Genetic assessment of lake sturgeon ( Acipenser fulvescens ) population structure in the Ottawa River

Genetic assessment of lake sturgeon (Acipenser fulvescens)population structure in the Ottawa River

Kristyne M. Wozney & Tim J. Haxton &

Shawna Kjartanson & Chris C. Wilson

Received: 29 December 2009 /Accepted: 11 October 2010 /Published online: 27 October 2010# Springer Science+Business Media B.V. 2010

Abstract Lake sturgeon (Acipenser fulvescens) are ofconservation concern throughout their range. Manypopulations are dependent on fluvial habitats whichhave been increasingly impacted and fragmented bydams and human development. Although lake sturgeonwere once abundant in the Ottawa River and itstributaries, historical commercial harvests and otheranthropogenic factors caused severe declines and lowcontemporary numbers in lake sturgeon populations.Contemporary habitat fragmentation by dams may beincreasing isolation among habitat patches and localrates of decline, raising concerns for persistence of local

populations. We used microsatellite DNA markers toassess population structure and diversity of lakesturgeon in the Ottawa River, and analyzed samplesfrom 10 sites that represent more than 500 km ofriverine habitat. To test for evidence of anthropogenicfragmentation, patterns of genetic diversity and con-nectivity within and among river segments were testedfor concordance with geographic location, separationby distance and obstacles to migration, consideringboth natural and artificial barriers as well as barrier age.Despite extensive habitat fragmentation throughout theOttawa River, statistical analyses failed to refutepanmixia of lake sturgeon in this system. Althoughthe long generation time of lake sturgeon appears tohave effectively guarded against the negative geneticimpacts of habitat fragmentation and loss so far,evidence from demographic studies indicates thatrestoring connectivity among habitats is needed forthe long-term conservation and management of thisspecies throughout this river system.

Keywords Lake Sturgeon (Acipenser fulvescens) .

Habitat fragmentation .Microsatellite .

Genetic diversity

Introduction

Anthropogenic fragmentation of riverine systems is asignificant concern for the persistence of residentaquatic species, particularly migratory fishes or those

Environ Biol Fish (2011) 90:183–195DOI 10.1007/s10641-010-9730-x

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10641-010-9730-x) containssupplementary material, which is available to authorized users.

K. M. Wozney (*) :C. C. WilsonOntario Ministry of Natural Resources,2140 East Bank Drive,Peterborough, Ontario K9J7B8, Canadae-mail: Kristyne.wozney@ontario.ca

C. C. Wilsone-mail: chris.wilson@ontario.ca

T. J. HaxtonOntario Ministry of Natural Resources,300 Water St., 4th Floor S.,Peterborough, Ontario K9J 8M5, Canada

S. KjartansonAECOM,99 Commerce Drive,Winnipeg, MB R3P OY7, Canada

with large home ranges. Large river systems are someof the most intensively fragmented ecosystems thatexist, with most of the world’s large rivers nowfragmented by dams and hydroelectric generatingstations (Jager et al. 2001). Habitat isolation andalteration by dams may put fragmented fish popula-tions at greater risk of extirpation, as demographicisolation and reduced recruitment cannot be offset byimmigration (Winston et al. 1991; Jager et al. 2001;Knaepkens et al. 2004). This is particularly true forlarge-bodied, highly vagile species, which may requirelarge areas of continuous or contiguous habitat tocomplete their life cycle (Auer 1996; Knaepkens et al.2004). While the effects of fragmentation by dams hasbeen most extensively investigated for salmonids(Gosset et al. 2006; Heggenes and Roed 2006; Moritaet al. 2009), their impacts are likely more pronouncedfor species with long generation times and episodicspawning, as their longevity limits opportunities toadapt to altered conditions (O’Grady et al. 2008). Asa group, sturgeon are particularly vulnerable to alteredriverine conditions due to their long generation times,large body size, and need for large stretches ofcontinuous riverine habitat (Billard and Lecointre2001; Lenhardt et al. 2006).

Lake sturgeon (Acipenser fulvescens) rely on riverinehabitat across most of their range (Peterson et al. 2007),and can migrate in excess of 200 km in the absence ofbarriers (Kempinger 1988; Rusak and Mosindy 1997;Auer 1999). Lake sturgeon were historically abundantthroughout much of North America but are currentlyconsidered threatened or endangered in many states orprovinces (Welsh and May 2006; COSEWIC 2007).Commercial harvests which began in the late 1880scaused severe decline in lake sturgeon populations in avery short period (Harkness and Dymond 1961;Ferguson and Duckworth 1997; Welsh et al. 2003;McQuown et al. 2003) primarily as a result of theirgeneration time, periodic spawning and low naturaladult mortality (Boreman 1997; Pikitch et al. 2005).Coinciding with commercial harvest, damming ofrivers for logging and hydroelectric developmentexacerbated other anthropogenic stressors by fragment-ing and altering formerly continuous habitats andnatural flows and reducing water quality (Fergusonand Duckworth 1997; Hay-Chmielewski and Whelan1997; Billard and Lecointre 2001).

Fragmentation of rivers by hydroelectric and watercontrol dams is a major contemporary obstacle

inhibiting lake sturgeon recovery (Haxton 2006;Hay-Chmielewski and Whelan 1997; Haxton andFindlay 2008). Despite restrictive harvest controlssince the early 1900s, improvements in water quality,and pollution control, sturgeon populations have notrecovered significantly across their range (COSEWIC2007). By acting as barriers to upstream migration,dams disrupt formerly continuous habitats into alteredfragments and may limit adult fish to spawning inareas of suboptimal habitat (McQuown et al. 2003).The impact by dams on lake sturgeon may becompounded by historical preferences to constructdams over existing rapids which may formerly haveprovided ideal spawning habitat for sturgeon (Fergusonand Duckworth 1997; Haxton 2006). As well as directhabitat loss, altered flow patterns from artificialimpoundments often turn fast-flowing waters intoperched reservoirs of poor quality lentic habitat forsturgeon eggs and larvae (McQuown et al. 2003).Larval drift downstream may decrease during lowflow periods, and both larvae and adults may sufferfrom asphyxia as the result of being trapped in shallowpools of water (Ferguson and Duckworth 1997).Conversely, dams can cause high flow in some areaswhich may result in larvae or eggs being washed intopoor habitat (McQuown et al. 2003; Dudley andPlatania 2007) and may also increase surface runoffdegrading overall water quality (McQuown et al.2003). These compounded impacts may act synergis-tically on multiple life stages to reduce populationviability, raising concerns for persistence of localpopulations (COSEWIC 2007). It is unknown whateffect dam-induced fragmentation has had on thegenetic integrity of formerly connected populationsand whether isolation/fragmentation has resulted ingenetic drift and/or loss.

Lake sturgeon in the Ottawa River and tributarieswere heavily impacted by fragmentation and habitatdegradation from logging and hydroelectric dams aswell as overharvesting in the late 19th century,resulting in their precipitous decline (Harkness andDymond 1961; Haxton and Chubbuck 2002). Lakesturgeon have been extirpated in many Ottawa Rivertributaries and vary in abundance and demographicstructure among river reaches, with unimpoundedreaches supporting the strongest populations (Haxton2002, 2006; Haxton and Findlay 2008). Recruitmenthas been impaired in many impounded reaches suchthat sturgeon abundance is low and skewed towards

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larger, older fish (Haxton and Findlay 2008, 2009).Lake sturgeon are now considered to functionallyexist as isolated populations in the different riverreaches, at best connected only by larval drift overdownstream barriers (Haxton and Findlay 2008).

We investigated the effects of river fragmentationby dams on the genetic structure and diversity of lakesturgeon within and among reaches of the OttawaRiver, using 14 published microsatellite DNA loci(McQuown et al. 2002; Welsh et al. 2003) to assesswhether population fragmentation of lake sturgeon inthe Ottawa River could be detected within a 580 kmportion of the river that includes seven majorhydroelectric dams. We hypothesized that the stur-geon subpopulations would show genetic differentia-tion between sample sites that were separated bydams, with the extent of differentiation predicted toincrease with physical separation and barrier age.

Methods

Study site

The Ottawa River is approximately 1,130 km inlength from the headwaters in northern Quebec to itsconfluence at the St. Lawrence River, with awatershed of 146,000 km2 and mean annual flow ofapproximately 2,000 m3 s−1 (Haxton and Chubbuck2002). Although major rapids and waterfalls formerlyexisted on the Ottawa, a review of historical con-ditions prior to dam construction suggests that onlyChaudière Falls and Chats Falls may have been barriersto upstream movement, with downstream larval dis-persal likely occurring (Haxton and Chubbuck 2002).Crib dams were first constructed in the 1870s tofacilitate log drives, which likely impacted waterquality as well as impeding upstream migration(Haxton and Chubbuck 2002). The first hydro-electricdam was constructed in the 1880s at Chaudière Falls(Fig. 1) and continued until 1964 (Table 1), making itone of the most highly regulated rivers in Canada(Telmer 1997) and fragmenting the river into a seriesof isolated reaches (Fig. 1).

Sampling

Lake sturgeon were sampled by gillnets or trapnetsduring standard index netting programs (Haxton

2002; Haxton and Findlay 2008) or during spawningassessment programs (Haxton 2006). Capture gearprimarily targeted fish ≥60 cm, with high captureefficiency for fish ≥80 cm (Haxton 2008). Sturgeonwere non-lethally sampled at ten sites that cumula-tively represent more than 580 km of riverine habitatin the Ottawa River and its tributaries (Fig. 1).Minimally invasive tissue samples were taken frompectoral rays and preserved in 95% ethanol forsubsequent genetic analysis. Sample sites listed fromnorth to south included Lac la Cave, Holden Lake,Allumette Lake, Muskrat River (lower AllumetteLake), Lac Coulonge, Lac du Rocher Fendu, Chenauxrapids (upper end of Lac des Chats), Lac des Chats(distributed sampling), Lac Deschênes and LacDollard des Ormeaux (Fig. 1). Samples were alsocollected from the St. Lawrence River near Île Madameeast of Quebec City as a neighboring population forcomparison to the Ottawa River, as well as from theMattagami River in northern Ontario to serve as anoutgroup population.

Data collection

DNA was extracted from individual finclips and finrays using a simple lysis extraction in 96 well platesusing 250 μL lysis buffer [50 mM Tris pH 8,1,000 mM NaCl, 1 mM EDTA, 1% sodium dodecylsulphate (weight per volume), and 1 mg proteinase K]per well and incubated overnight at 37°C. DNA wasprecipitated using 500 μL of 80% isopropanol perwell and centrifugation at 2,000 gravities for 30 min.Supernatant was removed and the remaining pelletswere rinsed with 1 mL of 70% ethanol, followed byre-centrifugation. DNA pellets were air dried at roomtemperature for 20 min, then dissolved in 150 μL 1×TE (10 mM Tris pH8, 1 mM EDTA). Extractionyields and quantity were tested using electrophoresisin 1.5% agarose gel stained with SybrGreen (CedarLane Laboratories, Burlington, Ontario) alongsidea molecular mass ladder (Bioshop, Burlington,Ontario).

Samples were amplified and genotyped at 14microsatellite loci that were previously identifiedfor lake sturgeon (McQuown et al. 2002; Welsh etal. 2003). Multiplex reactions were performed in15 μL reactions containing 1× PCR Buffer (Qiagen,Mississauga, Ontario), 2 mM MgCl2 (Qiagen,Mississauga, Ontario), 2 mM each dNTP (Bioshop,

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Burlington, Ontario), 0.2 mg·ml−1 BSA (Bioshop,Burlington, Ontario), 0.025U Taq DNA polymerase(Qiagen, Mississauga, Ontario), and approximately10 ng of template DNA. Multiplex reactions containedthe following primer concentrations Multiplex 1-Afu68 [0.25 μM], AfuG63 [0.22 μM], AfuG122[0.25 μM], AfuG195 [0.25 μM], AfuG74 [0.28 μM]and AfuG67 [0.25 μM]; Multiplex 2- AfuG160[0.25 μM], AfuG204 [0.23 μM], Afu68b [0.3 μM],

AfuG61 [0.25 μM] and AfuG71 [0.25 μM]; Multiplex3- AfuG9 [0.35 μM] and AfuG112 [0.38 μM]. Thelocus AfuG56 was amplified singly with a primerconcentration of 0.3 μM due to problems with non-specific amplification. PCR cycling was 95°C for11 min followed by 94°C for 1 min, 55°C for 1 minand extension of 72°C for a total of 35 cycles and finalextension of 60°C for 45 min. Amplified products forall samples were run on an AB 3730 automated

Barrier Year built Height (m) Purpose Natural rapids

Carillon 1959–1964 16.8 Hydro-electric power Yes

Chaudière Falls 1880s 9.8 Hydro-electric power Yes

Chats Falls 1929–1931 15.2 Hydro-electric power Yes

Chenaux 1948–1951 18 Hydro-electric power Yes

Bryson 1923–1925 21.3 Hydro-electric power Yes

Des Joachims 1946–1950 37 Hydro-electric power Yes

McConnell Lake 1946–1949 37 Hydro-electric power No

Otto Holden 1948–1952 40 Hydro-electric power Yes

Table 1 Summary of damson the Ottawa River withinthe study area, showingdate of construction, height,purpose, and sites withhistorical natural rapids.Adapted from Haxton(2002) and Haxton andChubbuck (2002)

Fig. 1 Different Ottawa River (Canada) reaches depicted along with location and date of service of hydro generating stations (G.S.).Inset map shows the location of the Ottawa River in Canada

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sequencer with a ROX-350 size standard (AppliedBiosystems, Foster City, California). Microsatellitegenotypes were scored using GeneMapper version 3.1(Applied Biosystems, Foster City, California) andvisual proofreading. The program CREATE (Coombset al. 2007) was used to format the resultant multi-locus genotypes for analysis by several geneticsoftware packages. Samples that were missing dataat four or more loci were excluded from analyses.

Analysis

To avoid bias from highly unequal sample sizesamong sites, a random subsample of the Allumettefish was chosen for further analysis. In order to ensurethat this subsample accurately represented the within-reach genetic diversity and composition despite itsreduced sample size, we tested for within-reachsubstructure using individual assignment tests (seebelow).

All loci were tested for Hardy-Weinberg equilibriumand linkage disequilibrium within sampling sites usingFisher’s exact tests in GENEPOP version 4.0 (Rousset2007) and FSTAT version 2.9.3.2 (Goudet 1995) andevaluated following a Bonferroni correction factor(Rice 1989). Allele frequencies within river sectionswere calculated in FSTAT (Goudet 2002). Total allelecounts and standardized allelic richness were calcu-lated in HP-Rare (Kalinowski 2005); the number ofprivate alleles and heterozygosity estimates for eachsample site were calculated in GENALEX version 6.2(Peakall and Smouse 2006). The inbreeding coeffi-cient (FIS) within populations was calculated inGENEPOP version 4.0 (Rousset 2007). To determineif directional (downstream) gene flow could be detected,standardized allelic richness was plotted against thelongitude (flow order) for each sampling site, with theprediction that allelic richness should increase insampling sites further downstream (Morrissey and deKerckhove 2009).

Patterns of genetic differentiation within andamong river segments were assessed to explicitly testfor evidence of anthropogenic fragmentation, usingpopulation- and individual-based analyses. Geneticdivergence among populations within river segmentswas calculated using the Weir and Cockerham(1984) estimate of FST in GENEPOP version 4.0(Rousset 2007). We also tested for evidence ofpopulation structuring of individuals among sam-

pling sites using the program Structure version 2.3(Pritchard et al. 2000; Hubisz et al. 2009) to identifythe number of genetic groups (K) and proportionalmembership of individuals to each group. Initialtrials with Structure used multilocus genotypes ofindividuals from all sampling locations including theMattagami outgroup, excluding information on sam-pling location to avoid a priori bias (Pritchard et al.2000). Subsequent Structure runs used only theOttawa River and St. Lawrence River sampling sites.Runs were conducted using default settings foradmixture 100,000 burnin and resampling steps(50,000 steps each) and four replicates of each Kfrom 1 to 10. Multiple runs with different parametersincluding assuming no admixture, increasing both theburnin and run lengths to 150,000, and runningsubsets of loci were also performed. Further analysisof the Ottawa and St. Lawrence River study sites inStructure was conducted assuming no admixture andusing population information to inform the priorprobability distribution, which biased towards find-ing localized equilibria corresponding with knownpopulation designations. Group membership ofmultilocus genotypes for individual fish was inde-pendently assessed using GeneClass2 (Piry et al.2004), using sampled river segments to define apriori groupings.

Analysis of molecular variance (AMOVA) withinand among collection sites was performed usingGENALEX (Peakall and Smouse 2006). Potentialdifferences (variance partitioning) between data sub-sets were also tested for correlation between pairwiseestimates of FST divergence with (a) geographicdistance and (b) number of barriers using Manteltests implemented in GENALEX (Peakall andSmouse 2006). For the Mantel tests, samples weregrouped first by individual sampling site (excludingthe Mattagami River outgroup), with subsequentanalysis performed excluding the St. Lawrence Riverto test for patterns of population structuring withrespect to barriers within the Ottawa River itself.Further analysis to test for the presence of naturalbarriers as suggested by Haxton and Chubbuck(2002) was performed by grouping within-reachsample sets into three groups (above Chats Falls,below Chats Falls and above the oldest dam atChaudière Falls, and below Chaudière Falls) andtesting for correlations between FST divergence andputative barriers as described above.

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Results

A total of 548 lake sturgeon were captured and analyzedfrom the different sampling sites on the Ottawa River(Table 2). Of these, a subset of 316 sturgeon from thefull dataset was used for analysis after excludingsamples with incomplete data (missing data at 4 ormore loci) as well as reducing the sample sizes fromthose sites with large numbers of captured individuals(Table 2). The sampling site at Lac la Cave had onlytwo individuals with adequate data, which did notshow any significant difference from neighbouringpopulations when tested for population assignment inGeneClass (Piry et al. 2004), and were therefore pooledwith the closest neighboring population (Holden Lake).

Genetic diversity

All microsatellite loci detected substantial diversitywithin Ottawa River sturgeon (Table 2). Allelicrichness ranging from two (AfuG195 and AfuG204)to 12 alleles per locus (AfuG68), with a mean value ofsix alleles per locus. Allele frequencies for thefourteen microsatellite loci are reported for all twelvesample sites in Appendix 1.

All loci were in Hardy-Weinberg equilibriumacross all populations with the exception of AfuG122in some populations. This locus was also reported toshow deviation from Hardy-Weinberg in some GreatLakes populations, which was attributed to thepossibility of a null allele at this locus (Welsh and

McClain 2004). A null allele has also been reportedfor the locus AfuG68 (Pyatskowit et al. 2001), but noevidence of this was observed within our dataset. Inaddition, we did not observe deviation from Hardy-Weinberg equilibrium in the St. Lawrence Riverpopulation at either locus AfuG160 or AfuG204 aswas reported by Welsh et al. (2008). No significantlinkage disequilibrium was observed between anypairs of loci after Bonferroni correction (Rice 1989).

Genetic characteristics of the sampled lake stur-geon within each sample site are summarized inTable 2. Standardized allelic richness ranged from2.85 in the Mattagami to 3.57 in two Ottawa Riversites (Holden Lake and Lac Dollard des Ormeaux;Table 2). As the sample set from within the Allumettesection of the river showed no evidence of internalsubstructure when tested in GeneClass version 2(Piry et al. 2004) and Structure (Pritchard et al. 2000)(data not shown), the random subsample (reducedsample size) was considered to accurately representthe within-reach genetic diversity and composition.Private alleles were observed in a number of samplingsites, including Holden Lake, Muskrat River, LacCoulonge, Lac Deschênes and Lac Dollard desOrmeaux as well as in the St. Lawrence and MattagamiRiver samples (Table 2). For all sample sites, observedheterozygosity values were similar to expected values(Table 2). Despite our prediction, standardized allelicrichness showed no trend with sampling location(upstream vs. downstream) (Table 2) or size of theimpounded river segments (Fig. 1).

Table 2 Sampling sites for lake sturgeon with site abbreviations, adjusted number of samples (N), latitude and longitude shown indecimal degrees, mean number of alleles observed per locus (A), standardized allelic richness averaged over all loci (As), number ofprivate alleles (Ap), expected heterozygosity (HE), observed heterozygosity (HO) and inbreeding coefficient within populations (FIS)

Sampling site Abbr N lat. long. A As Ap HE HO FIS

Holden Lake HL 26 46.2587 −78.3119 4.57 3.57 1 0.556 0.563 0.019

Allumette Lake ALL 66 45.8833 −77.2167 4.64 3.44 0 0.597 0.561 0.054

Muskrat River MR 13 45.8250 −77.1167 3.57 3.21 1 0.630 0.546 0.112

Lac Coulonge LC 27 45.8843 −76.8219 4.29 3.37 2 0.592 0.552 0.036

Lac du Rocher Fendu LRF 14 45.6371 −76.6818 3.71 3.33 0 0.610 0.533 0.104

Cheneux (Lac des Chats) CHE 17 45.5006 −76.4264 3.71 3.27 0 0.598 0.529 0.097

Lac Deschênes LDC 40 45.4495 −75.9444 4.29 3.26 1 0.664 0.552 0.207

Lac Dollard des Ormeaux LDO 37 45.6342 −74.9762 4.64 3.57 2 0.554 0.563 0.027

St. Lawrence River STL 33 46.9749 −70.8076 4.50 3.48 3 0.546 0.555 0.024

Mattagami River MAT 43 50.7281 −81.4872 3.57 2.85 2 0.522 0.514 0.004

Table 2 Sampling sites for lake sturgeon with site abbrevia-tions, adjusted number of samples (N), latitude and longitudeshown in decimal degrees, mean number of alleles observed perlocus (A), standardized allelic richness averaged over all loci

(As), number of private alleles (Ap), expected heterozygosity(HE), observed heterozygosity (HO) and inbreeding coefficientwithin populations (FIS)

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Evidence of population structure

The analysis of molecular variance (AMOVA) indi-cated that 99% of the variance within the dataset wasattributed to individuals within populations, with only1% due to difference among populations. Estimates ofgenetic differentiation among sampling locations asestimated using FST (Weir and Cockerham 1984)similarly showed less divergences among sites (Table 3),although all Ottawa River sites showed significantdifferences from the Mattagami River outgroup. Littledifferentiation was observed between all other sam-pling sites, including pairwise comparisons with the St.Lawrence River. The highest FST value was observedbetween Holden Lake and the St. Lawrence River,which had the greatest separation in terms of bothdistance and number of barriers (Fig. 1), although thedifference was not significant (Table 3). Within theOttawa River itself, no significant differences weredetected between any of the sampling sites. Despite thelack of significant differentiation among sites, geneticdivergence was positively correlated with physicalseparation among sampled populations (Fig. 2). Pair-wise FST values among locations covaried with bothestimated geographic distances among populations(Fig. 2a) and number of barriers (Fig. 2b) based onMantel tests, with correlations of 0.489 and 0.165respectively (p<0.05).

Grouping sampling sites by potential natural barriers(above and below Chaudière and Chats Falls) similarly

failed to show significant differentiation among majorriver segments (Fig. 3). Although divergence valueswere greater for site pairs including the St. LawrenceRiver, no significant pattern was detected (R=0.08; p>0.05; Fig. 3a). When the same analysis was performedamong Ottawa River segments only, the results werealso non-significant (R=−0.03; p>0.05; Fig. 3b).

Analysis of individual multilocus genotypes simi-larly failed to find significant structuring among riversegments (Fig. 4). Results from Structure (Pritchard etal. 2000; Hubisz et al. 2009) using no a prioripopulation information showed that maximum reso-lution (minimum improbability) was achieved underthe null hypothesis of a single panmictic population(K=1) for sturgeon sampled from the Ottawa andSt. Lawrence Rivers. When an outgroup from theMattagami River was included in the dataset,Structure showed that the most likely number ofpopulations was two. At higher values of K, lakesturgeon from the Mattagami River comprised aclearly separate gene pool from the Ottawa andSt. Lawrence Rivers, whose membership probabilitieswere split more or less equally among the remainingnumber of groups defined (Fig. 4a). When theMattagami outgroup was removed from analysis andthe number of genetic clusters (K) was set to two(Fig. 4b) most individuals showed equal membershipprobabilities belonging to both groups. At values ofK greater than two, membership probabilities wereagain split more or less equally among the number of

Table 3 Pairwise genetic and geographic differences among sampling sites, showing pairwise FST values (below diagonal) and riverdistances in kilometers and number of barriers (in brackets) above diagonal for eight sites along the Ottawa River as well as the St.Lawrence and Mattagami Rivers. Asterisks (*) indicate values which are significant after table-wide sequential Bonferroni correction(Rice 1989)

HL ALL MR LC LRF CHE LDC LDO STL MAT

HL 94 (2) 104 (2) 122 (2) 144 (2) 169 (4) 204 (5) 267 (6) 578 (7) n/a

ALL 0.0109 10 (0) 31 (0) 50 (0) 75 (2) 110 (3) 176 (4) 506 (5) n/a

MR 0.0152 0.0045 24 (0) 40 (0) 65 (2) 100 (3) 167 (4) 500 (5) n/a

LC 0.0029 0.0048 0.0024 30 (0) 53 (2) 84 (3) 146 (4) 476 (5) n/a

LRF 0.0196 0.0015 0.0081 0.0043 25 (2) 61 (3) 133 (4) 475 (5) n/a

CHE 0.0002 0.0004 0.0123 0.0142 0.0120 38 (1) 114 (2) 462 (3) n/a

LDC 0.0070 0.0066 0.0033 0.0109 0.0109 0.0045 78 (1) 430 (2) n/a

LDO 0.0001 0.0065 0.0110 0.0058 0.0065 0.0013 0.0059 353 (1) n/a

STL 0.0326 0.0230 0.0173 0.0427 0.0257 0.0046 0.0281 0.0218 n/a

MAT 0.0660* 0.0906* 0.0936* 0.0775* 0.0852* 0.0832* 0.0718* 0.0617* 0.1106*

Table 3 Pairwise genetic and geographic differences amongsampling sites, showing pairwise FST values (below diagonal)and river distances in kilometers and number of barriers (inbrackets) above diagonal for eight sites along the Ottawa River

as well as the St. Lawrence and Mattagami Rivers. Asterisks (*)indicate values which are significant after table-wide sequentialBonferroni correction (Rice 1989)

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groups specified (data not shown). Multiple runswith different parameters (assuming no admixture,increasing burn-in and resampling iterations, usingvaried subsets of loci) did not alter these results(data not shown). When Structure was run usingpopulation information, the results showed that themost reasonable number of populations was three,representing the St. Lawrence River, Lac DollardDes Ormeaux as a distinct second group, and a thirdcomprised of all other Ottawa River sites, withHolden Lake and Chenaux appearing to be a mix ofboth the Ottawa River groups (Fig. 4c).

Discussion

Despite extensive fragmentation of the Ottawa River byhydroelectric dams, there was little evidence of geneticerosion or isolation among lake sturgeon demes in the

separate river fragments. Although sturgeon populationnumbers are known to vary among river segmentswithin the river (Haxton 2002, 2006), this was notevident from the genetic data. Population structure dueto isolation or differentiation was not detected betweenany of our sampling locations within the Ottawa River,with broad congruence among analytical approaches.Levels of genetic diversity within and among lakesturgeon sites in the Ottawa River (observed hetero-zygosity and allelic richness) were comparable to dataobtained from other lake sturgeon populations in theGreat Lakes basin (McQuown et al. 2003; Welsh et al.2003; Welsh and McClain 2004; Welsh et al. 2008), aswell as for other sturgeon species (Smith et al. 2002;Wirgin et al. 2002; Dugo et al. 2004; Zhao et al.2005). The lack of variation in genetic diversityamong river segments underscored the historical

Fig. 2 Scatter plot of pairwise FST values against a geographicdistance and b number of barriers between nine sampling siteswithin the Ottawa and the St. Lawrence Rivers, showingcorrelation coefficients from partial Mantel tests

Fig. 3 Scatterplot of pairwise FST values versus number ofbarriers for sampling groups based on putative natural barriers(also the two oldest dam sites) for a within the Ottawa and theSt. Lawrence Rivers and b within the Ottawa River only,showing correlation coefficients from partial Mantel tests

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connectivity of the river, as there was no associationbetween allelic richness or heterozygosity with size orflow order (upstream to downstream) of river segments.

The results of our study indicate that lake sturgeonformerly comprised a single panmictic population inthe Ottawa River and that upstream movement bysturgeon was not prevented prior to development forhydroelectric purposes. The slight but non-significantdifferentiation between the Ottawa River and the St.Lawrence River concurred with earlier work byGuénette et al. (1993) who reported weak differencesbetween the Ottawa and St. Lawrence Rivers basedon mitochondrial DNA. Evidence of low differenti-ation among lake sturgeon populations has beenobserved throughout most of the Great Lakes basin(McQuown et al. 2003; Welsh et al. 2008), despitevery large distances between tributaries. Welsh et al.(2008) observed similarly low levels of populationdifferentiation between the St. Lawrence and Des

Prairies River, which flows into the St. Lawrencenear Montreal, Quebec, and has also been impactedby dams, logging and pollution. Other genetic surveyshave similarly suggested that the St. Lawrence Riverand its tributaries form part of a regional gene poolthat also includes the lower Great Lakes (Fergusonand Duckworth 1997; McQuown et al. 2003; Welshet al. 2008).

The lack of genetic evidence for fragmentation in thissystem likely reflects the long generation time of lakesturgeon, and should not be interpreted as habitatconnectivity. Published studies indicate that geneticdifferentiation in fragmented habitats may only bedetectable in species with short generation times(Carlsson et al. 1999; Koskinen et al. 2002; Heggenesand Roed 2006). For example, populations of Europeangrayling (Thymallus thymallus) which have beenisolated from one another since the 1930s (approxi-mately 12 generations) exhibit genetic differentiation

Fig. 4 Structure results forindividual-based analysis ofgenetic structure within andamong sampled segmentsof the Ottawa River, as wellas outgroup populations,grouped by samplinglocation. a populationnumber (K) of 3 for theOttawa, St. Lawrenceand Mattagami Rivers;b K=2 for the Ottawa andSt. Lawrence Rivers; andc K=3 for the Ottawa andSt. Lawrence Riversassuming no admixture andusing a priori populationinformation to identifyputative genetic groups

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that is positively correlated with the number ofseparating weirs (Koskinen et al. 2002). By contrast,long generation times may buffer the genetic effects ofhabitat fragmentation and destruction (Gibbs 2001;O’Grady et al. 2008). Similar results have beenobserved in other studies of animals with longgeneration times, including other fish (Lippé et al.2006; Reid et al. 2007) and mammals (Goossens et al.2005). Given the long generation time of lakesturgeon (22–36 years; Haxton 2008), fragmentationhas probably occurred too recently for neutral geneticmarkers to detect significant changes in geneticstructure despite hydroelectric dams presenting abso-lute barriers to upstream movement. In at least onesection of the river, sampled adult sturgeon were olderthan the year of dam construction (Haxton 2006). It istherefore unlikely that we would be able to detectsignificant genetic consequences of barrier construc-tion on lake sturgeon, as genetic effects would bereflected in biological time (generations) rather thansidereal time (years) (O’Grady et al. 2008). Fragmen-tation effects on other sturgeon populations andspecies are therefore expected to be similarly difficultto detect based on genetic data (Guénette et al. 1993;Ferguson and Duckworth 1997; Welsh and McLeod2010), and are most likely to be detected usingabundance and demographic data (Lenhardt et al.2006; Haxton and Findlay 2008).

Although larval drift over dams could maintainsome degree of connectivity among river segmentsvia unidirectional (downstream) gene flow, assumingthat at least some larvae survive to maturity andreproduce, the lack of differentiation among riverreaches prevented testing our predictions of asymmetricgene flow. Based on studies of larval drift, however, itseems more likely that the remaining diversity reflectslongevity rather than larval dispersal and recruitment.In a multi-year study of larval drift in an unimpoundedportion of the Sturgeon River, Michigan, most sturgeonlarvae and juveniles travelled downstream 45 km or lesswithin 25 to 40 days after spawning (Auer and Baker2002). In the Des Prairies River, Quebec, D’Amours etal. (2001) observed a 30-fold reduction in larval catchsuccess between sampling transects 2 and 19 kmdownstream from a known spawning site. Thesemovement distances are substantially smaller thanthose separating the dams on the Ottawa River,suggesting that substantial downstream larval drift ordispersal is unlikely (Haxton 2002, 2006).

The persistence of historical levels of geneticvariability by lake sturgeon throughout the OttawaRiver despite the presence of dams is encouraging, asretention of genetic diversity and adaptive resources isessential for enabling adaptive responses to environ-mental changes and selective pressures (Willi et al.2006). Based on published levels of diversity forother sturgeon populations and species (Smith et al.2002; Dugo et al. 2004; Welsh et al. 2008), otherextant populations may have retained comparableadaptive potential. Retention of genetic diversitydespite fragmentation was similarly attributed tospecies longevity in copper redhorse (Moxostomahubbsi) (Lippé et al. 2006) and black redhorse(M. duquesnei) (Reid et al. 2007). This is encour-aging from a conservation perspective, and suggeststhat lake sturgeon and other long-lived species havesome protection against significant erosion of geneticresources despite documented historical populationcollapses and a troubled history of anthropogenicimpacts (Harkness and Dymond 1961; Haxton andChubbuck 2002).

Simulations of habitat fragmentation in whitesturgeon have indicated that local extinction risksare highly dependent on migration rates, with up-stream populations at higher risk of extinction (Jageret al. 2001). Presently, lake sturgeon among segmentsof the Ottawa River are connected only by potentialdownstream larval drift, and the prevention of up-stream migration by dams is thought to be inhibitingpopulation recovery in impounded reaches (Haxton2006; Haxton and Findlay 2008). Upstream popula-tions may be at risk of extinction before evidence ofpopulation differentiation is observed (Gibbs 2001;Jager et al. 2001; O’Grady et al. 2008). To preventthis, facilitating upstream movement of adult fish overdams would mimic natural migration of fish and mayhelp mitigate the effects of habitat fragmentation onfuture generations.

The rate at which fragmentation will impact aspecies is also related to the habitat volume andquality in each fragment. Demographic studies of lakesturgeon in the Ottawa River have shown populationdeclines up to 80% and potential localized extirpation(Haxton 2006). In sites such as Allumette Lake withlarge patches of connected habitat, sturgeon exhibithigh abundances and demographic diversity (Haxtonand Findlay 2008, 2009). In more limited anddisconnected habitats such as Lac des Chats, however,

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lake sturgeon are scarce and have size distributionsskewed toward larger adults suggesting little or norecruitment in these sites (Haxton and Findlay 2008,2009). Similar results have been observed for redhorsesuckers (Moxostoma sp.) in fragmented rivers, whereisolated populations show differing and habitat-linkeddemographic structure despite apparent genetic conti-nuity (Reid 2008). In simulations of habitat fragmen-tation in white sturgeon, localized populations weremore likely to become extinct when habitat degrada-tion was also occurring (Jager et al. 2001). In general,fragmentation simulations have shown that effectiveconservation should be based on both habitat andpopulation management (Gibbs 2001). Removal ormitigation of barriers and/or facilitated movement offish around dams have the potential to counteract theeffects of fragmentation, but will only be successful ifcritical habitat patches are available and connected(Jager et al. 2001).

Conclusions

Our study shows that lake sturgeon in the OttawaRiver represent a single genetic population thatreflects its formerly continuous (historical) habitat,rather than contemporary habitat patches. As it isunlikely that the existing dams will be removed inthe foreseeable future, some alternate means ofconnectivity between habitat patches should beestablished to maintain the genetic cohesion of lakesturgeon in the Ottawa River. These findings havesignificant implications for the sustainable manage-ment and rehabilitation of other sturgeon popula-tions and species (Ferguson and Duckworth 1997,Lenhardt et al. 2006). Altering existing barriers toallow fish passage, translocation of wild adult orjuvenile sturgeon, or stocking hatchery-reared stur-geon from local sources are all potential options andeach have their own associated pros, cons andcaveats (Jager 2006a, b). In addition to maintaininggenetic diversity, an emphasis should also be placedon maintaining high quality habitat throughout theriver and fostering linkages among critical habitatsfor all life stages. By restoring some degree ofconnectivity among habitat patches, these actionswill significantly mitigate existing fragmentationeffects and help ensure the long-term persistence ofthis unique species.

Acknowledgments We would like to thank the field crewsfrom the Ontario Ministry of Natural Resources AquaticScience Unit, Pembroke and Kemptville Districts and theQuebec Ministry of Natural Resources and Wildlife inGatineau, who assisted in collecting the tissue samples.Funding for this project was supplied by the Ontario Ministryof Natural Resources Science and Information, RenewableEnergy, and Applied Research and Development Branches.

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