Harbour seal

Distribution

Harbour seals have one of the broadest distributions among the pinnipeds, ranging through temperate areas as far south as southern California and the south of France to arctic waters of the North Atlantic and the North Pacific Oceans.

Svalbard marks the northernmost limit of their range. This genetically isolated population most closely resembles harbour seals from Greenland. In Svalbard most harbour seals occur on Prins Karls Forland, an island on the west coast of Spitsbergen where they can be seen year-round. This is their only known breeding site in the Archipelago.

However, during the summer months they can be found in fjords all along the west coast of Svalbard and recently they have been reported along Spitsbergen’s north coast as well during summer.

General ecology

Harbour seals occur at low densities throughout their range. The global population size is thought to be close to half a million animals.

Harbour seals are a coastal seal species that aggregates in small groups on rocky outcrops, beaches or inter-tidal areas. They are highly social and are rarely seen alone. It is unusual for harbour seals to haul out on ice in most populations, but there is evidence that they do use ice in some parts of their range if shore-based sites are inaccessible for periods in the winter. In Svalbard the harbour seals use ice as a haul-out platform extensively during the winter months. Harbour seals haulout to rest on an almost daily basis around low tides and tend to forage when the tide is high. The number of animals hauling out on land is variable depending on the time within the tide cycle, season, tidal height, weather conditions etc.

Harbour seals spend a lot of time scanning for predators when they are hauled out and rarely sleep for more than a few minutes at a time when on land.

Harbour seal in most places shift their preferred haul-out sites on a seasonal basis so that they have an annual movement pattern on a very small geographic scale although they are not truly migratory.

Adult males tend to remain apart from females and young animals most of the year, entering the birthing areas only toward the end of the nursing period when females become receptive to mating. Groups containing animals of all ages and both sexes can be found during the moulting period, which occurs in late August through early September in Svalbard.

After the moult subadults and adults begin foraging intensively to replenish their blubber layer for the winter. Harbour seals are relatively shallow divers that do most of their diving in the top 100 metres. They are opportunistic feeders that eat a wide variety of fish species and some cephalopods and crustaceans. Invertebrates, especially shrimp, are important in the diet of newly weaned harbour seals, while they learn to swim and dive and chase fish effectively. The diet of harbour seals in Svalbard has shifted in the last decade away from arctic fish species such as polar cod, towards more temperate fish such as Atlantic cod and haddock.

Large sharks and killer whales are the primary predators of harbour seals; terrestrial predators such as bears, wolves and eagles may take pups in some parts of the harbour seal’s range. In Svalbard, Greenland sharks and walrus are likely predators. Polar bears were previously rare on Prins Karls Forland, but are more and more common and certainly now represent a major predator of harbour seals in Svalbard.

Life history and reproduction

Across their range harbour seals give birth from January through to October, but within an area birthing is quite synchronous within a two to three week period. Harbour seal pups are born during early-mid June in Svalbard. Similar to harbour seals in other locales, they are born on inter-tidal areas, and normally swim within minutes or at the most hours of being born. Their mothers nurse them for the following three to four weeks.

The pups accompany their mothers in the water, often riding on her back when they are small. Mothers can also carry their pups between their fore flippers if a rapid escape becomes necessary. The diving done by pups is shallow and of quite short duration but harbour seal pups are very active and very aquatic, spending about 50% of their time during the nursing period in the water, accompanied by their mothers.

Despite these high activity levels, the pups grow quite rapidly, reaching 25–30 kg by the time that they are weaned. Male harbour seals come into breeding areas toward the end of lactation and perform underwater displays to attract females.

They vocalize in set places along routes that females travel for foraging and also fight with one another and display at the surface using kelp or flipper slaps. Off the California coast groups of males have been filmed with one singing male surrounded by a group of younger, subordinate males. But, the dynamics and relevance of such groups is not known.

Similar to other seals, harbour seals have delayed implantation. Following mating, the embryo rests for several months, before implanting in the uterine wall and actively growing for about eight months. Harbour seal females reach sexual maturity at three to five years of age, males at four to six years of age.

Harbour seals can live to be 30–35 years old, but in Svalbard this species appears to have an unusually short longevity. The oldest harbour seal thus far aged from this population was 22 years of age. The cause of this short life span is not currently known.

Management status and monitoring

Similar to other coastal seals, harbour seals have been hunted along most coastlines throughout their range. Bounties have been paid by various countries to reduce local harbour seal populations in areas where fisheries take place.

Additionally, harbour seals often die from entanglements in gill nets where there are in-shore fisheries using this type of gear.

The Svalbard population of harbour seals is on the National Red List, and is totally protected from hunting. The entire west coast of Prins Karls Forland was photographically surveyed in 2009 and again in 2010 to estimate the population size of harbour seals. A correction factor for seals in the water at the time of the survey was calculated based on radio-tagged animals and an extensive behavioural study. The resulting abundance estimate was just short of 2000 animals, which is a substantial increase from previous estimates of this population.