Long-distance movements of harbour seals (Phocavitulina) from a seasonally ice-covered area, theSt. Lawrence River estuary, Canada

Véronique Lesage, Mike O. Hammill, and Kit M. Kovacs

Abstract: Previous studies of harbour seal (Phoca vitulina L., 1758) movements indicate that this species is relativelysedentary throughout the year. However, few investigations have examined their movements and seasonal distributionpatterns in ice-covered areas. This study used spatial analysis of ice data and movement data from harbour seals col-lected via satellite (n = 7) and VHF radiotelemetry (n = 15) to explore this species’ spatial use patterns in a seasonallyice-covered region, the St. Lawrence River estuary, Canada. When solid ice formed within the bays of the estuary, fourof the seven satellite-tagged animals (all adult males) left their summer haul-out areas, migrating 266 ± 202 km (range65–520 km) to over-wintering sites. The seals exhibited preference for areas of light to intermediate ice conditions dur-ing the winter months; at least six of the seven seals occupied areas with lighter ice conditions than those that prevailedgenerally in the study area. Evidence of high abundance of potential prey for harbour seals in the estuary during wintersuggests that reduced availability of adequate food resources is not the primary factor which influences the movementand distribution patterns of harbour seals. Movement patterns observed during the ice-free period concur with previ-ously reported harbour seal behaviour; the seals remained near the coast (<6.1–11.0 km from shore) in shallow waterareas (<50 m deep in 100% VHF and 90% SLTDRs (satellite-linked time–depth recorders)) and travelled only shortdistances (15–45 km) from capture sites. None of the VHF- or satellite-tagged seals crossed the 350 m deep Laurentianchannel, which suggests that this deep body of water might represent a physical barrier to this coastal population.

Résumé : Des études antérieures des déplacements du phoque commun (Phoca vitulina L., 1758) indiquent quel’espèce est relativement sédentaire tout au cours de l’année. Cependant, peu de travaux ont examiné les déplacementset les patrons de répartition saisonnière dans les régions couvertes de glace. Notre étude effectue une analyse spatialedes données sur la glace et sur les déplacements des phoques récoltées par satellite (n = 7) et par radio-télémétrie VHF(n = 15) afin d’explorer les patterns d’utilisation de l’espace chez cette espèce dans l’estuaire du Saint-Laurent, Ca-nada, une région couverte de glace pendant une partie de l’année. Lorsque la glace solide s’est formée dans les baiesde l’estuaire, quatre des sept phoques porteurs d’un émetteur satellite (tous des mâles adultes) ont quitté leurs échoue-ries d’été pour migrer sur 266 ± 202 km (étendue, 65–520 km) pour rejoindre leurs sites d’hiver. Les phoques ont dé-montré une préférence pour les endroits où la glace est mince ou moyennement épaisse durant les mois d’hiver; aumoins six des sept phoques ont occupé des sites ayant des conditions de glace moins rigoureuses que celles qui préva-laient généralement dans la région d’étude. Il semble exister une forte abondance de proies potentielles pour les pho-ques dans l’estuaire durant l’hiver; il ne semble donc pas qu’une baisse de la disponibilité des ressources alimentairesadéquates soit le facteur explicatif principal des déplacements et des patrons de répartition des phoques. Les patrons dedéplacement observés durant la période sans glace concordent avec les comportements décrits antérieurement chez lephoque commun; les phoques restent près de la côte (<6,1–11,0 km de la rive) en eau peu profonde (<50 m de profon-deur chez 100 % VHF et 90 % SLTDR (enregistreur de durée et de profondeur relié à un satellite)) à <50 m de pro-fondeur) et effectuent seulement de courts déplacements (15–45 km) à partir des sites de capture. Aucun des phoquesporteurs d’un émetteur VHF ou satellite n’a traversé le chenal Laurentien profond de 350 m, ce qui laisse croire quecette masse d’eau profonde pourrait constituer une barrière physique pour cette population côtière.

[Traduit par la Rédaction] Lesage et al. 1081

Can. J. Zool. 82: 1070–1081 (2004) doi: 10.1139/Z04-084 © 2004 NRC Canada

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Received 13 May 2003. Accepted 16 June 2004. Published on the NRC Research Press Web site at http://cjz.nrc.ca on17 Septembert 2004.

V. Lesage1,2 and K.M. Kovacs.3 Department of Biology, University of Waterloo, Waterloo, ON N2L 3G1, Canada, and NorwegianPolar Institute, N-9296 Tromsø, Norway.M.O. Hammill. Maurice Lamontagne Institute, Department of Fisheries and Oceans, P.O. Box 1000, Mont-Joli, QC G5H 3Z4,Canada.

1Corresponding author (e-mail: LesageV@dfo-mpo.gc.ca).2Present address: Maurice Lamontagne Institute, Department of Fisheries and Oceans, P.O. Box 1000, Mont-Joli, QC G5H 3Z4,Canada.

3Present address: Norwegian Polar Institute, N-9296 Tromsø, Norway.

Introduction

The annual cycles of many mammals display a seasonallyvariable pattern of food acquisition and energy balance. Thispattern is often concomitant with seasonal migratory phasesthat take them between geographically different areas wherepeak periods of food ingestion occur (e.g., Cameron andWhitten 1979; Lockyer 1987). Pinnipeds have a somewhatunusual pattern of food acquisition and energy balance dur-ing their annual cycle. Although these animals forage in amarine environment, they require a solid substrate for moult-ing, parturition, and in some species, for mating (Bartholomew1970).

Most large phocid seal species temporally and spatiallyseparate periods of intensive feeding from reproduction bybuilding up energy reserves prior to the reproductive periodand then fasting or at least reducing food intake duringbreeding and moulting, using stored blubber reserves to bal-ance their energy demands during these times. Amongsmaller phocids (<100 kg), the ability to store energy is lim-ited by body size. As a result, feeding may be obligatoryduring the breeding or moulting periods because energy de-mands associated with these activities may exceed their stor-age capacity (Hammill et al. 1991; Bowen et al. 1992;Coltman et al. 1998). The need to locate sites for reproduc-tion in close proximity to abundant food resources may limitthe distribution and seasonal movements of small phocids(Boyd 1998).

The requirement for proximity to food resources in smallphocids may also reduce the need for extended migrationswithin these species. However, seasonal fluctuations in foodabundance and accessibility (e.g., through changes in ice-cover conditions) probably affect their seasonal distributionpatterns. These environmental factors vary between regions,and thus, might be expected to introduce variability in sea-sonal distribution and movement patterns among populationsor colonies of a given species.

The harbour seal (Phoca vitulina L., 1758) is a small(<100 kg), coastal phocid that hauls out regularly on a vari-ety of substrates. Owing in part to their small size, harbourseals are unable to store sufficient energy to sustain com-plete fasting during the breeding or moulting seasons (Bowenet al. 1992; Coltman et al. 1998). As a result, they must feedduring these periods of high-energy demand. Previous stud-ies of harbour seal movements indicate that these animalsundertake only limited seasonal movements, and that theyseem to find both their necessary food resources and haul-out substrates within relatively restricted ranges throughoutthe year at most locations (Harvey 1987; Stewart andYochem 1994; Bjørge et al. 1995; Thompson et al. 1996,1998). However, the sedentary nature of harbour seals maybe overemphasized in the literature as a result of the highlyseasonal nature of many studies, the limited means of thetechnology used in most studies for tracking animals at sea(reviewed in Thompson 1993), and the limited number ofstudies that have examined seasonal movements of populationsor colonies from seasonally ice-covered areas. Movements ofharbour seals in such habitats might be expected to occur overlarger scales (Gjertz et al. 2001; Lowry et al. 2001).

In this study, seasonal movements and characteristics ofpreferred habitats of harbour seals from a seasonally ice-

covered area were examined using a combination of satelliteand VHF telemetry.

Materials and methods

This study was conducted in the St. Lawrence River estu-ary, Canada (Fig. 1). In this area, harbour seals haul out insmall colonies during summer, principally in a ≈175-km por-tion of the estuary located downstream of the SaguenayRiver to about Pointe-des-Monts (hereinafter the “lower es-tuary”). Animals were captured at three of four major (>50individuals) haul-out sites in the lower estuary (Lesage et al.1995). Two sites, Bic Island and Métis Beach, were locatedalong the south shore of the estuary. The third site was asmall island (Ile Blanche) located in the middle of the estu-ary ≈150 km upstream from Bic Island.

Harbour seals were captured using gill nets set in the wa-ter close to haul-out sites. In the St. Lawrence River estuary,harbour seals are accustomed to dealing with fishing gearand small boats, and during our capture attempts the sealsoften climbed over the float line and swam away without be-coming entangled. This limited our ability to direct our cap-tures towards specific age or sex groups (Table 1). Twenty-three seals were captured between May and October 1994–1997. Seals were immobilized with an intramuscular injec-tion of Telazol® (Tiletamine and Zolazepam; 0.5 mg·kg–1;Fort Dodge® Laboratories, Fort Dodge, Iowa) and wereweighed (±0.5 kg). A lower incisor was extracted for age de-termination (Bernt et al. 1996). Animals were equipped witha VHF-radio transmitter (Lotek Engineering Inc., Newmarket,Ontario), a time–depth–velocity recorder (TDR Mk6; Wild-life Computers, Redmond, Washington), and a stomach tem-perature recorder (HTR; Wildlife Computers, Redmond,Washington) (n = 14) or a 0.5-W satellite-linked time–depthrecorder (SLTDR; 15 cm × 5.5 cm × 6 cm, 650 g; WildlifeComputers, Redmond, Washington) (n = 8). The telemetryand recording devices were glued to the middle of the back(TDR/HTRs) (Hammill et al. 1999), high on the neck on theback of the skull (SLTDRs), or on the top of the head(VHFs). Detailed dive data from the TDRs and SLTDRs, aswell as information from the HTRs, are reported elsewhere(Lesage 1999; Lesage et al. 1999). Only movement data andparameters relevant to the interpretation of position informa-tion are assessed in this study.

Summer movements of harbour seals bearing VHF trans-mitters were monitored between mid-May and September1996 and 1997, from a 7-m Boston Whaler equipped with a6-element Yagi antenna mounted 4 m above sea level. AVHF receiver was turned on when the tracking vessel leftthe marina and was programmed to scan successively (onefrequency per 3–4 s) the radio frequencies of the differentanimals. The first signal that was received determined theanimal which would be tracked on that particular day. Thetracking vessel remained 300–400 m away from each studyanimal to minimize disturbance. A computer, interfaced withthe boat’s global positioning system, and an echo sounderlogged the time and position of the boat each time the ani-mal surfaced or dove.

Harbour seal movements during the autumn, winter, andspring were documented using satellite telemetry by collect-ing information on the position of animals every other day in

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1994–1996 (seven of eight seals) or every 3rd day in 1997(seal No. 17903). Locations were validated using an iterativeforwards/backwards averaging filter adapted from McConnellet al. (1992) to identify erroneous positions. Pairs of succes-sive positions, which implied an average velocity of >2 m·s–1

(i.e., equivalent to about twice the mean speed recordedfrom the St. Lawrence River harbour seals fitted with Mk6TDRs when diving to ≥4 m; Lesage et al. 1999), were re-jected. A large proportion of locations (51%–70%) were ofpoor quality (classes 0, A, or B). Because the inclusion ofthese poor-quality positions in the calculation of the meanvelocity could increase the mean velocity of good-quality lo-cations to values >2 m·s–1 and could cause their rejection, anew velocity was calculated when a good fix (class ≥0) wasobtained during the preceding 24 h, using this fix and theposition in question. The position was retained if the newvelocity was ≤2 m·s–1. This approach reduced the proportionof locations rejected from 43% to 30%. Movements of sealsrelative to the shoreline (of either the mainland or the is-lands) were examined by calculating the distance betweeneach position and the shoreline using ArcView version 3.2(Environmental Systems Research Institute Inc. 2002).

Daily composite estimates of ice conditions in the estuaryand the northwestern Gulf of St. Lawrence were acquiredfrom the Canadian Ice Service, Ottawa, Ontario. The largenumbers of ice charts covering the 4 years of the study andthe need to extract the data from each chart manually neces-sitated a subsampling of the database. Hence, one ice chartwas systematically selected every 10 days between 20 De-

cember and 1 May of each year of the study. The study areawas divided into 406 cells (6 naut. mi. × 6 naut. mi. each,approximately 11 km × 11 km; 1 naut. mil. = 1.852 km). Anice index was calculated for each cell from the product ofthe percentage of ice concentration (in tenths) in the cell anda thickness code of the prevailing type of ice. Areas charac-terized by open water or thin ice (<10 cm) resulted in indi-ces of <25, whereas areas with intermediate ice coverage(4/10 to 8/10) of thin to medium-sized ice thickness(<30 cm) were described by indices of 25–60. Indices from60–72 were indicative of areas with ice coverage of 9/10 to9+/10 and thicknesses of 30–120 cm. Cells dominated bysolid ice were restricted to bays or other very shallow waterareas; these areas had indices of 75–100. Mean ice indicesover the 4 years of the study were calculated for successiveperiods of 20 days. Contours of areas with similar mean iceindices were produced using Surfer® version 8 (Golden Soft-ware, Inc. 2002).

Ice-data resolution permitted the determination of ice con-ditions over scales of 25–50 km2. However, ice data areknown to be biased upwards intentionally for navigationsafety reasons, and also because they neglect rapid deforma-tions of ice cover (e.g., wind-driven leads) or other morepersistent, small-scale phenomenon.

Harbour seal location data were examined first in terms oftiming of movements relative to ice formation and breakupusing a descriptive approach, and second, in relation withthe physical characteristics of ice using statistical analyses.Numerical analyses using multivariate statistical approaches,

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Fig. 1. Map of the estuary and the northwestern Gulf of St. Lawrence, Quebec, Canada, showing capture and haul-out sites of harbourseals (Phoca vitulina), as well as important geographical reference points. The broken line across the St. Lawrence River represents thelimit between the estuary and the Gulf of St. Lawrence.

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such as those used in some other studies (e.g., Ferguson etal. 1998; Wilson et al. 2001), were not possible in this studybecause of the small number of animals equipped withSLTDRs relative to the number of input variables (Hair etal. 1995). Habitat preferences of harbour seals in relation toice conditions were examined during the period when icewas present in the estuary or the Gulf of St. Lawrence (i.e.,between 20 December and 30 April). Biases associated withautocorrelation between successive positions were reducedby randomly selecting one position every 6-h period fromthe positions remaining after forwards/backwards filtration.Ice conditions coincident with each seal location were ex-tracted using Surfer® version 8 (Golden Software, Inc. 2002).Habitat preferences were examined on a monthly basis fromthe statistical comparison of two cumulative frequency dis-tributions: one being the ice conditions associated with the Npositions of seals and the second being a random sample ofN of the 406 previously characterized cells describing the“available” habitat (prevailing local ice conditions) in thestudy area during the selected period. Similarity in cumula-tive distributions was tested using a Kolmogorov–Smirnovtest (Siegel 1956). Patterns in data transmissions were exam-ined throughout a diurnal cycle to address possible biasesthat would result in discontinuity in the data, and thus, a vio-lation of an assumption of the Kolmogorov–Smirnov test.

Results

Six of the eight harbour seals equipped with SLTDRs

were caught at Métis, whereas the other two deploymentstook place at Bic and Ile Blanche. One of the SLTDRs failedshortly after its deployment on a seal from Métis, and hence,was not included in the data set (Table 1). The other sevenSLTDRs transmitted data for 111–295 d (245 ± 26 d) be-tween August and June 1994–1998. Six seals were locatedon 68–187 d over their respective deployment periods; theremaining individual transmitted only 12 positions, spreadover the 245 days of deployment. The forwards/backwardsfilter of positions eliminated 13%–46% of positions (mean =31%, n = 7) and reduced by 1%–29% (mean = 13%, n = 7)the number of days with usable positions.

Fifteen VHF-radio-equipped seals were tracked for 246 h,spread over 41 sessions (mean = 6.0 h of consecutive track-ing, range 1.5–16.5 h). Individual animals were tracked fromone to eight times.

Ice conditions in the estuary and in the Gulf of St.Lawrence

In each year of the study, ice formed in the estuary priorto its formation in the Gulf of St. Lawrence. Nearly com-plete coverage of drifting pack ice occurred by late Februaryeach year (Fig. 2). The three capture sites in the St. Law-rence River estuary and in other bays near Bic Island andMétis Beach were covered with fast-ice by December andwere clear of ice sometime in April. This fine-scale informa-tion was not apparent from the ice maps because of their rel-atively coarse resolutions. Ice conditions were consistentlylighter along the north shore than along the south shore of

Seal No. RegionaMass(kg) Sex

Age(year)

Packagedeployed

Period (SLTDR) or date(VHF) of deployment

Recordduration (d)

17905 ME 78 M 5 SLTDR 9 Sept. 1994 – 1 July 1995 29517906 ME 73 M 5 SLTDR 9 Sept. 1994 – 30 Apr. 1995 23317904 BI 49.5 M 3 SLTDR 20 Aug. – 9 Dec. 1995 1111851 ME 83 M 6 SLTDR 27 Oct. 1995 (failed) 01854 ME 82 M 6 SLTDR 25 Sept. 1995 – 23 May 1996 24117908 ME 80.5 M 8 SLTDR 18 Sept. 1995 – 12 May 1996 2371855 ME 76 F 6 SLTDR 16 Sept. 1995 – 28 May 1996 25517903 IB 47 M 2 SLTDR 28 Sept. 1997 – 31 May 1998 2453197 BI 75.5 M 5 VHF/TDR 13 May 1995 —3199 BI 36 M 1 VHF/TDR 20 Aug. 1996 —3860 BI 39 M 1 VHF/TDR 20 Aug. 1996 —4615 BI 102.5 M 10 VHF/TDR 10 June 1996 —4618 BI 100 M 8 VHF/TDR 11 June 1996 —4619 BI 69 F 10 VHF/TDR 12 June 1996 —4601 ME 83 M 7 VHF/TDR 7 June 1996 —4612 ME 83.5 M 6 VHF/TDR 7 June 1996 —2696 BI 38.5 F 1 VHF/TDR 29 May 1997 —2699 BI 96.5b F 6 VHF/TDR 3 June 1997 —2700 BI 35.5 F 1 VHF/TDR 3 June 1997 —3502 ME 38 F 1 VHF/TDR 18 June 1997 —3508 ME 38.5 M 1 VHF/TDR 8 July 1997 —3526 BI 33.5 F 1 VHF/TDR 9 Sept. 1997 —3531 IB 59.5 M 2 VHF/TDR 24 Sept. 1997 —

aSeals were captured at Métis (ME), Bic (BI), or Ile Blanche (IB).bPregnant.

Table 1. Deployment period and characteristics of the St. Lawrence River estuary harbour seals, Phoca vitulina, equipped with a radiotransmitter and a time–depth recorder / stomach temperature data logger package (VHF/TDR) or a satellite-linked time–depth recorder(SLTDR) during 1994–1998.

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Fig. 2. Mean ice conditions observed in the estuary and the northwestern Gulf of St. Lawrence between 20 December and 1 May of1994–1998 as indicated by an ice index that was calculated based on ice concentration and thickness indices.

the estuary, the Gulf of St. Lawrence, and the Baie desChaleurs (Fig. 2). Light ice conditions prevailed at the headof the Laurentian channel, near the mouth of the SaguenayRiver and north of Ile Blanche, throughout winter.

Seasonal movements relative to the timing of iceformation and breakup

Adult animals performed long-distance seasonal movements.Four of the seven seals equipped with SLTDRs left theirsummer haul-out sites during November to mid-Decemberand migrated distances of 520, 330, 150, and 65 km, respec-tively (Fig. 3). All of these animals that over-wintered awayfrom their summer haul-out sites were adult males (5–8 yearolds) and all were originally caught at Métis. Two of thefour animals that showed seasonal movements migrated westto Ile Blanche and Bic (Figs. 3a, 3b), where they remainedthroughout the winter. The other two animals migrated eastand over-wintered in the Gulf of St. Lawrence, returning to

the estuary in the spring (Figs. 3c, 3d). One of these twoseals migrated to Baie des Chaleurs in early November,where it visited the southern and western portions of the bay(Fig. 3c). The seal moved to the northern portion of Baie desChaleurs when solid ice began to form in the southern andwestern portions of the bay in early January (Figs. 2a, 2b).The spreading of ice into the northern sections of Baie desChaleurs in late January (Figs. 2c, 2d, 2e) was associatedwith a retreat of the seal northwards, moving from the bay tothe mouth of Gaspé Bay, where it remained until mid-March.A small area with light to intermediate ice conditions (indi-ces <50) prevailed at the mouth of the Gaspé Bay through-out the winter (e.g., Figs. 2c, 2d). Several dry positions(“on-land” rate) were recorded for this seal while it was as-sociated with a solid ice edge (Figs. 2c–2f), indicating thatthis seal probably hauled out at the ice edge of the frozenbay during this period. The second seal that migrated out of theestuary during the autumn over-wintered in the Cloridorme

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Fig. 3. Capture sites (indicated by asterisks) and seasonal movements of four adult male harbour seals (a–d) documented using satellitetelemetry. Periods of residency at the various sites are indicated within the boxes.

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area along the Gaspé peninsula, though it did make sporadicexcursions to Cap-des-Rosiers (Fig. 3d). Ice conditions wereintermediate in the Cloridorme area (indices 25–40), but ice

conditions were somewhat heavier both in thickness and incoverage in the Cap-des-Rosiers area (indices >60).

No long-distance seasonal movements were observed

Fig. 4. Seasonal distribution patterns of three harbour seals (seelegend) equipped with satellite transmitters that did not under-take seasonal migrations.

Fig. 5. Seasonal variation in distribution patterns of harbour sealNo. 17906 when ice was absent (+) and present (�) in the Bicarea.

Fig. 6. Ice conditions at real locations of harbour seals (solid bars and solid curve) and at a random sample of available locations inthe lower estuary and the northwestern Gulf of St. Lawrence (open bars and dotted curve). The bars represent frequency distributions,whereas the curves represent cumulative frequency distributions.

from the two subadults (2- and 3-year-olds) captured at Bicand Ile Blanche or from the single adult female captured atMétis. The records for these animals extended until earlyDecember, February, and May, respectively (Fig. 4). Somefine-scale seasonal patterns were observed in the local distri-bution of one seal, which spent the winter and spring nearBic. From January until March when ice coverage intensi-fied in areas near Bic (Figs. 2b–2e), a shift was observed inthe distribution of this seal’s locations from the area sur-rounding Bic Island to an area located a few kilometres tothe east (Fig. 5). No seasonal changes were detected in thelocal distribution patterns of the adult female that over-winteredat Métis, where ice concentration and thickness were high(indices >60). Local seasonal changes in distribution pat-terns could not be examined for the Ile Blanche / head of theLaurentian channel area because of the small number of po-sitions obtained during autumn from the harbour seal cap-tured at this site.

Distribution relative to ice conditionsFrom December until April, when ice was present in the

estuary and in the Gulf of St. Lawrence, 95% of harbour seallocations came from areas with ice indices <47 (i.e., with in-termediate to light ice conditions; Fig. 6). Harbour sealswere located in areas with ice indices >60 (i.e., with heavyice coverage and thick ice) in a few instances during Januaryand March (Fig. 6b, 6d). The comparison of frequency dis-tributions of ice conditions at the seal’s locations relative toa random sample of “available” ice conditions suggested thatthe harbour seals exhibited some selectivity with respect toice conditions (Kolmogorov–Smirnov test, Da = 2.13, P =0.0002, n = 253). Analyses on a monthly basis revealed apattern of preferences that was variable over the winter sea-son. Specifically, harbour seals were located in heavier iceconditions than expected by chance during periods of iceformation and breakup (i.e., in December, January, and April)(Kolmogorov–Smirnov test, Da = 1.79–3.17, P = 0.0001–0.007;Figs. 6a, 6b, 6e). During February and March when ice cov-erage was maximal in both the estuary and the Gulf of St.Lawrence, harbour seals were found in lighter ice conditionsthan the norm in these regions (P = 0.0025 and 0.006 forFebruary and March, respectively) (Figs. 6c, 6d). Theassumption of continuity, necessary for the use of theKolmogorov–Smirnov test of similarity of distributions, wasmet; some variability occurred, but no systematic break indata transmission from satellite transmitters was observedwith time of the day.

Daily movements and site fidelityThe distances moved were generally limited once seals

were in their winter or summer areas. During summer, anaverage of 87.8% (range 81.4%–94.4%) of the positions ofquality ≥0 from the four satellite transmitters still functioningduring May–June were within 10 km of summer haul-outsites. Similarly, harbour seals that were tracked by radio-telemetry during daylight hours between May–September1996 and 1997 remained within 3 km of their haul-out sitesduring the majority (83%, n = 34/41) of the tracking ses-sions, and 14 of 15 seals were relocated at the haul-out sitewhere they were first captured during the tracking efforts.However, satellite telemetry and VHF-tracking indicated in-

frequent but regular, more distant excursions (15–30 km)away from the haul-out sites during the ice-free period. Ex-cursions were made over somewhat larger distances duringspring and autumn (i.e., movements performed before or af-ter winter migrations in migrating individuals, or betweenMarch and October in nonmigrating seals). Repeated 1- to6-d trips to areas 15–65 km away from the summer haul-outsites were recorded in both spring and autumn from five ofthe seven seals equipped with satellite transmitters, includ-ing movements between Métis and Bic by three of the seals.Two seals also made short trips between their winter andtheir summer areas before abandoning one area or the other.These trips between Métis and Cloridorme and Bic and IleBlanche represented round-trip distances of up to 660 kmand lasted 6–12 d. Movements during the winter seasonwere limited to within a 20-km radius in four seals. SealNo. 1855, which remained near Métis all winter, routinelyventured 40–65 km away during winter, similar to hermovement patterns in the spring and autumn, whereas thewintering area of seal No. 17908 was spread over 60 km ofcoastline. This seal did not make series of short trips be-tween the extremes of its wintering area, but rather, movedprogressively through the region.

The site fidelity observed during a given season did notnecessarily hold from 1 year to the next. Only one of fourseals from Métis returned directly to its initial capture site inthe spring (Fig. 3a). Two seals, which over-wintered awayfrom their capture sites, adopted Bic rather than Métis astheir haul-out site the next spring (Figs. 3b, 3d). However,the last transmissions from their tags occurred on 30 Apriland 12 May, so it is not known if they returned to their ini-tial capture sites later in the spring prior to breeding. An-other male that over-wintered near Ile Blanche stopped atBic for 3 weeks in April before returning to Métis (Fig. 3c).

Harbour seal movements into areas where water depthswere ≥50 m were limited. Seals that were tracked usingVHF telemetry were not observed in waters >50 m, and anaverage of 90% (range 74%–100%, n = 7) of the locations ofquality ≥0 from animals equipped with SLTDRs were in wa-ters that were <50 m. Satellite telemetry also indicated thatseals followed coastlines, either of islands or of the main-land during short excursions (15–45 km) away from haul-outsites, staying within 5 km from the shore. The coastal habitsof harbour seals were also revealed through the median dis-tances of positions of individual seals relative to the shore-line (2.2–3.6 km), with 75% and 90% of the positions notexceeding 4.3–7.9 km and 6.1–11.0 km, respectively (n = 7animals). Although a few satellite locations suggested thatseals ventured into the Laurentian channel, there were no in-dications that seals crossed to the other side of the channel;no positions were obtained from the north shore (Fig. 3).

Discussion

Satellite telemetry has been widely applied to the studyof movements of marine and terrestrial wildlife. Its usewith marine mammals has generally been restricted to thestudy of relatively long-distance movements, given the non-negligible inaccuracies in locations that result from using theDoppler effect and the ARGOS system (Stewart et al. 1989;Goulet et al. 1999; White and Sjöberg 2002; but see Thomp-

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son et al. 2003). In this study, satellite telemetry was used tostudy movement patterns of a species that tends to reside inrestricted areas. The results revealed long-distance seasonalmovements among the adult male harbour seals that weretracked, and some patterns in the distribution patterns ofmost of the tagged animals in response to prevailing ice con-ditions. Migration distances that are reported in this studyare at the upper limit of those previously observed fromadult harbour seals. They indicate a capacity among at leastadult males of this species to undertake long-distance migra-tions as part of their annual activity/distribution cycle. Thelongest movements reported for this species prior to satellitetelemetry records were dispersal movements by immatureindividuals (up to 550 km; e.g., Bonner and Witthames1974; Boulva and McLaren 1979; Brown and Mate 1983;Whitman and Payne 1990; Thompson et al. 1994; de Jong etal. 1997) rather than seasonal migrations by mature seals (upto 280 km; Pitcher and McAllister 1981; Harvey 1987). Per-haps because of the limitations of VHF telemetry to detectlarge-scale movements, the seasonal nature of many studies,or the ice-free nature of waters in regions where most previ-ous studies were conducted, the harbour seal has beenthought to be sedentary (Bjørge et al. 1995; Thompson et al.1996; 1998). Previous studies that have used satellite telem-etry on this species have supported the relatively sedentarynature of some harbour seal populations (Stewart andYochem 1994; Lowry el al. 2001), but also have revealedlarger scale migrations by adult harbour seals in other popu-lations (e.g., Gjertz et al. 2001). The results that were ob-tained from the different studies over a broad geographicalrange emphasized the variability that existed in patterns andamplitudes of seasonal movements of mature harbour sealsboth within a colony and among colonies or populations.

A need for regular access to abundant food resources isprobably the most important factor explaining the variabilityobserved in seasonal movement patterns between and withinharbour seal populations. The periods preceding and follow-ing breeding and moulting are times of high energy intakebecause seals must forage to meet daily energy requirementsin addition to replenishing or maximizing their body stores(Pitcher 1986; Beck et al. 1993; Carlini et al. 1997; Krafft etal. 2002). Ice formation and other changes in physicaloceanographic parameters, and varying ecological needs ofprey species during their yearly cycles, represent factors thatmight regulate the seasonal distribution of prey, and there-fore, access to food resources for harbour seals (Bailey et al.1977; Smith and Nelson 1986; Stirling 1997; Swain et al.1998). Little information exists regarding the diet of harbourseals from the estuary or the Gulf of St. Lawrence. Althoughtheir diet is dominated by fish (Lesage et al. 2001), only afew otoliths have been recovered from stomach lavages (n =17 subadults of 0 to 2 years old and one adult female).These belonged to a limited number of species: rainbowsmelt (Osmerus mordax (Mitchill, 1814)), herring (Clupeaharengus L., 1758), capelin (Mallotus villosus (Müller,1776)), sand lance (Ammodytes americanus DeKay, 1842),and winter flounder (Pseudopleuronectes americanus(Walbaum, 1792)). Rainbow smelt are present during thewinter months as they are fished through the ice in severallocalities, including Rimouski (near Bic) and the Gaspé Bay.Capelin are also known to occur in abundance in the St.

Lawrence River estuary where they constitute a major preysource for the harp seals, Pagophilus groenlandicus(Erxleben, 1777), wintering in this area (Bailey et al. 1977;Anderson and Gagnon 1980; Sergeant 1991; Murie andLavigne 1991). Sand lance are nonmigratory (Mercille andDagenais 1987), but they may not be equally accessibleyear-round because they remain buried in the sand duringmost of the winter (Winslade 1974), except for a shortspawning period between October and December (Brêthes etal. 1992). Seals in other regions are thought to prey on sandlance even during the winter by disturbing the sediment(Tollit et al. 1998). Winter flounder is available in the estu-ary throughout the winter along the lower estuary’s southshore (Fisheries and Oceans Canada 1995), but they maymove into deeper waters during the winter period (Andersonand Gagnon 1980). The winter availability of these potentialprey species and the limited movements observed amongsome of the harbour seals captured at Bic, Ile Blanche, andMétis suggest that a reduced availability of adequate foodresources is unlikely to be the primary factor influencingmovement and distribution patterns of harbour seals in theSt. Lawrence River estuary. The presence of some ice covermight in fact serve to concentrate food resources in areas ofopen water or at ice edges, and increase ease of access toprey (Smith and Nelson 1986; Stirling 1997; Swain et al.1998). However, whether this phenomenon occurs or not inthe estuary or the Gulf of St. Lawrence and its role in thedistribution patterns observed in harbour seals in this studyare not known.

Ice cover may act as a physical barrier that limits accessof harbour seals to food resources. In the estuary and theGulf of St. Lawrence, the seasonal distribution and move-ment patterns of harbour seals were influenced by ice condi-tions. Patterns in ice conditions that are documented duringthis study are consistent with historical information on icecoverage and modeling results of the formation and circula-tion processes of water masses and sea ice in the lower estu-ary and the Gulf of St. Lawrence (reviewed in Saucier et al.2003). Saucier et al.’s (2003) study indicates that the head ofthe Laurentian channel remains ice free during most of thewinter because the area has warmer, more saline surface wa-ters and upwellings resulting from northwesterly winds andtidal turbulence. These authors also demonstrated the pres-ence of a persistent gyre structure in the currents along theGaspé peninsula near Cloridorme, which resulted in some-what lighter ice conditions in this area, and the presence ofwind-driven leads that persisted throughout the winter alongthe north shore of the estuary and the Gulf of St. Lawrence.Harbour seals have been observed at cracks or at ice-edges,but they are incapable of maintaining breathing holes andare not normally associated with extensive ice cover(Mansfield 1967; Calambokidis et al. 1987; de Jong et al.1997; Gjertz et al. 2001). The absence of movements by theseal captured at Ile Blanche near the Laurentian channelhead, and the autumn migrations of the seals from Métis thatoccupied Ile Blanche, Cloridorme, northern Baie desChaleurs, and Gaspé Bay areas during the winter may resultfrom the predictable presence of persistent, lighter ice condi-tions. Although these observations must be interpreted withcaution given the small number of animals captured at sitesother than Métis, they support previous reports of intoler-

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ance of heavy and persistent ice cover by harbour seals. Theapparent selection of heavier ice conditions by harbour sealsduring ice formation and breakup as opposed to duringmonths when ice coverage is the most extensive is counter-intuitive. Two phenomena might be involved: first, becauseof the tendency for this species to prefer coastal areas, whichis also where ice forms first and disappears last, and second,ice conditions early and late in the season may be below thethreshold of tolerance for harbour seals, and thus, not exten-sive enough during these periods to limit their access to foodresources.

Harbour seals were observed at Bic and Métis during win-ter. These two areas had high ice indices. These observationssuggest that harbour seals can tolerate heavy ice conditionsat least for short periods of time. The change in the distribu-tion of harbour seals from Bic towards an area located at theeastern end of Bic Island indicates the influence of local iceconditions. Near Bic, residual movements of ice and watermasses are towards the east (Saucier et al. 2003), and thus,might create a persistent opening in the ice cover east of BicIsland coincident with the area where the animals werefound during winter, but the opening is too small to be re-vealed on low-resolution ice charts. Métis is an area wherethe ice cover is known to be very dynamic, with leads open-ing and closing along with current movement and changingtidal and wind conditions. The reasons for the presenceof one seal in this area throughout the winter (Fig. 4) are notas obvious as for the other seals that occupy predictable ar-eas of light ice conditions. We speculate that the presence ofthis seal might be related to the presence of a persistent ice-free microhabitat (Mansfield 1967), which is not captured onice charts because of the charts’ low spatial and temporalresolutions.

Harbour seals showed strong fidelity to their summeringand wintering haul-out sites, undertaking only occasionalshort (1–6 d) trips to areas ≤65 km away from these sitesonce they were established on them. These relatively limitedmovements were within the range of those previously re-ported for this species, and supported the suggestion thatfood resources were readily available near haul-out sites (re-viewed in Thompson 1993; see also Harvey 1987; Thomp-son et al. 1996; Suryan and Harvey 1998; Tollit et al. 1998;Lowry et al. 2001). Most of the harbour seals remained inthe shallow waters of the south shore and none of the taggedanimals spent significant amounts of time on the north shoreof the lower estuary opposite to their tagging sites despitethe fact that colonies occurred on both sides of the river only13–50 km apart (Lesage et al. 1995). A few satellite loca-tions were recorded from the Laurentian channel, which sug-gested that the harbour seals occasionally moved into thisarea of deep water (350 m). However, these positions mightalso be artefacts of the filtering methodology, which doesnot eliminate all erroneous positions. In areas such as the St.Lawrence River estuary where movements of harbour sealsare essentially coastal and where shallow diving is the norm(Lesage 1999; Lesage et al. 1999), the deep water might rep-resent a physical barrier to dispersal and dictate to someextent the direction and extent of seasonal movements. Sig-nificant genetic differentiation and differences in contami-nant profiles have been observed among harbour sealcolonies located only a few hundred kilometres apart,

including those from the estuary and the Gulf of St. Law-rence (Stanley et al. 1996; Goodman 1998; Härkönen andHarding 2001; Westlake and O’Corry-Crowe 2002; Lebeufet al. 2003). A greater genetic similarity between neighbour-ing colonies of harbour seals in the North Pacific has led tothe suggestion that dispersal movements follow an axis coin-cident with coastlines and the continental shelf (Westlakeand O’Corry-Crowe 2002).

In summary, harbour seals in the St. Lawrence River estu-ary exhibited typical summer movement patterns for the spe-cies, with the seals spending most of their time in shallowwaters near their haul-out sites. They also appeared to ex-hibit quite strong seasonal and interannual patterns in site fi-delity at least to a small geographic area. However, in thisseasonally ice-covered area, approximately half of the ani-mals (all of the adult males) tagged with SLTDRs exhibitedseasonal migrations of 65–520 km, indicating that not allharbour seal populations were sedentary. The persistent pres-ence of some seals during the winter at haul-out sites exten-sively used during the summer months indicates thatadequate food resources are available throughout the year inthe St. Lawrence River estuary. The preference exhibited byharbour seals for areas with lighter ice conditions during pe-riods when ice cover was most extensive indicated that iceconditions could have an important effect on their winterdistribution. Larger sample sizes will be required to establishwhether sex and age trends exist, but this study suggests thatadult males have a greater tendency to perform long-distanceseasonal migratory patterns.

Acknowledgements

Special thanks are extended to W.D. Bowen and Y. Dubéfor stimulating discussions about this work, and to K.E.Bernt, M. Deakos, J.-F. Gosselin, C. Mursell, S.-E. Picard,and I. Welch who helped with the fieldwork. J.-F. Gosselinaged the teeth and provided a preliminary version of the fil-tering program. G. Fortin helped construct the maps. Thanksare also extended to R. Emond for allowing the installationof a VHF station on Bic Island. Financial support was pro-vided to V.L. by the Fonds pour la Formation de Chercheurset l’Aide à la Recherche, Canadian Wildlife Foundation,University of Waterloo Graduate scholarship program, DavisMemorial Fund, and to K.M.K. by the Natural Sciences andEngineering Research Council of Canada research grants.This study was funded under the St. Lawrence Action Plan –Vision 2000 research program of Fisheries and Oceans Can-ada.

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