, preserving andunderstanding the diversity in Arctic charr will provideinstructive models of the drivers and range ofresponses possible for northern freshwater ecosystemsand their contained fish faunas. Furthermore, animproved understanding of diversity is fundamentallyimportant to the design of appropriate conservationand management initiatives.

Conclusions

The degree and nature of diversity discussed here forArctic charr in the narrow taxonomic sense is by nomeans confined to this taxon. It exists and is beingdescribed for Dolly Varden, bull charr (S. confluentus)(Reist, Johnson, and Carmichael 1997; Reist et al.2002) and lake charr, S. namaycush, at least in largernorthern lakes (Blackie, Weese, and Noakes 2003).Moreover similar variation is observed in other groupsof northern fishes (e.g. lampreys – Docker 2009;sticklebacks – Olafsdottir, Snorrason, and Ritchie2006; ciscoes – Vescei et al. 2012; whitefishes –Østybe et al. 2006). High and multiple intersectinglevels of variation appear to be a consistent phenom-enon in northern freshwater fishes. Much of thevariation appears to represent the legacy of significanteffects associated with Pleistocene glaciations (i.e. arisefrom evolutionary-scale events and allopatric differen-tiation), however, much also arises from more recentphenomena that promote diversification (i.e. sympat-ric, peripatric and parapatric evolution associated withhabitat diversity and ‘vacant’ niches) with some level ofgenetic basis. Much of the existing phenotypic

variation also appears to be largely non-genetic inorigin and superimposed upon genetic variability (e.g.facultatively determined life history forms, growthvariants). Nevertheless, most of the known variationappears to be functionally relevant, contributing toecosystem structural and functional properties relevantto stability and resistance. Standard approaches toconservation and management, however, typicallyfocus only upon genetically based groups at highertaxonomic levels, do not account for the relevance ofnon-genetic variation, and almost completely ignoreprocesses by which diversity is maintained or gener-ated. Arctic charr diversity (and by extension diversityof other northern fishes) is threatened by anthropo-genic stressors such as climate change. Accordinglymanagement paradigms must be revised to adequatelyaccount for and protect diversity and underlyinggenerative processes at all relevant levels, particularlyin the context of substantive and pervasive threats suchas climate change.

Acknowledgements

This contribution is an outgrowth of work funded in part bythe Canadian International Polar Year project, Chars andClimate Variability and Change. We also acknowledgeongoing funding from the Natural Sciences andEngineering Research Council of Canada and Fisheries andOceans Canada which has supplemented many of theindividual studies referred to here. Paul Vescei is thankedfor his extraordinarily accurate renditions of individualcharrs. As always Chantelle Sawatzky of Fisheries andOceans Canada has been instrumental in aiding manuscriptpreparation and in generating Figure 2.

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