Harbour seal (Phoca vitulina) COSEWIC assessment and status report: chapter 6

Biology

Harbour seals have the broadest distribution and occur in more different habitats than any other pinniped species. They exhibit considerable plasticity in their biology and behaviour (Burns 2002).

Life cycle and reproduction

The sex ratio for harbour seals after the first year is close to one, with some evidence of a higher mortality rate in subsequent years for males (Boulva and McLaren 1979; Härkönen and Heide-Jørgensen 1990). Females reach sexual maturity at around 4 years (y) of age, with males becoming sexually mature slightly later (Boulva and McLaren 1979; Härkönen and Heide-Jørgensen 1990). Average age of mature females (generation time) was estimated to be about 9 years (based on an annual survival rate of 81% for harbour seals in the Maritime provinces and reproductive rates of 27% at age 4 y, 55% at 5 y, 79% at 6 y and 95% for ages 7+ y – from Boulva and McLaren 1979).

Females generally have one pup every year on land, and the pupping season in any particular area can extend for one to two months with a two-week peak (Bigg 1981). Pups often follow their mothers into the water within hours of their birth (Lawson and Renouf 1985) and evidence from Sable Island suggests that pups of younger females gain mass at a lower rate than those of older females through mid-lactation (Bowen et al. 2001b). Pups are weaned at about four weeks of age (on Sable Island at around 24 days), and mating occurs in the water at around this time (Bigg 1981; Muelbert and Bowen 1993). In 2000 in the St. Lawrence River estuary, the median date of weaning was 26 June (Dubé et al. 2003).

Unlike harbour seals in Europe, which pup mainly in late June and show no significant latitudinal variation, pupping in P. v. concolor is more variable and is positively related to latitude (Bigg 1969; Temte et al. 1991). The southern limit of pupping appears to be Maine where pupping occurs in late May. It becomes progressively later by about 1.7 days per degree latitude, with the latest being late June or early July on Southampton and Baffin islands (Bigg 1969; Temte et al. 1991; Stewart and Lockhart 2005), though the median pupping date in the St. Lawrence River estuary (26 May in 2000) occurs somewhat earlier than that predicted by this relationship (Dubé et al. 2003). Pupping in the Churchill River is thought to occur around the beginning of June (Bernhardt 2005). Twomey (1938) found afterbirth on a rock at Kasegalik Lake on the Belcher Islands around this time.

Temte (1994) was able to statistically differentiate crania of local populations of P. v. richardsi, the harbour seal subspecies on the Pacific coast of North America. He found a significant relationship between differences in cranial morphometry and population differences in birth timing, and suggested that these “allochronic”, as well as allopatric factors, may be acting to promote population differentiation.

With respect to P. v. mellonae there is evidence that, in addition to the population’s apparent geographic isolation from other harbour seal populations, similar allochronic mechanisms may be at play. From his conversations with the indigenous peoples of Northern Québec, Doutt (1942) reported that the mean pupping date of P. v. mellonae occurred in early May. This date has also been corroborated by more recent observations (Consortium Gilles Shooner & Associés et al. 1991). Using data collected by Doutt (1942) and growth curves calculated by Boulva and McLaren (1979) and Bigg (1969), Smith et al. (1994) arrived at two different estimated pupping dates for Lacs des Loups Marins: Between 6 and 31 May; and 10 May. Both of these estimates for the mean birth date of P. v. mellonae are substantially earlier than for other harbour seal populations at similar latitude (Smith et al. 1994). If Temte et al. (1991) and Temte (1994) were correct in their conclusion that birth timing in harbour seals has a heritable component and therefore functions to create reproductive barriers between adjacent populations, these data support the hypothesis that P. v. mellonae is reproductively isolated from P. v. concolor. However, it is unclear to what extent timing of pupping in harbour seals is an inherited characteristic or one that varies in response to local environmental conditions.

The moult generally occurs during midsummer to early autumn, within 2 or 3 months of the pupping season (Burns 2002). In Lacs des Loups Marins, animals hauled out in the spring months are usually in small groups, whereas at the end of the summer, they are usually hauled out singly or in pairs, a behaviour that has been linked by some observers to the moulting process (Consortium Gilles Shooner & Associés et al. 1991). However, Krieber and Barrette (1984) noticed an increase in group size from spring to autumn at a haulout in Forillon National Park, Québec.

As was noted by Härkönen and Heide-Jørgensen (1990) it is difficult to compare mortality rates between different harbour seal populations because in some areas hunting is included in the mortality. Boulva and McLaren (1979) tried to factor out hunting in their mortality rate calculation of about 17.5% for 1+ animals in one harbour seal population in Nova Scotia. On Sable Island, Lucas and Stobo (2000) estimated shark-inflicted mortality of pups to be under 10% during 1980-93, roughly 25% in 1994-95, and 45% in 1996, with shark-related deaths in all age groups recorded year-round. The maximum lifespan of harbour seals appears to be around 30 years (Härkönen and Heide-Jørgensen 1990).

Diet

Published stomach content and scat analyses for northwest Atlantic harbour seals indicate a broad diet consisting of invertebrates as well as planktivorous and omnivorous fish (Boulva and McLaren 1979; Payne and Selzer 1989; Lesage et al. 1999). Based on their examination of isotopic signatures, Lesage et al. (2001) concluded that harbour seals occupy the highest trophic position in the St. Lawrence River estuary and that these animals fed primarily on estuarine, as opposed to Gulf of St. Lawrence, prey species. In their comparison of two areas off Nova Scotia and New Brunswick, Bowen and Harrison (1996) documented geographic and interannual variability in prey taken by harbour seals. Approximately 40 prey species were identified in harbour seal stomachs sampled from near shore areas of Newfoundland and Labrador, including 32 fish species and 18 invertebrate species (Sjare et al. 2005). Little has been documented regarding the diet of harbour seals in the Arctic. Beck et al. (1970) found lake whitefish (Coregonus clupeaformis) and lake trout (Salvelinus namaycush) in the stomach of one animal killed in the Thlewiaza River (located south of the western Hudson Bay Inuit community of Arviat).

The Cree of Whapmagoostui have long contended that the Lacs des Loups Marins seals feed in fresh water and that these animals taste different than oceanic harbour seals, making them a favoured target for hunting (Twomey 1938; Posluns 1993; Petagumskum 2005). The Cree also consider that this freshwater diet is one of the reasons why the Lacs des Loups Marins seals’ pelage is darker and more lustrous than its saltwater counterpart (Posluns 1993; Petagumskum 2005).

An examination of the only four P. v. mellonae stomachs available indicated that the diet of these animals consisted, in large part, of resident lake whitefish, lake trout, and brook trout (S. fontinalis) (Smith et al. 1996). Power and Gregoire (1978) conducted a study in which fish from Lacs des Loups Marins were compared with samples caught in nine nearby Ungava peninsula lakes. After determining that brook trout was the dominant fish species in Lacs des Loups Marins, and that lake trout and lake whitefish populations in the lake were depressed compared to the other lakes, they concluded that seal predation on the latter two species was responsible for the observed alterations in the Lacs des Loups Marins fish community.

Smith et al. (1996) analyzed stable-carbon and nitrogen isotope values in three groups of seals: P. v. concolor from Kasegalik Lake, Belcher Islands, Nunavut; P. v. concolor from the northwest Atlantic Ocean; and P. v. mellonae. Fatty acid values were examined for the latter two of these groups, in addition to P. v. richardsi from the Pacific Ocean. Results from both the stable-isotope and fatty acid analyses confirmed that the diet of Lacs des Loups Marins seals was of freshwater origin. Carbon isotopic signatures (d13C) of hair and blood sampled over three years of collection indicated freshwater feeding over a prolonged period.

In the same study, P. v. concolor from northwest Atlantic marine populations showed d13C values that are typical of marine mammals measured elsewhere in north-temperate regions (Smith et al. 1996). The Kasegalik Lake sample was intermediate in d13C signature between the marine and Lacs des Loups Marins values, indicating that seals collected from Kasegalik Lake had access to both freshwater and marine-derived carbon, either because of their use, or because of their prey’s use, of both kinds of aquatic habitat.

The stable-nitrogen isotope values found by Smith et al. (1996) for the marine P. v. concolor sample indicated that it occupied at least two trophic levels, which is consistent with a broad diet. The P. v. mellonae and Kasegalik Lake P. v. concolor were similar in d15N values, suggesting that they occupy similar trophic positions, and the freshwater seals showed less variation in trophic position, probably due to fewer dietary alternatives in high-latitude lakes compared with Arctic marine environments.

Physiology

A variety of studies have investigated the relationship between environmental variables and thermoregulation in harbour seals. Hind and Gurney (1998) provided experimental evidence that the timing of pupping may be influenced by the thermoregulation cost of hauling out. This study indicated that the most energetically favourable time for lactation is June and July, a period coincident with the timing of pupping in the European harbour seal population they examined. The winter survival rate in harbour seal pups has been significantly correlated with their autumn body mass (Harding et al. 2005). Because there is increasing thermal stress with decreasing body size of pups, low winter water temperatures induce a negative energy balance in these smaller animals. Colder air temperatures may also be partly responsible for lower numbers of seals appearing on the haul-out sites during late autumn and early winter (Pauli and Terhune 1987).

A recently published study found a significant relationship between the weaning mass of harbour seal pups and their body condition in their first weeks of independence (Muelbert et al. 2003).

Movements

Harbour seals are often sedentary, exhibiting considerable fidelity to one or a few haul-out sites. They have also been recorded to move great distances. Burns (2002) suggested that generalizations are inappropriate given the harbour seal’s wide distribution and varied population dynamics, and that their kinds of movement include “migrations, juvenile dispersal, seasonal shifts, shifts related to breeding activity, responses to seasonal habitat exclusion, responses to acute or chronic disturbance, and immigration/emigration, occasionally on a relatively large scale.”

Only a few studies have provided insight into the movement patterns of eastern Canadian harbour seals.

Phoca vitulina concolor

In Hudson Bay, harbour seals tagged in Churchill ranged over a fairly large area, extending from the Nelson River to the south and the vicinity of Arviat to the north (Bernhardt 2005). During the autumn, animals made 1-8 day long excursions from the Churchill River haul-out site to offshore areas. Home ranges became smaller and movement patterns less linear during winter, likely because land fast ice prevented animals from returning to haul-out sites in river mouths. Home ranges were comparable in size to previous reports for harbour seals in other jurisdictions (100-55,000 km²) (Bernhardt 2005).

On the Atlantic coast, significantly greater numbers of seals were seen in Saint John harbour at high tide and with lower temperatures. Seal abundance peaked in early May and coincided directly with the presence of alewife (Alosa pseudoharengus) (Browne and Terhune 2003). A southward movement from the Bay of Fundy to southern New England waters occurs in autumn and early winter (Rosenfeld et al. 1988), and a northward movement from southern New England to Maine and eastern Canada occurs prior to the pupping season in May and June (Whitman and Payne 1990).

Lesage et al. (2004) reported four of the seven harbour seals they were monitoring left their summer haul-out areas and migrated 266 ± 202 km (range 65-520 km) to over-wintering sites. During the ice-free period, the seals remained near the coast (generally between 6 and 11 km from shore), in shallow water (generally less than 50 m in depth), and travelled only short distances (15-45 km) from capture sites. None of the monitored seals crossed the 350 m deep Laurentian channel, indicating that this feature may restrict the animals’ movements.

Juvenile harbour seals that have been tagged on Sable Island have been subsequently observed at various mainland sites (Bowen 2005). There is some evidence of immigration to Sable in the early 1980s, which led to elevated pup production. An important contributor to decreased pup production in recent years could be emigration of adult females or female recruits (Bowen et al. 2003; Bowen 2005).

Phoca vitulina mellonae

Smith et al. (2006) tracked eight seals in the area of Lacs des Loups Marins between August and January (1995 and 1996). Daily distances travelled averaged 1.5 to 9.8 km. The overall ranges (95% probability distribution) of seals varied from 82.9 km² to 890.8 km² (median 368.4 km²). All seals exhibited considerable site fidelity to particular areas along the Lake’s shore during the tracking periods. Overall, the eight seals used two primary zones. Seven of them used Lacs des Loups Marins/Lac Bourdel proper, and one animal used the river between Lacs des Loups Marins and Petit Lac des Loups Marins. The total area used (95% probability distribution) was 672.3 km².

No other data on the seasonal movements of P. v. mellonae exist, though the sporadic observations of Consortium Gilles Shooner & Associés et al. (1991) hint at seals spending the winter months in larger bodies of water like Lacs des Loups Marins, Lac Bourdel, and Petit Lac des Loups Marins, with some dispersal into outlying, smaller bodies of water upon the melting of the ice. These investigators reported finding many worn trails between bodies of water frequented by the seals, some as long as 0.15 km, and on inclines as steep as 25°.

There is no evidence that harbour seals are able to navigate the waterfalls on the Rivière Nastapoca and move between the area of Lacs des Loups Marins and Hudson Bay, or into Ungava Bay (Smith 1999).

Additional censuses of the waterways of this region of Québec are warranted to better document the presence or absence of the Lacs des Loup Marins harbour seals. Spatial distribution and seasonal dynamics should also be determined to verify the sedentary hypothesis and identify the areas most frequented.

Interspecific interactions

Shark-inflicted mortality on pups and adult females reduced pup production on Sable Island by 43 to 154 pups annually between 1993 and 1997 and occurred in all months except December, January and February (Lucas and Stobo 2000). It has also been speculated that harbour seals on Sable have been affected adversely by interactions with grey seals (Bowen et al. 2003).

Adaptability

In recent years on Sable Island, the mean birth date of harbour seals has become significantly later (by approximately 2 weeks) suggesting nutritional stress of females and slower fetal growth (Bowen et al. 2003; Bowen 2005). At the same time, the grey seal (Halichoerus grypus) population of the island has been pupping earlier, indicating that this species may be outcompeting harbour seals for scarce resources (Bowen 2005).

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