ORIGINAL PAPER

Zoe Lucas Æ Pierre-Yves Daoust

Large increases of harp seals (Phoca groenlandica) and hooded seals(Cystophora cristata) on Sable Island, Nova Scotia, since 1995

Accepted: 11 April 2002 / Published online: 4 June 2002� Springer-Verlag 2002

Abstract Beach surveys for harp (Phoca groenlandica)and hooded (Cystophora cristata) seals documented adramatic increase in their numbers on Sable Island in themid-1990s. From late 1994 to 1998, 1,191 harp and 870hooded seals,mostly young animals, were recorded on theisland whereas, in the 1980s, no more than 5 animals ofboth species were observed each year. Of the 2,061 harpand hooded seals examined, 41.7% were found alive,26.7% were killed by sharks, and 31.6% were found deadbut intact. This increase in numbers of harp and hoodedseals on Sable Island, which is south of their historicnorthern range, is consistent with the recent increase ofextralimital occurrences of these species along the eastcoast of North America. However, the large number ofseals recorded in this study provides more information ontheir demography than has previously been possible.

Introduction

Harp and hooded seals are ice-breeding species thatmigrate annually between the subarctic and arcticregions of the Atlantic. Harp seals in the northwest At-lantic are normally found no further south than the Gulfof St. Lawrence (approximately 47�N) (Sergeant 1991).Hooded seals are a more offshore species, again notusually found south of the Gulf of St. Lawrence(Hammill 1993). Young hooded seals have a tendency towander and have been reported in areas far outside the

normal range of this species (Burns and Gavin 1980;McAlpine et al. 1999b; Mignucci-Giannoni and Odell2001). Stevick and Fernald (1998), McAlpine et al.(1999a, b) and Harris et al. (2001) reported an unprece-dented increase in numbers of extralimital harp andhooded seals in the Gulf of Maine in the 1990s, partic-ularly since 1994. These seals, most of them observedannually between late January and mid-May, were pri-marily immatures and seemed to include a mixture ofboth healthy and unhealthy animals. McAlpine et al.(1999a) also noted that numbers of harp and hoodedseals had increased along the Atlantic coast of NovaScotia and the New England states. They concluded thatthe collapse of fish stocks in the western North Atlanticin recent years, occurring concurrently with an increasein size of harp and hooded seal populations resultingfrom decreased commercial harvest of these two species,explained the sudden increase in extralimital distribution.Lacoste and Stenson (2000) reported a southern shift inwinter distribution of harp seals off eastern Newfound-land and southern Labrador in the 1990s.

Sable Island is an isolated, crescent-shaped sandbar,42 km long and up to 1.4 km wide, located in the westernNorth Atlantic at about 44�Nand 60�W, the only landfallon the Scotian Shelf, and approximately 160 km south-east of Canso, the nearest landfall in Nova Scotia,Canada. In January 1994, increased numbers of harp andhooded seals were noted on the island, and surveys todocument this increase began in December 1994. Due toits location far offshore and because its entire shorelinecan be surveyed, the island provides an opportunity togather information on the demography of the animalsinvolved in extralimital movements. This paper reportsthe results of a surveyofnumbers of harpandhooded sealson Sable Island between December 1994 and May 1998.

Materials and methods

Prior to 1995, records of harp and hooded seals on Sable Islandwere anecdotal. Although surveys for these two species were not

Polar Biol (2002) 25: 562–568DOI 10.1007/s00300-002-0393-8

P.-Y. Daoust (&)Department of Pathology & Microbiology,Atlantic Veterinary College,University of Prince Edward Island,Charlottetown, PE C1A 4P3, CanadaE-mail: daoust@upei.caTel.: +1-902-5660667Fax: +1-902-5660851

Z. LucasP.O. Box 64, Halifax CRO,Nova Scotia B3J 2L4, Canada

carried out, beach monitoring and research on the island have beenunderway since the 1970s. Research on Sable Island harbour(Phoca vitulina) and grey (Halichoerus grypus) seals began in theearly 1970s (e.g. Mansfield and Beck 1977; Boulva and McLaren1979), and throughout the 1980s and 1990s the beaches were fre-quently and extensively travelled during all seasons by researchersinvolved in harbour and grey-seal tagging programmes andbehavioural studies (e.g. Stobo and Zwanenburg 1990; Bowen et al.1994; Ellis et al. 2000), marine litter and cetacean stranding studies(e.g. Lucas 1992; Lucas and Hooker 2000), research on seal mor-tality caused by shark predation (Lucas and Stobo 2000), andstudies of the Sable Island horses (Lucas et al. 1991). Since the mid-1980s, the senior author has lived on Sable Island for most of theyear. During the 1980s, sightings of harp and hooded seals wereexceptional enough to be noted anecdotally. Beach surveys for harpand hooded seals were carried out on Sable Island duringDecember to May from December 1994 to May 1998 by the seniorauthor. All 80 km of the island’s beach were searched at approxi-mately 7-day intervals, except during several periods of 3 weeksannually when the observer was off island. The survey effort andtiming were roughly the same during each of the 4 years. Whilesurveys and markings of live harp and hooded seals were not car-ried out during June to November, travel and activity on the beachfor other projects continued at a similar level of effort, and recordswere kept of harp and hooded seals sighted.

During December to May, live young seals were captured onthe beach and individually marked with animal-marking crayons ofdifferent colours (Viehzeichenstift Raidex, Kane VeterinariansSupplies, Cambridge, Ontario) before release. Adult seals were notcaptured and marked, but in most cases it was possible to identifythem on the basis of pelage-pattern characteristics. The subsequentfate of marked or otherwise identified seals, if known, wasrecorded. Dead seals, comprising both those that died on the beachafter coming ashore alive and those washed ashore as carcasses,were also examined. Carcasses with shark-inflicted wounds weredistinguished from scavenging by the presence of characteristicpatterns of punctures and cuts (Z. Lucas, unpublished data).Depending on the condition of the seal carcass (e.g. presence ofinjuries, scavenging or decomposition), measurements were takenfollowing the recommendations of the American Society ofMammalogists (1967). Information recorded included sex, size(standard length, axillary girth), and estimated date of death. Jaws(for aging) were collected from some specimens. Seals were groupedinto three age categories: pups (1–3 months old, born in March orApril of the same year that they were found on Sable Island);immatures (10 months to approximately 4 years old, the youngestborn in March or April of the year prior to the year that they werefound on the island); and adults. Age category was determined onthe basis of one or more of the following variables: standard length,based on its published relationship with age of normal harp andhooded seals collected from the North Atlantic (Wiig 1985;McLaren 1993; Chabot et al. 1996); pelage pattern, colour andwear (Lavigne and Kovacs 1988); presence of lanugo (in harp seals)(Lavigne and Kovacs 1988); and tooth sections (G. Stenson,Department of Fisheries and Oceans Canada, St. John’s, New-foundland, personal communication). Pups born in the Gulf of St.Lawrence could reach Sable Island during the first week of April,and because immature molt normally occurs at 13–14 months ofage (Sergeant 1991) and there is much overlap in standard length ofpups and young immatures (Kovacs and Lavigne 1986; McLaren1993; Chabot et al. 1996), distinguishing between pups and smallimmatures was in some cases difficult. Therefore, young sealswhose age category was uncertain were categorized as ‘‘pup/im-mature’’.

A one-proportion test was used to determine whether the sexratio deviated significantly from parity in the groups of immatureharp and hooded seals and in a group of 326 shark-killed harp sealpups found in 1997. A chi-square test of independence was used todetermine whether there was a significant difference in sex ratioamong the groups of immatures of either species that had beenkilled by sharks or that had been found either alive or dead on thebeach. For immatures, only seals found between December and

March were used in the calculations, in order to avoid any potentialconfusion with pups in April and May.

Results

During the 1980s, no more than five alive and dead harpand hooded seals (combined) were observed each yearon Sable Island, the hooded seal being the species mostoften seen. These numbers increased in the mid 1990s; inJanuary 1994, the carcasses of 3 immature harp sealsand 17 immature hooded seals were found, and a smallnumber of live animals were sighted. From December1994 to May 1998, 1,191 harp seals and 870 hooded sealswere recorded on Sable Island between December andMay. These totals include live seals, intact carcasses, andthe carcasses of shark-killed animals found on the beach(Table 1). While some adults and pups were found, themajority of harp and hooded seals were immature:64.0% and 93.6%, respectively. The difference betweenthese two percentages was magnified by a single event in1997 in which an unusual number of shark-killed sealpups, mostly harp, washed ashore between 22 April and8 May (Table 1), greatly inflating the shark-kill total forthat year. Some of these carcasses were very fresh; otherswere decomposed and may have drifted into the areafrom some distance. While shark-killed harp seal pupsand immatures were found during winter and spring inall 4 years of the study, the April 1997 observationssuggested that many pups killed elsewhere were cast upon Sable Island along with carcasses of those killednearby. Excluding the large number of pups foundduring this event, the overall proportion of immaturesfound during December 1994 to May 1998 was similarfor both species, 88.1% and 94.2% of harp and hoodedseals, respectively. Although the number of these im-

Table 1. Total numbers of harp and hooded seals found on SableIsland, Nova Scotia, during beach surveys carried out during De-cember to May from December 1994 to May 1998

Adult Immature Pup Pup/immaturea

Total

HarpShark-killed 0 142 346b 22 510Alive 16 333 1 9 359Alive-deadc 13 49 0 1 63Dead 15 238 3 3 259Total 44 762 350 35 1191

HoodedShark-killed 0 24 6b 10 40Alive 4 299 0 0 303Alive-deadc 10 118 3 3 134Dead 18 373 1 1 393

Total 32 814 10 14 870

aAnimals in this group could not be assigned with certainty toeither the pup or the immature age groupb326 of the 346 harp-seal pups and all 6 hooded-seal pups killed bysharks were found in 1997; many of the carcasses may have been inthe water for several weekscFound alive on the beach and later found dead

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mature animals fluctuated annually (Fig. 1), it wasnoticeably higher in any of these years than had beenobserved prior to 1995.

Live seals were found throughout January/April,with smaller numbers in May and December (Table 2),and with occasional sightings in June to November(fewer than ten harp and hooded seals combined duringthis 6-month period in each of the 4 years). During thestudy period, the earliest confirmed sightings of live pupson the island occurred on 22 April 1997, for a harp seal,and on 5 April 1997, for a small, thin hooded seal. Ex-cluding the small number of live immature harp andhooded seals which may have been included in the pup/immature group (a maximum of 13 harp and hoodedseals combined, Table 2), most live immature animals(77.2% of harp seals and 90.4% of hooded seals) re-corded on Sable Island during December/May werefound ashore during January to March. Similarly,83.8% of immature harp seals and 83.3% of immaturehooded seals killed by sharks were found during January

to March. Of live adult harp and hooded seals, 89.7%and 64.3%, respectively, were found during January toMarch.

Most of the immature harp seals were likely 10–14 months old (i.e. born during March of the previousyear), and probably none were older than 2 years. Of 12harp seals whose tooth sections were collected in 1995,10 were 1 year old, and 2 were 2 years old. Most hoodedseals categorized as immatures were likely 3 years old oryounger. All but four immature hooded seals had anunspotted blueback pelage. According to Lavigne andKovacs (1988), the blueback pelage is lost during thefirst post-natal moult. However, 2- or 3-year-old hoodedseals with a full blueback pelage are not uncommon(G. Stenson, personal communication). Of the fourimmature hooded seals having a spotted pelage, two(a live, large individual of unknown sex, and a dead, thin120-cm-long female) had a blueback pelage with darkerspots showing on the back and head, and two (a live,large female, and a dead 144-cm-long male) had a fullyspotted pelage similar to adults. There was a slight butstatistically significant (P £ 0.001) predominance ofmales among the total numbers of immatures in bothspecies: 59.7% in harp seals (95% confidence interval,0.55–0.64, n=574) and 56.6% in hooded seals (95%confidence interval, 0.53–0.60, n=648). However, therewas no statistically significant difference (P>0.05) in sexratio among the groups of immatures of either speciesthat had been killed by sharks or that had been foundeither alive or dead on the beach. There was also nosignificant sex predominance in the group of 326 shark-killed harp-seal pups washed ashore in spring 1997.

Of 382 immature harp seals and 417 immaturehooded seals found alive on the beach and markedduring December 1994 to May 1998, only 3.4% (13animals) and 9.4% (39 animals), respectively, were seenalive again on the island after the first sighting (1 harpseal as long as 39 days later; 1 hooded seal as long as30 days later), and 12.8% (49 animals) and 28.3% (118animals), respectively, were found dead after the firstsighting. The majority of live immature seals marked(83.8% of harp seals and 62.4% of hooded seals) werenot seen again, alive or dead. These proportions for eachspecies changed little from year to year (Fig. 2). The lownumber of resightings suggests that many seals did notlinger long on the beach; some may have died, however,and were buried by wind-blown sand. Of the 2,061 harpand hooded seals of all age groups examined on SableIsland during December 1994 to May 1998, only 2 boretags. One, an immature female harp seal found dead inApril 1995, had been tagged as a newborn near theMagdalen Islands, Gulf of St. Lawrence, on 1 March1994 (P. Carter, Department of Fisheries and OceansCanada, Mont-Joli, Quebec, personal communication).The other, a 1-year-old male hooded seal observed alive,robust, and very active on 3 May 1996, had been taggedand released on 29 February after rehabilitation at theMarine Mammal Stranding Center, Brigantine, NewJersey (B. Schoelkopf, personal communication). Also,

Fig. 1. Numbers of immature harp and hooded seals found onSable Island, Nova Scotia, during December to May fromDecember 1994 to May 1998 (unshaded animals found alive ordead on the beach; black shading animals killed by sharks)

Table 2. Monthly occurrence of harp and hooded seals when firstfound alive on Sable Island, Nova Scotia, as indicated by beachsurveys carried out during December to May from December 1994to May 1998

Dec Jan Feb March April May Total

HarpAdult 0 10 11 5 2 1 29Immature 10 129 103 63 53 24 382Pup – – – – 1 0 1Pup/immaturea

– – – – 3 7 10

Total 10 139 114 68 59 32 422HoodedAdult 1 0 5 4 4 0 14Immature 1 171 156 50 30 9 417Pup – – – – 2 1 3Pup/immaturea

– – – – 2 1 3

Total 2 171 161 54 38 11 437

aAnimals in this group could not be assigned with certainty toeither the pup or the immature age group

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an immature male harp seal that we crayon-marked onSable Island on 17 January 1998, was found in NewJersey on 14 March in an emaciated and dehydratedstate (B. Schoelkopf, personal communication). The lasttwo animals had each covered at least 1,200 km in theapproximate 60-day interval between sightings.

Discussion

Harbour and grey seals breed on Sable Island and areyear-round residents (Mansfield and Beck 1977; Boulvaand McLaren 1979; Stobo and Zwanenburg 1990). Inaddition, during winter and spring, adults and young ofharp and hooded seals, and young ringed seals (P.hispida) (Z. Lucas and D.F. McAlpine, unpublisheddata) have been observed on the island. Surveys begin-ning in winter 1994–1995 documented greatly increasednumbers of harp and hooded seals as compared toprevious years. The total numbers of seals of these twospecies that actually occurred on the island were cer-tainly underestimated by the beach surveys. More than70% of immature harp and hooded seals marked duringDecember 1994 to May 1998 were seen only at the timeof marking and were not found again, alive or dead.Thus, some seals which came ashore between surveysand remained on the beach only briefly or died and wereburied in the sand were likely missed by the surveys.Observations of increased numbers of harp and hoodedseals on Sable Island confirmed reports by others of adramatic increase in the numbers of these species outsidetheir historic northern range during the 1990s (McAl-pine and Walker 1990; Stevick and Fernald 1998; Mc-Alpine et al. 1999a, b; Harris et al. 2001). Extralimitalrecords of harp and hooded seals in the northwest At-lantic were rare until the 1990s when there was a markedincrease in numbers of these seals in areas south of whatis considered their normal range (Kovacs and Lavigne1986; McAlpine and Walker 1990; Sergeant 1991). In

1988–1993, 7 harp seals and 3 hooded seals, all live, werereported from the New Jersey area but, during 1994–1996, 72 harp seals and 11 hooded seals were found inthe same region (B. Schoelkopf, personal communica-tion). Stevick and Fernald (1998) documented an in-crease in the frequency of extralimital records of harpseals in Maine since 1994, noting that, prior to that year,harp seals were almost unknown along the coast ofMaine, and that the increase in the numbers of harp andhooded seals was sudden and dramatic and did not re-flect an increase in survey effort. McAlpine et al. (1999a,b) reported a more than 90-fold increase in extralimitaloccurrences of harp and hooded seals in the Bay ofFundy and the northern Gulf of Maine region since1994. Lacoste and Stenson (2000) reported that south-eastern shifts in the winter distribution of harp sealsseem to have occurred since the early 1990s, movingmore to the south in 1994 and 1995, and they noted thatthis southern shift agrees with the increase in extralimitaloccurrences of harp seals along the east coast of NorthAmerica.

Harp and hooded seals have broadly similar life his-tories. Distribution and migration patterns of harp sealshave been described by Sergeant (1976, 1991). Sergeant(1991) considered the harp seal to be a nearshore species,but recent findings (Stenson and Sjare 1997; Lacoste andStenson 2000) indicate that Newfoundland harp sealsutilize offshore waters during winter. Evidence frommarked young in the Gulf of St. Lawrence has shownthat harp seals are capable of swimming long distancesat 4 weeks of age (Sergeant 1976). Distribution ofhooded seals is similar to that of harp seals (Sergeant1976; Kovacs and Lavigne 1986) although, traditionally,they have been less common in the Gulf of St. Lawrenceand are thought to occupy deeper waters. All age groupsof this species, particularly young in their first summer,have a greater tendency to wander outside their normalrange than do harp seals (Sergeant 1974, 1976; Burnsand Gavin 1980; McAlpine et al. 1999b; Mignucci-Giannoni and Odell 2001). Tag recoveries indicate thatmovements of young hooded seals can be erratic(Hammill 1993).

The majority of harp and hooded seals reportedfrom the Bay of Fundy and the Gulf of Maine since1994 were young animals (Stevick and Fernald 1998;McAlpine et al. 1999b; Harris et al. 2001). Similarly,most harp and hooded seals found on Sable Islandduring December 1994 to May 1998 were immatures,and the combined recoveries of live, intact-dead, andshark-killed seals indicated that the overall proportionof immatures was similar for both species. There was ahigher proportion of males among immatures (80.0%in harp seals, Stevick and Fernald 1998; 83.3% and67.3% in hooded seals, McAlpine et al. 1999b andHarris et al. 2001, respectively) in the Bay of Fundyand Gulf of Maine than on Sable Island, but theseratios were based on smaller sample sizes. Sergeant(1991) found considerable variation in sex ratio amongharp seals 1-year-old or older depending on time and

Fig. 2. Fate of immature harp and hooded seals found alive andindividually identified on Sable Island, Nova Scotia, duringDecember to May from December 1994 to May 1998 (total of382 harp seals and 417 hooded seals) (unshaded animals seen onlyonce; grey shading animals seen alive more than once; black shadinganimals found dead after the first sighting

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region of sampling, but reported more males (53%)among juvenile animals.

The seasonal timing of the extralimital sightings ofharp seals corresponds with the timing of migrations toand from traditional whelping areas. While harp sealswere observed on Sable Island in small numbersthroughout the year, most occurred during the 3-monthperiod of early January to late March. The arrival ofharp seals at Sable Island in January is consistent withthe southward movement from arctic summering areas.Young harp seals arrived approximately 1 month laterin more southerly Maine areas (Stevick and Fernald1998). Harp-seal sightings on Sable Island diminishedduring April and May. Similarly, Stevick and Fernald(1998) reported an abrupt drop in number of sightings inMaine in April and suggested that this drop corre-sponded to the normal moult period when young andadults converge at the southern extremity of the pack icein preparation for the northerly migration in May(Sergeant 1991). On Sable Island, the temporal distri-bution of hooded seals was similar to that of harp seals.Movements of marked and tagged extralimital harp andhooded seals generally reflect these seasonal migrationpatterns of the young of these two species. The harp sealmarked on Sable Island in January and found in NewJersey in March suggests that, early in the season, someseals may pass through the Sable Island area and per-haps linger before continuing to more southerly regions.Also, harp and hooded seals tagged in extralimitalsouthern areas have been reported later in normalnortherly areas. Of five harp seals tagged in Maine,Massachusetts or New Jersey and seen later in tradi-tional harp-seal feeding areas in Newfoundland andGreenland (McAlpine and Walker 1990; Stevick andFernald 1998; B. Schoelkopf, personal communication),four were tagged in southern regions in April and werefound in waters off Newfoundland in May. At leastthree of these four seals were caught in nets set forlumpfish (Cylopterus lumpus Linnaeus) (McAlpine andWalker 1990; Stevick and Fernald 1998; B. Schoelkopf,personal communication). The fifth seal was released inMaine in June and caught in southwest Greenland inNovember of the same year (McAlpine and Walker1990). Likewise, a hooded seal tagged in New Jersey inlate February (B. Schoelkopf, personal communication)was seen on Sable Island in May, presumably headingnorth.

Harp and hooded seals spend most of their time atsea (Kovacs and Lavigne 1986; Stenson and Sjare 1997),and therefore individuals found ashore may be biasedtowards those in poor body condition. McAlpine andWalker (1990) noted that most extralimital harp sealsfound were animals in poor health but suggested that itis difficult to assess the significance of this because sickanimals are more likely to be encountered on beachesand reported. However, most of the 26 harp seals re-ported by Stevick and Fernald (1998) appeared healthy.McAlpine et al. (1999b) reported that about one-third ofthe hooded seals in the Bay of Fundy and northern Gulf

of Maine (a minimum of 25 animals recorded for 1994–1998) were in poor condition. The suggestion that sealsin poor condition are more likely to be encountered onbeaches and reported (McAlpine and Walker 1990) issupported by the Sable Island observations whereemaciation was considered the dominant and primarydisease problem in the immature harp and hooded sealsthat died on the beach or that were washed ashore intact(Z. Lucas, unpublished data). A higher average sternalblubber thickness in shark-killed immature harp seals(22.5 mm, as compared to 13.26 mm in intact carcasses)and shark-killed immature hooded seals (20.75 mm, ascompared to 11.56 mm in intact carcasses) indicatedthat they were in relatively better body condition thananimals that came ashore and died (Z. Lucas, unpub-lished data). However, comparison of the sternalblubber thickness of shark-killed immature harp sealswith values (35–45 mm) provided by Chabot et al.(1996) in immature harp seals collected at the same timeof year in the northwest Atlantic suggests that mostanimals in waters surrounding Sable Island were thin.There is no evidence of food shortage for harbour andgrey seals using Sable Island. Data on body mass ofharbour-seal mothers and pups were collected during thesummer breeding season in 1987–1996 (Ellis et al. 2000),and body mass and composition for adult grey seals atthe start of the December-February breeding season andweaning mass of grey-seal pups have been measured inmost years from 1986 to the present. None of these dataindicate a decline in body condition of these two speciesover time (W.D. Bowen, Department of Fisheries andOceans Canada, Dartmouth, Nova Scotia, personalcommunication). The diet of grey seals near SableIsland, however, is composed primarily of sand lance(Ammodytes spp.), Atlantic cod (Gadus morhua) andsilver hake (Merluccius bilinearis) (Bowen and Harrison1994), whereas capelin (Mallotus villosus) is a very largecomponent of the normal diet of harp seals (Hammilland Stenson 2000). Although less is known about thediet of hooded seals, Greenland halibut (Reinhardtiushippoglossoides) constitutes a large proportion of the fishspecies consumed by hooded seals (Hammill andStenson 2000).

The occurrence of large numbers of harp seals inregions beyond their normal distribution as a result of anincreasing population size, and the poor condition of suchanimals, were predicted by Brodie and Pasche (1982).These authors also noted that the effects of intraspecificcompetition in harp seals would be exacerbated by thedepletion of prey resources caused by commercial fisher-ies. McAlpine et al. (1999a) suggested that collapse of thefish stocks on which harp and hooded seals prey, com-bined with seal-population growth, may have been themost significant recent change, correlating with the sud-den increase in extralimital records of these two species.There is good evidence that the population of harp sealshas increased in the recent past, but data are insufficient toreach a similar conclusion for hooded seals (McLarenet al. 2001). Chabot et al. (1996) andHammill et al. (1995)

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reported a decline in body condition (reflected by changesin bodymass and length) of harp seals collected in easternNewfoundland and theGulf of St. Lawrence, respectively,in the early 1990s, as compared to earlier periods, perhapsreflecting a chronic food shortage for these animals.However, commercial fish species such as Atlantic cod(G. morhua) do not appear to have been a major compo-nent of the diet of harp seals, even prior to the collapse ofthe stocks of these species (Hammill and Stenson 2000).Capelin, a species of greater importance as a food sourcefor harp seals, underwent significant changes in its biologyin the 1990s, including a southward expansion of itsdistribution onto the eastern Scotian Shelf and an increasein population numbers (Carscadden et al. 2001). Theabundance of Greenland halibut, a large component ofthe diet of hooded seals, showed a marked decline in thelate 1980s and early 1990 period, particularly in northernareas off the coast of Labrador. The stock started torecover in the mid-1990s, but currently remains belowhistoric highs (Bowering 2000).

The high proportion of immatures observed on SableIsland is consistent with Sergeant’s (1991) description ofmovements of harp seals, in which there is a generalsegregation between adults and younger age classes(0–3 years) in this species. These two age groups mayrespond differently to pressures for extralimital migra-tions, whatever the nature of these pressures may be.There is evidence in other pinniped species that animalsin their first 3 years of life are more susceptible tonutritional stress brought about by changes in theoceanic environment or by increased population densi-ties (Fowler 1990; Bickham et al. 1998). It is not clear,however, that density-dependent factors are responsiblefor the changes seen in the distribution of harp andhooded seals. The influence of other biological andphysical factors is suggested by the concurrence of recentchanges in harp and hooded seal distribution withanomalous meterological and hydrographic conditions,which may have, in turn, influenced distribution of preyspecies (McAlpine et al. 1999a; Lacoste and Stenson2000; Rowe et al. 2000; Carscadden et al. 2001).

Acknowledgements We thank G. Forbes (Sable Island Station,Meteorological Service of Canada), the Nova Scotia PetroleumDirectorate, ExxonMobil Canada Energy, PanCanadian Resourc-es, and J. Conway (Department of Fisheries and Oceans Canada)for logistical assistance, and W. Stobo and D. Cairns (Departmentof Fisheries and Oceans Canada) and three anonymous reviewersfor helpful comments on the manuscript.

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