Seal Island:  the Island, the Seals, the Sharks

Geography and Marine Biota of Seal Island

Seal Island is a small, elongate rocky islet with its long axis oriented roughly north-south, measuring approximately 400 by 50 metres, and with a maximum elevation of about 7 metres above the high tide line. The island is centered at approximately 35° 8' 6" S, 18° 35' 00" E. The island has a narrow western shelf, with the water depth dropping off to a depth of 20 metres within 5 metres of shore, and a broad eastern shelf, where the water does not drop off to comparable depths until about 20 metres from shore. Off the southern tip of the island is a small, craggy outcrop, the portion which protrudes above the surface at high tide is approximately 4 metres long and 2 meters tall. Water temperature off Seal Island varies little throughout the year, hovering between 13 and 14°C at the surface and a degree or so cooler at the bottom where the depth is 20 m. From March through September, the predominant winds at Seal Island are northwesterly, changing to southeasterly from October through February.

Seal Island is populated by approximately 60,000 Cape Fur Seals (Arctocephalus pusillus pusillus) and roughly 200 Kelp Gulls (Larus dominicanus). On the Island, there are also small groups of Cape Cormorants (Phalacrocorax capensis), White-Breasted Cormorants (P. carbo) as well as a few Subantarctic Skuas (Catharacta anatarctica) and Jackass Penguins (Spheniscus demersus). The shallow waters immediately surrounding the Island support large shoals of Southern Mullet (Liza richardsonii), Red Roman (Chrysoblephus laticeps) and Hottentot (Pachymetopon blochii). Occasionally, Jackass Penguins find their way to Seal Island from the breeding colony [about 200 breeding pairs] at Boulders Beach, located along the eastern shore of the Cape Peninsula, about 2 kilometres south of Simon's Town. The offshore waters surrounding Seal Island are inhabited by a variety of cetaceans, including Bryde's Whales (Balaenoptera edeni), Southern Right Whales (Eubalaena australis), Common Dolphins (Delphinus delphis) and Dusky Dolphins (Lagaenorhynchus obscurus). Large smacks of rhizostome jellyfishes are quite common throughout the waters of False Bay. Large flocks of Cape Gannets (Morus capensis) often follow pods of Common and Dusky Dolphins, plunge diving and feeding on shoals of small teleosts swimming a metre or two beneath the surface. During February and March of each year, Bronze Whaler Sharks (Carcharhinus brachyurus) are relatively abundant in False Bay, where they are caught in nets to be sold locally as a food fish.

Life History Sketch of Cape Fur Seals at Seal Island

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.

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.

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 northeastern 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.

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 traveling efficiency by 'porpoising'. At Seal Island, two basic types of porpoising have been noted: 1) high porpoising, in which the animal leaps completely from the water and, 2) 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. Possible functional significance of these behaviors is discussed elsewhere on these pages.

Life History Sketch of White Sharks at Seal Island

Breeding - As individuals less than 1.7 metres in length are conspicuously absent, White Sharks do not appear to breed at Seal Island nor anywhere within False Bay. In southern Africa, this species is believed to give birth during late spring or early summer in Eastern Cape waters, but details on precise pupping grounds are sketchy.

In South African waters, male White Sharks become sexually mature at a length of 3.5 to 3.6 metres and an age of 9 or 10 years, females mature at a length of 4.5 to 5 metres and an age of 14 to 16 years. The females grow larger than the males, reaching a maximum length of at least 7 metres and a mass in excess of 2,000 kilograms, although individuals over 6 metres in length are extremely rare.

Gestation period in the White Shark is not known, but may be longer than one year, with mature females breeding only every third year. Fetal nutrition features oophagy but there is no evidence of adelphophagy, as in the distantly-related Sandtiger Shark (Carcharias taurus). White Shark pups measure between 1 and 1.5 metres in length and about 60 (?) kilograms at birth. Litter size in this species ranges from 5 to at least 10 and possibly to 17 pups.

Foraging - Diet of the White Shark at Seal Island has received little quantitative investigation. A 3.9-metre male specimen that washed ashore in False Bay near Simon's Town in July 1998 was necropsied by Leonard J.V. Compagno in Capetown and found to contain remains of Cape Fur Seals only. The relatively fresh carcass of an 11-metre Bryde's Whale that washed ashore at Glencaim Beach, about 3.5 kilometres north of Simon's Town, on 5 July 2000 was towed to Seal Island the following morning and promptly attracted the scavenging attentions of at least 28 individual White Sharks. Further details will eventually be reported elsewhere on this website.

Elsewhere in its range, the White Shark is a potent predator-scavenger combining a broad prey spectrum with opportunism in a highly successful way. Juveniles feed predominantly on demersal teleosts and small elasmobranchs. Adult White Sharks feed predominantly on relatively large elasmobranchs and teleosts, consuming pinnipeds, dolphins, whale blubber, squid, seabirds, marine turtles, crabs and gastropods when the opportunity presents itself.

Movements - White Sharks appear to arrive at Seal Island at early-to-mid-May each year, remaining in the immediate area until at least September. During this time, they feed heavily on Cape Fur Seals, with peak predations occurring during July and August. In 1999, perhaps as a consequence of the unusually mild winter, White Sharks were in evidence at Seal Island until the end of October. From December through January, White Sharks are conspicuously absent from the waters surrounding Seal Island and are most often sighted farther north in False Bay and closer to shore, off Macassar Beach.

At Seal Island, White Sharks typically cruise near the bottom but often investigate boats and other objects at the surface. The sharks seem to spend relatively little time in the mid-water column, but this needs to be confirmed through depth telemetry. A study of space utilization by White Sharks at Seal Island is presently underway by Rocky Strong.

Although they often aggregate when and where prey is seasonally abundant, White Sharks are highly nomadic. Individuals tagged at Seal Island have been re-sighted at Dyer Island and Mossel Bay, distances of about 175 and 350 kilometres, respectively. A nearly 5-metre female  individual tagged at Struisbaai was recaptured off Natal, having traveled 720 kilometres in 27 days.

[ Note from R. Aidan Martin ]   I would like to acknowledge that much of the information on Seal Island  presented here was provided by two very talented South African naturalists, Chris Fallows and Rob Lawrence, of African Shark Eco-Charters. I would also like to acknowledge my debt to South African diver Alison Kock, who provided valuable information on the Island's underwater topography.  Were it not for Chris's, Rob's, and Ali's generosity in sharing what they have learned, I could not have gotten a conceptual handle on what's going on at Seal Island as quickly as I did.