Trophic relationships, seasonal diving activity and movements of harbour seals, Phoca vitulina concolor, in the St. Lawrence River Estuary, Canada

Abstract

An understanding of community structure and the ecological requirements of a species are desirable for responsible management practices/decisions. In the St. Lawrence Estuary, the harbour, or common seal is the seal species that is most frequently observed from shore but, ironically, it is also the least abundant pinniped within the region. The number of harbour seals in the Estuary remains low despite decades of protection from hunting and the absence of other obvious anthropogenic factors which might limit their expansion. Reasons for their low population levels are unclear. Several other marine mammal species are found in the St Lawrence Estuary, and one hypothesis to account for the low number of harbour seals may be competition for food resources with these other species. However, given the scarcity of information on seasonal distribution, trophic relationships, and activity patterns of harbour seals in this region, it is difficult to evaluate this hypothesis. This information is lacking partly because of 1) the large amount of time that harbour seals, and pinnipeds in general, spend underwater and away from shore, 2) the difficulty of studying their foraging behaviour while they are in these areas, and 3) the difficulty of obtaining sufficient and unbiased information on their diet. In this thesis, recent techniques that overcome some of these limitations are used to examine the diet of harbour seals, trophic relationships in the Estuary community, their seasonal movements, diving and foraging behaviour, and preferred habitats to determine whether the competition hypothesis warrants further investigation.

The structure of the St Lawrence Estuary and Gulf communities was examined using stable carbon and nitrogen isotopes, with a special focus on trophic positions and relationships among harbour seals and the other marine mammal species in the area. Different body tissues were used to examine diet over variable periods of time for a number of marine mammal species. The analyses suggested that most harbour seals likely remain in the Estuary year round, and occupy top trophic positions along with other marine mammal species. Harbour seals and hooded seals occupy the highest trophic positions. Grey seals, male beluga whales and Gulf harp seals were intermediate and Estuary harp seals and female beluga whales occupied the lowest trophic postitions. These differences in trophic positions, along with subtle variations in diving behaviour and distributions among the different marine mammal species in the Estuary, may minimise overlap in food resource utilisation. Stable isotope analyses also showed that harbour seals are largely piscivorous by the time they are one year old, but that yearlings may feed at lower trophic levels than older seals.

Information on diving and foraging activity were obtained by deploying time-depth-velocity recorders and stomach temperature sensors or satellite transmitters on individual seals. The dive records of harbour seals were classified using a combination of principal components, complete linkage and K-means cluster analyses in order to examine diving behaviour. These combined methods were shown to be slightly more efficient than the use of a K-means cluster analysis alone. The analysis identified five dive types on the basis of dive depth and swim speed characteristics. Four dive types were U-shaped, while another was V-shaped. Dives associated with stomach temperature drops, indicative of feeding, were spread among all five dive types. However, most successful foraging took place during U-shaped dives where seals swam along the bottom at speeds near the minimum cost of transport with occasional burst s of faster swimming.

The analysis of diving records also indicated that harbour seals in the Estuary rarely performed deep dives. Despite considerable variation among individuals, 95% of dives were on average less than 23 m deep (range 5-50 m; n = 20 seals). Many dives were very shallow, with dives less than 4 m representing on average 51% of all dives (range 15-93%; n = 13 seals) and 35% of dives associated with foraging success (range 1-72%; n = 11 seals). Patterns of seasonal variation in diving behaviour were explored. Diving activity, depicted by diving rate and time spent diving, intensified while haul-out activity declined during winter. Haul-out frequency and time spent diving also varied according to the reproductive or moulting status of individuals. Except for a single breeding male, harbour seals hauled out on most days during summer. Most seals hauled out preferentially near daytime low tides and dove most intensively during twilight or night. Diving depths were generally shallower at night than during the day, which might be a consequence of feeding on vertically migrating prey. The analysis of stomach contents from seals < 2 yrs-old (n = 17) indicated that capelin, herring, rainbow smelt, sand lance and winter flounder are eaten by harbour seals in the Estuary, at least on a seasonal basis. Adult females decreased diving activity, but did not cease feeding during early lactation. They may take advantage of the abundance of spawning capelin in bays located near their haul-out sites while they are nursing young pups. Adult males fed regularly during the pre-mating season when underwater display areas were likely being established. Juveniles decreased diving activity with the approach of the moult and intensified effort following its completion. When foraging success was examined in relation to overall diving activity, 5 of 7 juveniles fed at depth that were on average deeper than more efficient seals.

Satellite telemetry and in situ tracking of individuals indicated that harbour seals are largely coastal. While away from their haul-out sites, they frequent river mouths, bays and near-shore areas where depths are usually less than 50 m. This distribution pattern was observed both during summer and winter, although ice formation in bays forced animals to abandon some near-shore areas during winter. Most harbour seals remained in the Lower Estuary throughout the year. However, 4 of 7 seals over-wintered in areas 65-520 km (x + SD = 266 km + 202) away from their summer haul-out areas. Heavy ice conditions may have contributed to the relatively large amplitude of movements observed in these animals.

Harbour seals appear to find both their necessary food resources and haul-out substrates within relatively small geographic areas throughout the year. They may overlap in their food resource utilisation with some other marine mammals from the Estuary, but direct source competition is likely minimised by differences in distribution and feeding areas. The coastal nature of the harbour seal, its use of riverine resources and its position as a top predator in the Estuary community might increase its vulnerability to the development of man-made infrastructures and human activity and to the effects of contaminants that tend to bioaccumulate within the St Lawrence River Estuary ecosystem.