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Site description (2001 baseline):
Site location and context
Dyer Island is one of two low-lying islands situated 4.7 km south-east of Danger Point. The nearest harbour is at Kleinbaai/Franskraal, just south of Gansbaai. The coastline is rugged with some low rocky areas spreading inland. The island is flat and low-lying, with a pebbly surface. The vegetation consists primarily of Mesembryanthemaceae and non-native weeds (including conspicuous stands of
Lavatera). In the south-eastern part of the island are several buildings that house the island staff, boats and stores.
See Box for key species. The discovery of
Oceanodroma leucorhoa in several of the stone walls on the island in October 1995 was the first evidence of a procellariiform bird breeding in southern Africa. In November 1996 it was estimated that 8–9 pairs were breeding. In 1996, after an absence of some 25 years,
Sterna dougallii, which breed regularly only at two other islands in southern Africa (Bird and St Croix islands in Algoa Bay; IBA ZA074), attempted to breed again at Dyer Island. The breeding attempt failed owing to suspected human disturbance. The population of
Spheniscus demersus at Dyer Island is in rapid decline, and although the island supported some 72,500 birds in 1976, it is thought that the population may have fallen below 3,000 pairs in 1997.
Many other species breed on this island, including large numbers of
Phalacrocorax capensis and small numbers of
P. carbo,
P. neglectus and
P. coronatus. Several large colonies of
Larus hartlaubii and
Sterna bergii breed at the island.
Sterna sandvicensis,
S. hirundo,
S. paradisaea and
S. vittata form large, mixed tern roosts with the breeding
S. bergii. Dyer Island is estimated to hold over 1% of the world population of
Haematopus moquini.
Non-bird biodiversity: The waters around the island hold a large population of the shark Carcharodon carcharias (VU).
Pressure/threats to key biodiversity
Dyer Island is state-owned and administered and managed by the Hermanus branch of Cape Nature Conservation. This island used to support the largest colony of
Spheniscus demersus in the world, but since the 1980s numbers have decreased greatly. The average number of penguins between 1978 and 1986 was 18,231, but between 1990 and 1992 it had decreased to 5,325. Events such as the mass abandonment of nests in February 1991 have accelerated the decline. The most likely cause of these desertions and consequent breeding failure is scarcity of food.
Competition with commercial fisheries, especially purse-seining for surface-shoaling fish such as anchovy
Engraulis capensis and pilchard
Sardinops sagax, has been implicated as one of the most significant factors causing seabird population declines in the region. Anchovy recruitment was impaired and stocks were greatly reduced between 1989 and 1990, which may have triggered mass penguin emigration. It is thought that birds which used to breed at Dyer Island may have relocated to Stony Point, Boulders Bay (IBA ZA096), Robben Island (IBA ZA089) and as far east as St Croix in the Algoa Bay group (IBA ZA074) where the populations are growing steadily. It has been recommended that marine reserves with a radius of 25 km be created around important breeding islands. Commercial fishing should be banned or restricted within these zones. Regulations of this type may prevent local depletion of food resources that contribute to low breeding success and which may precipitate mass desertion of these colonies.
The population of the seal
Arctocephalus pusillus on the nearby Geyser Island has been growing steadily, and there has been an increase in the number of cases of seals mauling and killing penguins. Certain ‘rogue’ individuals seem to be the principal agents of this mortality and it is important that management action be introduced to eliminate the responsible animals. Furthermore, increased seal numbers, coupled with their ability to outcompete and displace birds at breeding islands, pose a major threat to all breeding seabirds.
Certain problems are known to affect seabirds throughout their ranges. An unpredictable threat, which is difficult to control, is chronic pollution by crude oil or other pollutants that spill into the ocean when tankers break open, wash their tanks, dump cargo or pump bilge.
Spheniscus demersus is particularly susceptible to these events and a single oil disaster can severely impact populations. The South African National Foundation for the Conservation of Coastal Birds (ZANCCOB) cleaned, rehabilitated and returned some 3,000 penguins to the wild between 1981 and 1991.
Tourism to the reefs around the island to see the shark
Carcharodon carcharias is increasing, and the industry needs to be carefully monitored. Landings on the island are not currently permitted. Several introduced species occur on the treeless Dyer Island, including 15 exotic plants, which are mostly small weeds and forbs.
Recommended citation
BirdLife International (2024) Important Bird Area factsheet: Dyer Island Nature Reserve (South Africa). Downloaded from
https://datazone.birdlife.org/site/factsheet/dyer-island-nature-reserve-iba-south-africa on 23/12/2024.
