Dassen Island, South Africa’s second largest coastal island, lies 9 km from the mainland between Saldanha Bay and Cape Town. This island reaches 19.2 m at its highest point, and is generally flat or gently sloping, with extensive sandy areas and a few patches of exposed rock. It is richly covered with vegetation in winter. Several buildings occur in the north-east, as does a large manned lighthouse in the south-east. The island is partially enclosed by a low solid concrete wall. Hedges of non-native manitoka Myoporum occur near the buildings.
See Box for key species. Owing to its proximity to the mainland, comparative isolation, and suitable cover, the island offers sanctuary to a variety of land and seabirds. The most important resident is Spheniscus demersus. Numbers have been stable since 1989, following a 26% decrease during the late 1970s. Dassen Island also holds up to 4.6% of the global population of Haematopus moquini—the largest island population in South Africa. Dassen Island and Lake St Lucia (IBA ZA044) in KwaZulu-Natal are the only two sites in South Africa supporting breeding Pelecanus onocrotalus. Unlike most pelican populations, the Western Cape population has increased substantially during the twentieth century. The numbers on Dassen Island’s Boom Point have increased from less than 100 pairs in the mid-1970s to c.550 pairs in 1996. The island also supports healthy breeding populations of Phalacrocorax coronatus, P. capensis, Larus dominicanus, L. hartlaubii and Sterna bergii, as well as supporting many Arenaria interpres and other migratory waders during summer. Phalacrocorax neglectus, which used to breed in large numbers, has decreased dramatically over the last five years. Oceanodroma leucorhoa breeds on this island in very small numbers.
Non-bird biodiversity: Scelotes gronovii (LR/nt), a reptile endemic to the west coast, occurs on the island.
Pressure/threats to key biodiversity
Dassen Island was proclaimed a Provincial Nature Reserve in 1988, and an island warden who enforces regulations currently mans it. Many non-native plants have been introduced, as have mice Mus musculus, cats Felis catus and rabbits Oryctolagus cuniculus. The cats, which eat mostly rabbits, are also known to prey on Haematopus moquini and Sterna hirundo, and in 1983 cats were estimated to take some 2,000 nestlings of Spheniscus demersus each year. Cat numbers have been reduced considerably since 1983 and now only a handful remain. These should be eliminated from Dassen Island as soon as possible. Cat predation on Dassen probably results in lowered breeding success compared to cat-free islands.Certain problems are known to affect seabirds throughout their ranges. Competition with commercial fisheries, especially purse-seining for surface-shoaling fish such as anchovy Engraulis capensis and pilchard Sardinops sagax, has been implicated as the most significant factor causing seabird population declines, especially in populations of Spheniscus demersus and Phalacrocorax capensis. A recommendation has been made 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.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 could severely affect 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.