Journal of Fish Biology (1999) 55, 84–93Article No. jfbi.1999.0969, available online at http://www.idealibrary.com on

Evidence for anadromy in a southern relict population ofArctic charr from North America

R. R. D*, M. P†¶, G. P*, F. C‡ J. D. R§*Department of Biology, University of Waterloo, Waterloo, ON, N2L 3G1, Canada;

†Department of Agricultural Economics, University of Manitoba, Winnipeg, MB,R3T 2N2, Canada; ‡Ministere de l’Environnement et de la Faune, Service de la FauneAquatique, 675 Boulevard Rene-Levesque Est. (11e), Bte 92, QC, G1R 5V7, Canada

and §Canada Department of Fisheries and Oceans, Freshwater Institute, Winnipeg, MB,R3T 2N6, Canada

(Received 30 November 1998, Accepted 9 March 1999)

Stable isotope analysis of Arctic charr from a southern relict population in Quebec showed thatsome individuals adopted anadromy as a life-history strategy, but the strategy did notpredominate in the population. Differences in length-at-age, ages and sex ratios betweenidentified freshwater residents and anadromous individuals were consistent with previousdescriptions of partially migratory populations. This suggests that anadromy in relict popu-lations in Quebec is associated with niche shift and occurs as a conditional response toopportunities for maximizing the rewards (e.g. fitness) of habitat use.

? 1999 The Fisheries Society of the British Isles

Key words: Arctic charr; stable isotope analysis; anadromy; partial migration.

¶Author to whom correspondence should be addressed. Tel.: 204 474 9655; fax: 204 261 7251; email:power@cc.umanitoba.ca

INTRODUCTION

Populations of Arctic charr Salvelinus alpinus (L.) with access to the sea oftencontain both anadromous and non-anadromous forms that are typically indis-tinguishable on the basis of morphometric and meristic characteristics.Although it is known that in southern regions resident, non-anadromous formsof Arctic charr predominate, exact limits for anadromy have not been estab-lished. Hammar et al. (1989) argued that the southern limit for anadromy inNorth America was the Sandhill River system (53)34*N) in Labrador, althoughscattered populations are reported from insular Newfoundland. Similar reports,however, have not been made for southern Quebec where Arctic charr arethought to exist exclusively as landlocked populations (Vladykov, 1957;Dumont, 1982).

The separation of populations into their respective anadromous and non-anadromous components is vital for both proper management of the species(Halden et al., 1995) and for understanding differences in life-history strategieswithin and between populations. Accordingly, a simple, rapidly applicablemethod is needed to identify individuals that migrate to and from the marine

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0022–1112/99/070084+10 $30.00/0 ? 1999 The Fisheries Society of the British Isles

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environment. Stable-isotope analysis (SIA) meets these conditions. In addition,it offers a quick and inexpensive alternative to contemporary techniques (e.g.biological tags and scanning proton microprobe imaging) for the determinationof anadromous life histories, particularly where there is only anecdotal evidenceof the behaviour.

Detailed laboratory feeding experiments have shown that predictable relation-ships exist between the stable-isotope ratios of animals and their food sources(DeNiro & Epstein, 1978, 1981), and many field studies have confirmed thegeneral utility of SIA in describing food-web relations (Fry & Sherr, 1984).Because marine and freshwater primary producers tend to be isotopicallydistinct, there are opportunities to delineate between animals feeding from thetwo environments. Recently, the method has also been used to identify thepresence of anadromous fishes in fresh water (Kline et al., 1998; Limburg, 1998;Doucett et al., 1999).

Here, SIA is applied to resolve questions about the migratory status of Arcticcharr from a southern relict population in the Riviere de la Trinite (49)25*N,67)18*W) Quebec. The river forms part of the index salmon Salmo salar L. riverssystem operated by the Quebec Government to assess the status of Atlanticsalmon stocks. Repeated observation of small numbers of Arctic charr mixedwith samples of emigrating salmon smolts raised obvious questions aboutwhether anadromy was occurring in a region where it had been assumed not tooccur.

MATERIALS AND METHODS

Smolt traps are operated annually 1·5 km from the mouth of the Riviere de la Trinitecoincident with salmon smolt migration and sample data are used to estimate totalnumbers of migrating smolts. Since Arctic charr were first observed in the traps in 1986,captures have ranged from a low of 0 in 1990 to a high of 84 in 1996. Owing to the earlierArctic charr migration period, and trapping of only a portion of all migrating fish, actualnumbers of migrating Arctic charr will be higher than recorded catch sizes indicate.

Of 29 individuals captured between mid May and June 1998, 17 were retained for SIAanalysis. A sample of lake resident Arctic charr was obtained from the Lac Davidsontributary system for comparative purposes. The system is located c. 22 km upstream ofthe river mouth (Fig. 1). Although the connection between the two sample sites is notknown, samples are thought to be drawn from a common population in so far as the LacDavidson system is the only one known to contain Arctic charr in the lower de la Trinitewatershed.

Wet weight and fork length were taken for all samples at capture. Sagittal otolithswere removed for ageing. Ageing was completed by personnel at both the Ministere del’Environnement et de la Faune, Quebec, and the University of Waterloo and checked forcorrespondence. There was complete agreement. Dorsal muscle for SIA was removedfrom fish and stored in 85% ethanol for transport back to the laboratory. All sampleswere dried at constant temperature (60) C for 48 h), pulverized to a fine powder using aRetch MM 2000 ball mill grinder, and stored in clean, glass vials in a desiccator to awaitisotopic analysis. Approximately 1 mg of dried, ground muscle tissue was used in thesimultaneous analysis of stable C and N isotopes. Due to low sulphur levels in animaltissues, stable S isotope analysis required 6 mg tissue.

Stable-isotope ratios are expressed as delta values (ä) and are measures of a parts perthousand difference (‰) between the isotope ratio of a sample and that of aninternational standard according to the formula:

ä13C, ä15N, or ä34S=[(Rsample"Rstandard)R"1standard]#1000

86 . . .

where R=13C/12C, 15N/14N, or 34S/32S. Samples that are more negative are depleted andcontain less 13C, 15N, or 34S. Samples that are less negative are enriched and containmore of the heavier isotopes. International standards are carbonate rock from the PeedeeBelemnite formation (Craig, 1957), nitrogen gas in the atmosphere (Mariotti, 1983),and primordial sulphur from the Canyon Diablo meteorite (Rees et al., 1978). Allinternational standards are, by definition, set at a value of 0‰.

Isotopic analyses were performed exclusively on a Micromass VG Isochromcontinuous-flow isotope-ratio mass spectrometer connected to a Carlo Erba elementalanalyser at the Environmental Isotope Laboratory (University of Waterloo, Waterloo,Ontario, Canada). Repeat analyses of common laboratory standards yielded results thatwere both accurate and precise [IAEA–CH6: ä13C= "10·5&0·2‰ (mean&1 ..)(n=132); IAEA–N1: ä15N=0·6&0·3‰ (n=225); NBS–122: ä34S=0·8&0·5‰ (n=25)].Replicate samples of a lipid-extracted fish standard (EIL-70) also produced reli-able ä values for carbon, nitrogen, and sulphur [ä13C= "20·7&0·2‰ (n=38);ä15N=16·4&0·3‰ (n=38); ä34S=5·3&1·0‰ (n=31)].

Statistical analyses were performed using SYSTAT 7·0 (Wilkinson, 1997). Maximaltype one error rates were set at á=0·05. Normality and homogeneity of varianceassumptions were checked using plots of the residuals or with F-tests. Significant analysisof variance (ANOVA) results were followed by multiple comparisons using the conserv-ative Tukey’s HSD post hoc test (Sokal & Rohlf, 1995). Differences in mean length-at-age and ages were established using two-sampled t-tests (Larsen & Marx, 1986).

Lac DAVIDSON

Scale

RIVIERE de la TRINITE

BAIE-TRINITE

Hwy 138

0 5 km

–

––

F. 1. The Lower Riviere de la Trinite system depicting the relationship of the river system to the StLawrence River and the position of Lac Davidson with respect to the river mouth.

RESULTS

Three of 17 Arctic charr caught in Riviere de la Trinite, Quebec, in summer1998 had average (&1 ..) stable-isotope ratios (ä13C= "22·1&1·3‰;ä15N=12·3&0·5‰; ä34S=13·1&1·3‰) that were more enriched thanother river-sampled Arctic charr (ä13C= "29·2&1·1‰; ä15N=8·9&0·4‰;ä34S=5·2&0·5‰). The ä13C values reported for the isotopically enriched

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Anadromouscharr

–20

14

6

–32

δ13C (‰)

δ15N

(‰

)

12

10

–28 –24

Freshwaterresidents

8

Riviere de la TriniteLac Davidson

´´

F. 2. Stable carbon (ä13C) and nitrogen (ä15N) isotope profiles of individual Arctic charr Salvelinusalpinus L. collected in the Riviere de la Trinite system, Quebec, in May and June 1998. The dashedlines represent an isotopic distinction between resident and anadromous fish.

–20

15

3

–32

δ13C (‰)

δ34S

(‰

)

11

7

–28 –24

Anadromouscharr

Freshwaterresidents

Riviere de la TriniteLac Davidson

´´

F. 3. Stable carbon (ä13C) and sulphur (ä34S) isotope profiles of individual Arctic charr Salvelinusalpinus L. collected in the Riviere de la Trinite system, Quebec, in May and June 1998. The dashedlines represent an isotopic distinction between resident and anadromous fish.

individuals fall within the range of ä13C values reported for marine particulateorganic carbon ("24 to "18‰) in the Gulf of St Lawrence (Tan & Strain,1983). The SIA ratios for those same individuals were also more enriched thanthe 20 Arctic charr collected for comparative purposes in the Lac Davidsontributary system (ä13C= "27·9&0·7‰; ä15N=7·3&0·7‰; ä34S=6·7&0·3‰)(Figs 2 and 3). The stable C, N, and S isotope ratios of all three sample groupswere statistically different from one another (Tukey’s HSD, P<0·05).

Differences in the age of lake- and river-sampled specimens were also evident.Lac Davidson fish averaged (&95% confidence interval half lengths) 3·73&1·02(n=20) years. River sampled fish averaged 4·71&0·77 (n=17) years. F-tests

88 . . .

established the equality of sample variances (P>0·05) and a two-sampled t-testdemonstrated a significant difference in age between the two samples (P<0·05).Two-tailed t-tests for differences in length-at-age of lake and river samplesshowed similar results (P<0·05). River sampled fish were separated intopreviously anadromous and non-anadromous groupings based on isotopeprofiles. One-tailed t-tests showed significant differences in length-at-ageindicative of the benefits of anadromy (Fig. 4).

7

30

153

Age

Len

gth

(cm

) 25

20

5 6

Anadromous

4

Non-anadromous

F. 4. Comparison of length-at-age for Arctic charr taken in the Riviere de la Trinite system, Quebec, inMay and June of 1998. Specimens were separated into anadromous and non-anadromousgroupings based on their isotope profiles. Fish that have been to sea (/) and those that have not(.) are shown. The range of values defined by the associated 95% confidence intervals for groupedmeans are plotted as boxes for all n>2.

DISCUSSION

Three of 17 Arctic charr, caught in the Riviere de la Trinite system, Quebec,had stable carbon, nitrogen, and sulphur isotope ratios indicative of fish thathad fed in the marine environment. Stable C and S isotope ratios, which areused most often to identify food sources (Peterson & Fry, 1987), showed thegreatest variation among the Arctic charr (range= "31·5 to "20·6‰ and 3·9to 14·5‰ for ä13C and ä34S, respectively). These isotopic differences pointtowards an alternative life history and/or feeding strategy for the three Arcticcharr that were 7–8‰ more enriched (Fig. 3), and whose SIA ratios weresimilar to values reported for marine fishes in other studies (Fry, 1988; Jarmanet al., 1996). Stable N isotope ratios (range=6·1–12·9‰) were not as variableas those of carbon and sulphur, but the same three individuals were still more15N-enriched than all other Arctic charr (Fig. 2). ä15N values are used mosttypically to define trophic position (Vander Zanden et al., 1997), and so it ispossible that the more enriched stable nitrogen isotope ratios of the threeArctic charr were simply reflecting their higher trophic position. This expla-nation seems likely as these fish were also the three largest individuals in thesample population.

Although the 14 other river-sampled Arctic charr had stable isotope ratios thatwere statistically distinct from those of fish sampled at Lac Davidson, individuals

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in both samples fell well within the range of values expected of freshwater fishes(Hesslein et al., 1991; Doucett et al., 1998). The small difference in the stable Cand S isotope ratios of the fish from the two environments may be attributed todifferences in lake and river biogeochemical cycles (Wada et al., 1993). It is alsopossible that biogeochemistry had an effect on the stable N isotope ratiosof individuals at the two sites. Probable overwintering in a common lakeenvironment, however, suggests it is more likely that the 15N-enriched values ofriver-sampled Arctic charr were due to their larger size (Fig. 4), and subsequentlyhigher trophic position.

The presence of Arctic charr in southern Quebec has been known since the endof the 19th century (Dumont, 1982). Arctic charr colonization of the region isthought to have occurred coincident with the retreat of the last Pleistoceneglaciers and existing populations are considered to be vestiges of the anadromouspopulations that inhabited the Champlain Sea and Atlantic Ocean, i.e. glacialrelict populations (Behnke, 1972). Populations that colonized glacial head-water systems subsequently became resident or physically landlocked inNewfoundland, Quebec and New England. Previous studies, therefore, havegenerally treated these populations as uniquely lacustrine (Vladykov, 1957;Saunders & Power, 1969; Qadri, 1974; Kircheis, 1976; Rombough et al., 1978;Dumont, 1982). There are scattered reports of anadromous Arctic charr ininsular Newfoundland. Scott & Crossman (1964) have documented anadromyin Parker’s River (51)27*N), on the tip of the Northern Peninsula. Hammar(1987) provides anecdotal evidence of anadromy as far south as Parson’s PondRiver (49)56*N). Other anecdotal reports of anadromy exist (J. B. Dempson,pers. comm.). Until now, however, little definitive evidence of anadromy insouthern North American relict populations has been presented and no evidencehas been forthcoming for Quebec.

The sporadic occurrence of anadromy in southern Canadian Arctic charrpopulations raises obvious questions about the ecological mechanisms respon-sible for triggering anadromy. Jensen (1981) and Berg & Berg (1989) provideevidence suggesting that sea surface temperatures are one of the main factorscontrolling the southern limit of anadromous behaviour in Norway. Averagecoastal water temperatures measured as 10-day means exceeding 14) C at 4–5 mdepth are thought to provide an effective abiotic barrier to anadromousbehaviour, particularly as Arctic charr are the least resistant of salmonids to hightemperatures (Baroudy & Elliott, 1994). Sea surface temperatures taken atSept-Iles (50)10·3*N, 66)25·8*W) at 2·0 m over the period 1993–1997, however,show sustained periods of temperatures in excess of 14) C in August andSeptember, with daily peaks as high as 21) C (Gilbert et al., 1997). Hydrographicreadings taken at Mont-Louis A, in the middle of the Gulf of St Lawrence(49)33·6*N, 65)45·6*W), at 0·5 m over the same period show similar results. Thepresence of anadromous Arctic charr in such environments suggeststhat temperature per se does not provide an effective barrier to anadromousbehaviour in southern Canada.

Differences in food resources, decreases in predation or disease risks andfavourable abiotic conditions have been suggested as other possible reasons formigratory behaviour (Northcote, 1992). Gross (1987) argues that migrationshave evolved to track productivity differences between fresh and salt water and

90 . . .

are most likely to occur where such differences are largest. Differences in aquaticproductivity correlate well with the incidence of anadromy on a latitudinal basisin general (Gross et al., 1988), and the pattern of anadromy observed insalmonids in particular (McDowall, 1987). In the de la Trinite, however, thegrowth advantage of anadromy is very slight and differences in food resources inmarine and freshwater environments are unlikely to provide the only explanationfor anadromy in populations of Arctic charr bordering the Gulf of StLawrence.

Jonsson & Jonsson (1993) have noted that partially migratory populations offish are often polymorphic with residents being smaller than their migratorycounterparts. Statistical tests of the differences in size-at-age betweenanadromous and non-anadromous individuals captured in the Riviere de laTrinite showed statistically significant differences between the two groups(Fig. 4). Because fecundity increases with body size, switching to potentiallyricher feeding grounds can increase the reproductive fitness of individuals. Fishattaining maturity near the asymptotic body size determined by the food rationconsumed in the freshwater niche may thus attempt to avoid the growthstagnation associated with that niche and delay maturation by 1 or more yearsby migrating (Jonsson & Jonsson, 1993). Statistically significant differences inmean age of resident and migrating individuals in the Riviere de la Trinite areconsistent with the hypothesis of niche shift which has been documented to occurelsewhere (Forseth et al., 1994).

Where migration is concerned, the niche shift tactic is likely to be useddifferentially by the sexes because of sex-specific differences in the relative costsand benefits of migration. Male size at maturity tends to be more variable thanfemale size (Forseth et al., 1994). The reason may be that male reproductivesuccess is less dependent on size than female success. For females the advantagesof larger size include increases in fecundity and improved competitive ability(Fleming & Gross, 1992). Gains in size, even small gains, therefore, are morelikely to induce females to migrate and the resulting sex ratio of migrants shouldbe skewed toward females. In the Riviere de la Trinite 82·4% of potentialmigrants were female, whereas in sampled freshwater populations femalesaccounted for only 44·6% of all individuals captured. The pattern is consistentwith the sex ratios reported for the migratory components of other partiallymigrating populations (Jonsson & Jonsson, 1993).

CONCLUSION

The universal distinction between marine and freshwater environmentsfavours the use of SIA as a means of identifying individuals in a population thatadopt an anadromous life history. It should now be possible to discover whetherother river systems along the Quebec/Labrador and Newfoundland coastlineshost partially anadromous populations of Arctic charr similar to that found inthe Riviere de la Trinite, where anadromy appears to arise as a conditional tacticdependent on the biotic and abiotic conditions prevailing at a point in time. Thisuse of SIA will help further in attempts to isolate the environmental conditions(e.g. densities and co-existing communities, differential habitat rewards and

91

migration costs) critical to determining the transition between resident andanadromous life-history strategies in eastern North America.

The authors thank C. Gauthier, D. Deschamps and D. Fournier for samplingassistance and B. Drimmie and B. Mark for technical assistance. Support for the workwas provided by the Ministere de l’Environnement et de la Faune, Department ofFisheries and Oceans, Ocean Climate Program, and NSERC operating grants to M.P.and G.P.

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