Harbour seals

Sable Island National Park Reserve is home to a small number of harbour seals.

On this page

• Morphology
• Behaviour
• Population
• Population decline
• More information

Morphology

Harbour seals differ from other seal species in many ways:

• They are smaller.
• They mate at sea.
• They are born without a white birth coat (lanugo).

Male harbour seals are larger than the females. On average, the males on Sable Island weigh 110 kg and measure 156 cm in length. At the beginning of the breeding season, females weigh an average of 85 kg and measure 150 cm in length.

Behaviour

Harbour seals give birth on the island throughout May and June. They occasionally haul out to rest on the island for the rest of the year.

Courtship and mating

Harbour seals mate at sea rather than on land.

During the breeding season, males reduce their foraging activities to focus on courtship rituals and competitive behavior to find female partners. The males spend increasing amounts of time in shallow habitats, including the waters around Sable Island, where they perform shallow display dives. Though this behaviour is often seen in birds, it is not common with seals and is another thing that makes this breed unique!

Births

Harbour seals give birth on the island from early May until the middle of June. In recent years, evidence suggests a 7-day delay in the birthing period on Sable Island (in comparison to previous years). Competition against grey seals may be causing this delay.

Harbour seal pups are born without a white birth coat (lanugo). They shed it while still in the womb. Therefore, the presence of lanugo at birth signifies a premature birth and is usually related to younger mothers.

Harbour seal pups enter the water with their mothers within hours of birth, which is unique to this species. Pups from other seal species enter the water months after birth, without their mothers.

Pups

In the 24 days following birth, the pups double in weight by feeding only on their mother’s milk.

Mother harbour seals continue to forage during lactation, which is rare for seal breeds. Beginning halfway through the lactation period, mother harbour seals leave for feeding trips lasting seven hours or more. One reason for this behaviour is that almost all seals lack the stored body energy to feed their offspring, so they must supplement with feeding.

Despite foraging during lactation, females still lose about 33% of their body mass.

Diet

Like grey seals, harbour seals feed on a variety of fish and invertebrates.

Population

Unlike grey seals, harbour seals generally prefer to stay close to the coast, so it is interesting to find them on Sable Island, hundreds of kilometers from the mainland!

Harbour seals found on Sable Island and along the east coast of Canada are likely part of the same population.

Population decline

The harbour seal population on Sable Island has declined since the 1990s. In 1989, a survey observed more than 600 pups on the island. By 2000, those numbers had dropped to fewer than 20.

Two reasons for this population decline are increased competition for food and heightened predation. Both reasons are likely related to the increase in the grey seal population.

Competition with grey seals

Grey seals and harbour seals have similar diets. As the grey seal population on the island has grown, competition for food has increased. This increased competition has likely added pressure to the harbour seal population.

Predation by sharks

In the late 1990s, predation by sharks was a major mortality factor for harbour seals. The huge growth in the grey seal population may have attracted more sharks to the island, increasing predation rates. Prior to the 1990s, shark predation was infrequent.

More information

Harbour seal (Phoca vitulina) COSEWIC assessment and status report: Chapter 3 – Canada.ca.

Freedman, B., Lucas, Z.and S. Blaney. 2016. Marine Mammals. In Freedman, B. (Ed.) 2016.Sable Island, the Ecology and Biodiversity of Sable Island. Fitzhenry and Whiteside, Markham ON.