Dassen Island

Site description

Dassen Island, South Africa’s second largest coastal island (273 hectares), lies 9 km from the mainland between Saldanha Bay and Cape Town. 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 are located in the north-east and there is a large manned lighthouse in the south-east. The island is partially enclosed by a low solid concrete wall. Hedges of alien manitoka Myoporum serratum grow near the buildings.

Birds

Its proximity to the mainland, comparative isolation and suitable cover enable Dassen Island to offer sanctuary to a variety of land and seabirds. The most important resident is the African Penguin Spheniscus demersus. Numbers stabilised in 1989 following a 26% decrease during the late 1970s. They have been declining ever since, however, and have dropped by almost 50% since the publication of the previous IBA directory. This decline is mirrored across the species’ range, emphasising the importance of this IBA, which currently hosts the largest African Penguin colony in the Western Cape (according to the 2012 population census).

Having previously supported 4.6% of the global population of the African Black Oystercatcher Haematopus moquini, Dassen Island now holds almost 8%. The species’ breeding population has remained stable over the past 30 years, although the number of birds in roosts has increased (Loewenthal 2007). Island populations of African Black Oystercatcher are mostly free of major predation and disturbance events and they fare much better than adjacent mainland populations. It has been suggested that the islands act as source populations, and the mainland, where breeding success is lower, as sink populations.

Dassen Island and Lake St Lucia IBA in KwaZulu-Natal are the only two sites in South Africa that support breeding Great White Pelican Pelecanus onocrotalus. Unlike most pelican populations, the one in the Western Cape has increased substantially during the 20th century. The numbers on Dassen Island’s Boom Point have risen from fewer than 100 pairs in the mid-1970s to approximately 550 pairs in 1996. The increase in numbers of breeding pelicans has been attributed to several factors, including reduced disturbance on the island and an increase in the number of coastal waterbodies suitable for pelican foraging in the Western Cape. The building of farm dams and the dredging of seasonal vleis in the 20th century, combined with the introduction of exotic carp Cyprinus carpio to waterbodies, have led to an increased food supply for pelicans in the province.

The island also supports healthy breeding populations of Crowned Cormorant Phalacrocorax coronatus, Cape Cormorant P. capensis, Kelp Gull Larus dominicanus, Hartlaub’s Gull Chroicocephalus hartlaubii and Swift Tern Thalasseus bergii. Large numbers of Ruddy Turnstone Arenaria interpres as well as other migratory waders visit in summer. Bank Cormorant P. neglectus, which used to breed in large numbers, has decreased dramatically from about 200 breeding pairs in the 1980s to only 35 in 2010 (Crawford et al. 2012). Leach’s Storm Petrel Oceanodroma leucorhoa breeds on the island in very small numbers.

The status of the seabird populations that use Dassen Island is well known thanks to regular counts by CapeNature and the Oceans and Coasts Division of the DEA. There have been severe declines in a number of species and the implementation of a 20-km small pelagic fishing closure around the island has been proposed to reduce fishing pressure on those fish stocks utilised by the seabirds.

IBA trigger species

The globally threatened species in this IBA are African Penguin (16 000 individuals and 2 678 breeding pairs), Bank Cormorant (70 breeding pairs), Crowned Cormorant (280 breeding pairs), Cape Cormorant (765 breeding pairs) and African Black Oystercatcher (280 individuals). Regionally threatened species are Great White Pelican (450 breeding pairs) and Leach’s Storm Petrel (4–5 breeding pairs). The island also hosts 1% or more of the populations of African Penguin, African Black Oystercatcher, Kelp Gull (1 739 breeding pairs), Hartlaub’s Gull (1 327 breeding pairs), Swift Tern (1 477 breeding pairs) and Ruddy Turnstone. (Data from the Dassen Island Management Plan and Crawford et al. 2012.)

Other biodiversity

The west coast endemic Gronovi’s dwarf burrowing skink Scelotes gronovii occurs on Dassen Island. The IBA also hosts angulate tortoise Chersina angulata at what is regarded as one of the highest densities of tortoises anywhere in the world.

Conservation issues

Threats

The principal threat to the survival of the trigger species for this IBA is competition with commercial fisheries for food resources, in particular the decrease in abundance of small pelagic fish such as sardine Sardinops sagax and anchovy Engraulis capensis. This decrease on the west coast is due in part to a shift in sardine and anchovy stocks southward. The reasons for the shift are unclear, but climate change and fishing pressure have been implicated. The lack of food availability has led directly to declines in seabird numbers on the island. This is associated with a larger-scale decline in population numbers of different species of marine biodiversity due to the fishing industry and its associated impacts. A recommendation has been made that marine reserves with a radius of 20 km are created around important breeding islands. Commercial fishing should be banned or restricted within these zones.

Many exotic plants have been introduced to the island, as have the house mouse Mus musculus, domestic cat Felis catus and European rabbit Oryctolagus cuniculus. In 1998, domestic cats were estimated to take some 2 000 African Penguin nestlings each year. Cat numbers have been reduced considerably since 1983 and the species has now been completely eradicated from the island.

Other threats directly affecting seabirds include oil and other hazardous and noxious substances at sea or washing up on the island. African Penguins are particularly susceptible to oiling events and a single oil disaster can severely affect populations.

Conservation action

The Dassen Island Nature Reserve was established as a provincial nature reserve in 1998. The boundaries cover the land surface area and extend 500 m seawards from the high-water mark. In accordance with the legislative requirements of the designation of a nature reserve, a Dassen Island Nature Reserve Management Plan has been drafted and received ministerial sign-off in June 2013. This plan details all aspects of the management of the reserve and potential expansion of the marine buffer zone in the near future.

The conservation activities currently under way include environmental monitoring, law enforcement and compliance, infrastructure maintenance and alien vegetation eradication, among other tasks. Research projects have been undertaken to determine the extent of a Marine Protected Area or similar exclusion zone around the Island and it is hoped that such an extension to the protected area will be proclaimed.

Related webpages

http://www.capenature.co.za/wp-content/uploads/2014/05/Dassen-Island-Nature-Reserve-PAMP.pdf

Contact

If you have any information about the IBA, such as a new threat that could impact on it, please send an e-mail to iba@birdlife.org.za or call BirdLife South Africa +27 (11) 789 1122.

Page last updated

Monday, 19 January 2015