Life History Sketch of Cape Fur Seals at Seal Island
Cape Fur Seals Breeding – Adult male Cape Fur Seals measure up to 2.7 metres in length and 350 kilograms in mass; adult females are generally less than 2 metres long and 100 kilograms in mass. Females mature at 2 to 3 years of age, while males mature at about 3 years, but generally do not become large and powerful enough to defend breeding territories until they are 10 to 14 years of age. Experienced bulls often attempt to reclaim the same breeding territories they held the previous year.
In late October, adult male Cape Fur Seals come ashore on Seal Island to compete for and establish breeding territories, followed shortly thereafter by the adult females. After a gestation period of about 11.5 months (including 3.5 to 4 months delayed implantation), pups are born on Seal Island between mid-November and mid-December. Cape Fur Seals pups measure about 75 centimetres long and 5 to 6 kilograms in mass at birth and, between the ages of 3 and 7 weeks, have black fur that is quite distinct from the olive-brown pelage of the adults.
The pups suckle for almost a full year, although they begin taking solid food at an age of about 6 months. Perhaps 2,000 Cape Fur Seals pups are born each season but, before their first year, a large percentage drown, fall victim to infection, or are crushed by adult males. As a result, for several weeks each summer, hundreds of dead seal pups are found floating in the waters around Seal Island. Adult male Cape Fur Seals compete vociferously and violently to establish a breeding harem of about 20 adult females. Successful bulls mate with their harem of cows about six days after the pups are born. Adult seals may spend as much as half of each year at their rookery.
Cape Fur Seals foraging – Cape Fur Seals at Seal Island travel far out to sea in search of food, having been observed as much as 50 kilometres offshore. Adult males may stay out at sea for months at a time, whereas adult females tend to remain at sea for a few days. At sea, Cape Fur Seals hunt singly or gather together at pelagic shoals of schooling teleosts (74% of the diet) and squids (17% of the diet). In near-shore waters, they also consume small sharks, octopus, rock lobster, crabs, and other crustaceans. Fast and agile predators, Cape Fur Seals are capable of swimming speeds of up to 16 km/hour, can reach a depth of at least 200 metres (although 90% of dives are shallower than 150 metres and 70% of dives are shallower than 50 metres and last less than 2 to 3 minutes) and can hold their breath underwater for at least 7.5 minutes. When prey is plentiful, they consume 6 to 8% of their body weight each day.
Cape Fur Seals are notorious scavengers, often stealing fish or squid from lines and nets, which earns them no affection among commercial fishermen. In retaliation for the seals’ misdeeds – both real and imagined – fishermen often shoot them at sea.
Cape Fur Seals resting – During daylight hours, Cape Fur Seals at Seal Island spend much of their daylight hours either resting on the island, aggregating in ‘rafts’ or cavorting over water shallower than 2 metres. Aggregations of rafting and cavorting seals occur primarily near the edge of the drop-off at either the south end of Seal Island or over the broad shallow bank at the north eastern side of the island. Individual Cape Fur Seals at Seal Island are sporadically vigilant of potential threats beneath the surface, plunging their large, sensitive eyes beneath the waves every few minutes; collectively, the aggregated seals provide nearly constant sub-surface vigilance. At night, those seals near the Island are presumed to sleep or rest ashore, but this needs to be confirmed by observation.
Cape Fur Seals Movements – Cape Fur Seals at Seal Island are not migratory, although even young individuals travel great distances to forage. Eight-month-old pups tagged at Seal Island have been recaptured at Cape Cross in Namibia, a distance of over 1,600 kilometres.
Cape Fur Seals often increase their travelling efficiency by ‘porpoising’. At Seal Island, two basic types of porpoising have been noted:
- high porpoising, in which the animal leaps completely from the water and,
- low porpoising, in which the animal does not completely leap from the water but, rather, undulates along the surface with the head alternately above and below the waves.
