|Avian Demography Unit
Department of Statistical Sciences
University of Cape Town
Seabird Islands of South Africa
This small, 20-ha island is the easternmost of the chain of seabird islands of the Western Cape. Moving east along the coast, there are no more significant seabird islands until the cluster of islands in Algoa Bay, near Port Elizabeth in the Eastern Cape is reached. A tiny island near Mossel Bay is called Seal Island and, as its name implies, has rather few seabirds.
Dyer Island is about 5 km offshore, 12 km southeast of Danger Point. The nearest fairly well-known places are Hermanus and Gansbaai. Cape Agulhas is another 60 km farther east along the coast.
In the 1970s, this island supported the largest colony of African Penguins, about 25 000 pairs. By 2000, the population had plummeted to around 2 000 pairs, and the Dyer Island colony was the sixth largest. The breeding population has remained more or less stable at this level between 2000 and 2005. Scarcity of food is the most likely cause of the decline. This is, in part, due to overfishing, but is also likely to be due to changes in the migration patterns of pilchards and anchovy, the fish species on which they mainly feed. Seals, mainly from the adjacent Geyser Island, also take a heavy toll on penguins. It is thought that penguins which fledge from Dyer Island do not return there to breed, but recruit to the new colonies at Stony Point, the Boulders and Robben Island, and even to Dassen Island. At the three new colonies, the rates of increase seem to be greater than what could be sustained by the breeding populations at these colonies.
Roseate Terns were first recorded breeding on the island in 1907. The lighthouse keeper at the time was adamant that they had not bred there in the previous 30 years during which he had been based on the island. Although the records thereafter are somewhat erratic, it is likely that Roseate Terns bred on the island almost annually until the 1970s. No breeding was recorded between 1975 and 1995, although sight records of the species were occasionally made over the next 20 years. Breeding was again recorded in July 1996, when a single nest was found. Since then, a small number of pairs have bred each year, and several young are known to have fledged. Roseate Terns are especially sensitive to disturbance while breeding, and they desert their nests readily. The only other breeding islands in southern Africa are in Algoa Bay, and this species is classified as “Endangered” in the South African Red Data Book.
Phil Whittington, while a PhD student at the Avian Demography Unit studying penguins, went outside into the yard between the buildings late one night in October 1995, and instantly recognised a call he had only ever heard on tape before – Leach’s Storm Petrel. Together with Bruce Dyer, of Marine and Coastal Management, they realized that the calls were not only coming from the skies above, but also from deep inside the thick dry stone walls which mark the edges of the yard. A year later, breeding was proved when they found eggs at the end of a burrow in these walls. They had thus added a breeding species to the South African bird list, and the first proof that any species belonging to the procellariiforms bred in southern Africa. Annual surveys have shown that approximately 20 pairs breed annually on Dyer Island, and that several of the other offshore islands also support small populations. Previously Leach’s Storm Petrel had been regarded as a rare visitor to the region, from breeding grounds in North America and Europe.
A range of other seabirds breed on the island: Whitebreasted, Cape, Bank and Crowned Cormorants, Kelp Gulls and Hartlaub’s Gulls, and Swift Terns. Huge roosts of terns, mainly the migratory Common and Sandwich Terns, occur in summer. Antarctic Terns, which breed on the subantarctic islands, roost here in winter.
The earliest definite record of Great White Pelicans breeding on Dyer Island is dated about 1869, and there are also records from 1898, 1913 and 1919. From about 1930 they bred on Seal Island in False Bay. Sometime in the 1920s, persecution by gauno collectors on Dyer Island caused them to desert this island.
One seabird conspicuously missing from the Dyer Island is the Cape Gannet. The island is about halfway between the widely-separated colonies at Malgas Island in Saldanha Bay and Bird Island in Algoa Bay. Gannets forage at distances of up to about 150 km from their colonies while breeding, so there is certainly “space” for a colony here. Many gannets feed in the sea around the island, so there is apparently food for them. There is a record of a pair of Cape Gannets breeding on the island early in the 20th century, but there are now some doubts about the validity of the record.
Little Egrets bred on the island for the first time in 1999, when there was one nest. In 2000, there were four. A few pairs of Egyptian Geese have recently started breeding on the island.
African Black Oystercatchers breed along the shoreline of Dyer Island at a high density, and the island holds more than 1% of the world population of this "Near-threatened" species. Kittlitz’s and Whitefronted Plovers breed in the open areas in interior of the island.
Dyer Island is a nature reserve belonging to CapeNature, but has no visitor access facilities. This is understandable both in terms of the sensitivity of the island’s bird to disturbance, and because it is one of the trickiest of the offshore islands to reach – the best landing place on the island can only be used in fairly calm conditions. In the 1970s, several scientists were stranded on the island for nearly two weeks, because the boatman due to fetch them deemed the landing too risky. Even today, scientists planning visits to Dyer Island give themselves a window of several days and hope that a crossing will be possible on one of them.
Adjacent to Dyer Island is Geyser Island. This island supports one of the largest Cape Fur Seal colonies in the Western Cape. The seals attract Great White Sharks, and the channel between the two islands is popularly known as "Shark Alley".
Dyer Island also lies between two of the most important “whale bays” along the southern coast, and there is considerable whale traffic between them. Hermanus, with its spring Whale Festival, is on Walker Bay to the west; the large bay off the De Hoop Nature Reserve lies to the east. The population of Southern Right Whales which calves in the sheltered bays along this coastline, is increasing by about 7% per year. A growing marine ecotourism industry, operating out of Gansbaai and Kleinbaai, is based on these three big S’s: the seal, the shark and the Southern Right Whale.