Dassen Island

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General Information

Status:

Global IBA (A1, A4i, ii)

Province:

Western Cape

Protection:

Fully Protected

Size:

740 ha

Number:

SA109

Additional Info

  • 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

    Further Reading

    Adams NJ. 1991. Patterns and impacts of oiling of African Penguins Spheniscus demersus 1981–1991. Biological Conservation 68: 35–41.

    Berruti A. 1986. The predatory impacts of feral cats Felis catus and their control on Dassen Island. South African Journal of Antarctic Research 16: 123–127.

    Branch WR. 1991. The herpetofauna of the offshore islands of South Africa and Namibia. Annals of the Cape Provincial Museum (Natural History) 18: 205–225.

    Brooke RK, Prins AJ. 1986. Review of alien species on South African offshore islands. South African Journal of Antarctic Research 16: 102–109.

    Cooper J. 1981. Biology of the Bank Cormorant, Part 1: Distribution, population size, movements and conservation. Ostrich 52: 208–215.

    Cooper J, Berruti A. 1989. The conservation status of South Africa’s continental and oceanic islands. In: Huntley BJ (ed.), Biotic diversity in southern Africa: concepts and conservation. Cape Town: Oxford University Press. pp 239–253.

    Cooper J, Brooke RK. 1986. Alien plants and animals on South African continental and oceanic islands: species richness, ecological impacts and management. In: Macdonald IAW et al. (eds), The ecology & management of biological invasions in southern Africa. Cape Town: Oxford University Press.

    Cooper J, Hockey PAR, Brooke RK. 1983. Introduced mammals on South and South West African islands: history, effects on birds and control. In: Cooper J. (ed). Proceedings of the Symposium on Birds of the Sea and Shore, 1979. African Seabird Group, Cape Town. pp. 179–203.

    Cooper J, Williams AJ, Britton PL. 1984. Distribution, population sizes and conservation of breeding seabirds in the Afrotropical region. ICBP Technical Publication No. 2.

    Crawford RJM. 1995. Conservation of southern Africa’s breeding seabirds. Birding in Southern Africa 47: 106–109.

    Crawford RJM, Boonstra HGvD, Dyer BM, Upfold L. 1995a. Recolonisation of Robben Island by African Penguins, 1983–1992. In: Dann P et al. (eds), The penguins. Australia: Surrey Beatty. pp 333–363.

    Crawford RJM, Cooper J, Dyer BM. 1995b. Conservation of an increasing population of Great White Pelicans Pelecanus onocrotalus in South Africa’s Western Cape. South African Journal of Marine Science 15: 33–42.

    Crawford RJM, Cooper J, Shelton PA. 1982a. Distribution, population size, breeding and conservation of the Kelp Gull in southern Africa. Ostrich 53: 164–177.

    Crawford RJM, David JHM, Williams AJ, Dyer BM. 1989. Competition for space: recolonizing seals displace endangered, endemic seabirds off Namibia. Biological Conservation 48: 59–72.

    Crawford RJM, Dyer BM. 1995. Responses by four seabird species to a fluctuating availability of Cape Anchovy Engraulis capensis off South Africa. Ibis 137: 329–339.

    Crawford RJM, Dyer BM, Brooke RK. 1994. Breeding nomadism in southern African seabirds: constraints, causes and conservation. Ostrich 65: 231–246.

    Crawford RJM, Dyer BM, Kotze PGH, McCue S, Meÿer MA, Upfold L, Makhado AB. 2012. Status of seabirds breeding in South Africa in 2011. Cape Town: Department of Environmental Affairs, Branch Oceans & Coasts.

    Crawford RJM, Shelton PA. 1978. Pelagic fish and seabird interrelationships off the coasts of South West and South Africa. Biological Conservation 14: 85–109.

    Crawford RJM, Shelton PA. 1981. Population trends for some southern African seabirds related to fish availability. In: Cooper J (ed.), Proceedings of the Symposium on Birds of the Sea and Shore, 1979. Cape Town: African Seabird Group. pp 15–41.

    Crawford RJM, Shelton PA, Brooke RK, Cooper J. 1982b. Taxonomy, distribution, population size and conservation of the Crowned Cormorant Phalacrocorax coronatus. Gerfaut 72: 3–30.

    Crawford RJM, Shelton PA, Cooper J, Brooke RK. 1983. Distribution, population size and conservation of the Cape Gannet Morus capensis. South African Journal of Marine Science 1: 153–174.

    Crawford RJM, Williams AJ, Hofmeyer JH, Klages NTW, Randall RM, Cooper J, Dyer BM, Chesselet Y. 1995c. Trends in African Penguin Spheniscus demersus populations in the 20th century. South African Journal of Marine Science 16: 101–118.

    Crawford RJM, Williams AJ, Randall RM, Randall RM, Berruti A, Ross GJB. 1990. Recent population trends in Jackass Penguins Spheniscus demersus. Biological Conservation 52: 229–243.Frost PGH, Siegfried WR, Cooper J. 1976. Conservation of the Jackass Penguin (Spheniscus demersus). Biological Conservation 9: 79–99.

    Furness RW, Cooper J. 1982. Interactions between breeding seabird and pelagic fish populations in the southern Benguela region. Marine Ecology Progress Series 8: 243–250.

    Hockey PAR. 1983. The distribution, population size, movements and conservation of the African Black Oystercatcher Haematopus moquini. Biological Conservation 25: 233–262.

    Hockey PAR, Hallinan J. 1981. Effects of human disturbance on the breeding of Jackass Penguins Spheniscus demersus. South African Journal of Wildlife Research 11: 59–62.

    Loewenthal D. 2007. The population dynamics and conservation of the African Black Oystercatcher Haematopus moquini. PhD Thesis, Percy FitzPatrick Institute of African Ornithology, University of Cape Town, South Africa.

    Morant PD, Cooper J, Randall RM. 1981. The rehabilitation of oiled Jackass Penguins Spheniscus demersus, 1970–1980. In: Cooper J (ed.), Proceedings of the Symposium on Birds of the Sea and Shore, 1979. Cape Town: African Seabird Group. pp 267–301.

    Randall RM, Randall BM, Bevan J. 1980b. Oil pollution and penguins – is cleaning justified? Marine Pollution Bulletin 11: 234–237.

    Saul L, Visagie J, Cleaver G, Hayward N. 2011. Dassen Island Nature Reserve Management Plan. CapeNature Conservation.

    Shelton PA, Crawford RJM, Cooper J, Brooke RK. 1982. Distribution, population size and conservation of the Jackass Penguin Spheniscus demersus. South African Journal of Marine Science 2: 217–257.

    Siegfried WR. 1982. Ecology of the Jackass Penguin Spheniscus demersus, with special reference to conservation of the species. National Geographic Society Research Reports 14: 597–600.

    Summers RW, Cooper J. 1977. The population, ecology and conservation of the African Black Oystercatcher Haematopus moquini. Ostrich 48: 28–40.

    Underhill LG. 1992. Waders (Charadrii) and other shorebirds at Dassen Island, and the Algoa Bay Islands. Ostrich 63: 128–129.

    Williams AJ, Steele WK, Cooper J, Crawford RJM. 1990. Distribution, population and conservation of Hartlaub’s Gull Larus hartlaubii. Ostrich 61: 66–76.

    Wilson RP, Wilson MTP, Duffy DC. 1988. Contemporary and historical patterns of African Penguin Spheniscus demersus: distribution at sea. Estuarine, Coastal and Shelf Science 26: 447–458.

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