Environment of Dassen Island
West Coast South Africa
Contact us Cape Columbine Nature reserve
Sail to the wildlife paradise of Dassen Island, the home to 68 000 African penguins.
The African penguin is endangered as a result of oil spills and the collecting of their eggs in the first half of the 20 th century. The most recent threat for penguins are the fierce competition with some commercial fishing practices.
The following is an extract of the Dassen Island Management plan:
COMPILED BY: Anton Wolfaardt
Dassen Island Nature Reserve Manager ,Western Cape Nature Conservation Board
Date 1 June 2000
Topography and Geology
Dassen Island is approximately 220 hectares in size, and is about 3.2 kilometers long and 1.6 kilometers wide. The island is flat and low-lying, the highest point above sea level being only 19 meters.
The island is formed on a granite base and is predominantly sandy with patches of exposed rock mainly along the shore. The geology of the island comprises predominantly fine-grained tourmaline-granite with a few areas of biotite granite (McClachlan 1949)(Figure 2). The granites found in the interior of the island have weathered into flat exfoliated masses, some of which form temporary pans in winter. The granites found along the coast, to just above the high-water mark, consist of large, rounded boulders.
The central part of the island is covered primarily with coarse, shell-derived sediments, with small areas covered by recent limestone sediments. A small outcrop of ferricrete occurs about 180m south of House Bay
Dassen Island experiences a temperate, Mediterranean-type climate, with warm, dry summer and cool, relatively wet winter seasons. The warmest mean monthly temperatures are normally recorded from January until March, with the lowest being recorded in July (see Figure 3a). The cold Atlantic Ocean has a tempering effect on temperatures.
The rainfall occurs as a result of cold fronts moving in from the south Atlantic Ocean. Most of the rain occurs from May until September, normally peaking in July, although there is some inter-annual variation in the time of this peak (see Figure 3b). The mean annual rainfall for the period 1990-1999 at Dassen Island was about 330 mm.
Northerly and north-westerly winds predominate in winter. In summer southerly and south-westerly winds dominate. The incidence of calms is greatest in the spring months, from September until November.
Very little is known about the vegetation on the island before it was first visited by the European seafarers in the early 1600s. It is known that the island was naturally vegetated (Brooke and Prins 1986). The environment on Dassen Island is relatively disturbed compared to the continental mainland, due mostly to the impact of the large seabird colonies Ė both directly in the form of trampling, and indirectly due to the high levels of nutrients in the guano. Human-related disturbance, such as farming activities and the establishment of gardens in the past, rabbit activity and a relatively harsh climate have all contributed to disrupting the naturally occurring patterns of vegetation. Consequently, the flora that has developed on the island has had to be tolerant of the above factors.
Very little is known about the terrestrial invertebrates of Dassen Island. The taxa that have been recorded on the island are included in Appendix 2. A total of 7 alien invertebrates have been recorded (Brooke and Prins 1986, Appendix 2). A recent investigation of the Coleoptera (beetles) living in the nests of African Penguins has produced some interesting results. These include the discovery of a new species of Histeridae Atribalus wolfaardti, and the discovery of the unknown larva of Hypocaccus brasiliensis (Perreau, 2000 in litt.) A survey of the remaining orders of terrestrial invertebrates present on the island and their role in the ecological processes on the island is necessary.
Five reptile species have been reported in recent years (see Appendix 3). Three of these, the Angulate Tortoise Chersina angulata, Gronovis Dwarf Burrowing Skink Scelotes gronovi and the Marbled Leaf-toed Gecko Phyllodactus porphyreus, are presently common, although the dwarf burrowing skink is included in the South African Red Data Book for Reptiles and Amphibians due to its restricted distribution (Branch 1988). The island population of Angulate Tortoises is particularly dense in comparison with mainland populations. Branch (1991) suggests that the Angulate Tortoise was deliberately introduced to the island between 1896 and 1929, but others feel that the species is probably "indigenous" (Brooke and Prins 1986, E.Baard, pers. comm.). A research project aiming to, among other things, ascertain the origin of the tortoises on the Island is presently underway. The Rock Agama Agama atra has been recorded on the island in the past, but Branch (1991) considers this record to be doubtful. A Cape Skink Mabuya capensis was observed on the island in December 1997 (J. Kemper, pers. comm.) This is the only record of this species on the island.
Birds dominate the vertebrate fauna of Dassen Island, both in terms of diversity and numbers. Appendix 4 provides a checklist of birds that have been recorded on the Island. The African Penguin, the White Pelican, the Bank, Crowned, Cape and White-Breasted Cormorants, the African Black Oystercatcher, the Swift Tern , Hartlaubís Gull and Leachís Storm Petrel are identified for management purposes as the species with the highest conservation priority (not in order of priority).
African Penguins Spheniscus demersus are endemic to southern Africa. The African Penguin is the only penguin species that breeds in Africa, distributed from Hollamís Bird Island, off central Namibia, to Bird Island, Algoa Bay (Crawford et al. 1995a). The total breeding population has decreased markedly this century (Crawford et al. 1995a). Dassen Island used to be the most important breeding site for African Penguins, and it was estimated that there were a maximum of 1 500 000 penguins inhabiting the island in the 1930ís (Frost et al. 1976). The present global population is probably less than 10 % of that at the beginning of the century, with the population at Dassen Island estimated to be just over 48 000 individuals (approximately 15 000 breeding pairs) (WCNCB, unpubl. data). Due to the rapid decline of the total African Penguin population, it is listed as "Vulnerable" in the South African Red Data Book for Birds (Brooke 1984). At the international level it has been regarded as a species of "Special Concern" (Collar and Stuart 1985), but was reclassified as "Vulnerable"(to extinction) at the 1996 CAMP/IUCN meeting (Whittington et al. 1999a). Factors implicated in the decline include egg exploitation and habitat alteration and disturbance associated with commercial exploitation of guano (Frost et al. 1976). These threats no longer exist at Dassen Island. More recently, reduced availability of pelagic fish due to competition with commercial fisheries has been responsible for persistent declines (Crawford et al. 1990). Other factors include mortality in oil spills, predation by feral cats, seal predation of potential recruits, egg and chick predation by Kelp Gulls Larus dominicanus, and entanglement in fishing gear and other marine debris.
The population size at Dassen Island presently appears to be increasing slightly. Between 1990 and 1997 the population remained between 9000 and 10000 breeding pairs, increasing to 11 000 and 15 000 breeding pairs in 1998 and 1999 respectively (Marine and Coastal Management and WCNCB, unpubl. data). This increase could, in part, be an effect of greater synchronicity of breeding during the census period, i.e. in some years a higher proportion of the breeding population may be recorded because more of them are breeding at the same time. Over 25 % of the African Penguin breeding population breed at Dassen Island. Furthermore, Dassen Island is one of only two colonies with more than 10 000 breeding pairs, highlighting the global importance of Dassen Island for African Penguins.
At Dassen Island there are birds breeding throughout the year, with different sub-colonies showing different egg Ė laying peaks. Penguins moult annually, with a peak in moulting activity in November/December. African Penguins are prone to disturbance at all stages of the breeding cycle.
Dassen Island is one of only two localities in South Africa where the White Pelican Pelecanus onocrotalus breeds. The species has been listed as "rare" in the South African Red Data Book for Birds, with an estimated national population of approximately 2500 breeding pairs (Brooke 1984, Williams and Borello 1997). The breeding population at Dassen Island is approximately 500 pairs (Crawford et al. 1995b, C.N.C., unpub. data). The breeding season at Dassen Island is normally from September until February. The birds are very susceptible to disturbance during this period. Recent observations have shown that the pelicans are causing a large amount of disturbance to other protected species on the island. Pelicans have been recorded preying on the chicks of Kelp Gulls, Cape Cormorants Phalocrocorax capensis, Crowned Cormorants Phalocrocorax coronatus and African Penguins, and have probably disturbed Bank Cormorant Phalocrocorax neglectus, Hartlaubís Gull Larus hartlaubii and Swift Tern Sterna bergii bergii colonies on the island.
The Crowned Cormorant is endemic to the Benguela system and the western Agulhas Bank. It has a small world population (approximately 2700 pairs in the early 1980s, Williams and Cooper 1983). Dassen Island is close to the centre of the speciesí range and has a population of about 120 breeding pairs (WCNCB, unpubl. data). At Dassen Island the Crowned Cormorant breeds throughout the year with peaks in breeding activity occurring in January-February and August. There is also inter-annual variation in the time of the breeding season. The Crowned Cormorant was listed as "uncommon and vulnerable" in the first edition of the South African Red Data Book for Birds (Siegfried et al. 1976), but was not included in the following edition (Brooke 1984). However, due to its being endemic to South Africa and Namibia, persistent monitoring of its conservation status is important (Brooke 1984). Crowned Cormorants feed mainly on bottom-dwelling fish in the infratidal and intertidal regions, and are not threatened by competition with fisheries (Crawford 1997). The Crowned Cormorant is susceptible to human disturbance at breeding colonies. Fishing debris, such as pieces of net and nylon line, is often incorporated into nests and has resulted in the entanglement and starvation of birds.
The Bank Cormorant is endemic to southern Africa. The Dassen Island population has recently undergone a severe decrease, from about 200 pairs in the late 1980ís to about 40 pairs in 1999. The Bank Cormorant breeds throughout the year, with a peak in egg-laying occurring from June to October at Dassen Island. There is also inter-annual variation in the time of the breeding season. Bank Cormorants prey on bottom and reef-dwelling organisms occurring in inshore kelp beds, such as clinids, crustaceans (including rock lobster), and cephalopods (octopus and cuttlefish). The consumption of rock lobster results in a potential conflict with commercial and recreational rock lobster fisheries. Bank Cormorant breeding colonies are susceptible to human disturbance, especially as unattended eggs and chicks are preyed upon by Kelp Gulls. Another potential conservation problem is the drowning of Bank Cormorants in rock lobster traps, which they presumably enter in search of prey (Cooper 1985). The Bank Cormorant is listed as "Vulnerable" in the latest South African Red Data Book for birds.
The Cape Cormorant is endemic to southern Africa. It is the most numerous of all the cormorant species breeding on Dassen Island. However, there are large inter-annual fluctuations in the number of birds that breed, which is related to the abundance of Cape Anchovy Engraulis capensis (Crawford and Dyer 1995). This is highlighted by census figures for Cape Cormorants at Dassen Island; 48 182 active nest sites were counted in 1988-1989 and only two active nests were counted the following season, 1989-1990. Wise management of the anchovy resource is therefore important for this species. In the early 1990ís, close to 10 000 Cape Cormorants died from Avian Cholera at Dassen Island (Crawford et al. 1992). Breeding Cape Cormorants are very susceptible to human disturbance. The Cape Cormorant is listed as "near threatened" in the latest South African Red Data Book for birds.
The Whitebreasted or Great Cormorant Phalacrocorax carbo lucidus is a widespread species, which breeds both along the coast and inland. The South African coastal population is about 2500 pairs (Brooke et al. 1982). The Dassen Island population is about 18 pairs. At Dassen Island there is a peak in breeding activity from August until November. The major threat to this species is considered to be human disturbance at breeding colonies.
The nominate subspecies of the Swift Tern (Sterna bergii bergii) is endemic to the coastal waters of southern Africa. There are substantial inter-annual fluctuations of the number of birds breeding in the southwestern Cape, including Dassen Island. These changes are related to the abundance of Cape Anchovy, the most important prey item of Swift Terns in the region (Crawford and Dyer 1995). In 1988, there were about 3000 pairs breeding at Dassen Island. In other years they havenít bred at all. In addition to the large inter-annual fluctuations in the numbers of birds that breed, Swift Terns also exhibit a high degree of nomadism, and can alter their breeding sites from one year to the next (Crawford et al. 1994). At Dassen Island, the peak in breeding activity normally occurs in February/March. It has been suggested that the presence of feral cats on the island has prevented the birds from breeding in some years (Cooper et al. 1990). Swift Terns are also at risk from entanglement with fishing equipment and debris.
Hartlaubís Gull Larus hartlaubii is endemic to South Africa and Namibia. It is one of the worldís rarest gulls and has an overall breeding population of about 12 000 breeding pairs (Williams et al. 1990). Hartlaubís Gulls frequently breed in mixed colonies with Swift Terns. Egg-laying normally takes place in late summer and autumn, from February to April (Crawford 1997), although there is inter-annual variation in the timing of breeding. Hartlaubís Gulls are relatively nomadic, and can alter breeding localities from one year to the next (Crawford et al. 1994). One of the chief factors affecting the reproductive success of Hartlaubís Gulls is predation, especially by mammals (Williams et al. 1990).
The Kelp Gull is the most widespread and abundant species of gull in the Southern Hemisphere. The southern African Kelp Gull is a distinct subspecies, Larus dominicanus vetula (Brooke and Cooper 1979, cited in Crawford et al 1997). The largest breeding population occurs at Dassen Island, where approximately 4500 breeding pairs were counted in 1992 (Crawford et al. 1994). The number of Kelp Gulls in the southwestern Cape has increased since the late 1960s, probably as a result of increased post-fledging survival due to supplementary food from manís activities, especially at sites such as fishing harbours and waste disposal sites (Steele and Hockey 1990). It has recently been observed that the breeding success of Kelp Gulls at Dassen Island is significantly reduced by predation of young chicks by Great White Pelicans (Crawford et al. 1997, WCNCB, unpubl. data).
Leachís Storm Petrel
Leachís Storm Petrel, Oceanodroma leucorhoa, is a (austral) summer visitor to southern Africa. It is normally seen in oceanic waters and is only rarely observed closer inshore. A recent investigation found a Leachís Storm Petrels breeding colony at Dyer Island Ėthe only confirmed breeding locality for this species in the Southern Hemisphere (Whittington and Dyer 1995, Whittington et al. 1999). A number of sightings of Leachís Storm Petrels have recently been made on Dassen Island, and it is suspected that they may be breeding here. If this species is found to be breeding on the island, it will be the islandís rarest breeding seabird. However, it is an abundant breeding species in the Northern Hemisphere.
African Black Oystercatcher
The African Black Oystercatcher Haematopus moquini is endemic to the coast of southern Africa. The total world population is less than 5000 birds, over half of these occurring in the southwestern Cape Province (Hockey 1983). Dassen Island presently has a population of about 230 individuals. The population size appears to have changed little from the late 1970s, when a mean of 221 individuals was reported for the island (Hockey 1983). The main threats to oystercatchers on the island are the predation of chicks and eggs by Kelp Gulls, and the predation of chicks by feral cats. Another potential threat is paralytic shellfish poisoning following "red tides" (Summers and Cooper 1977). Oystercatchers are subject to severe disturbance on the mainland during the breeding season. Offshore islands (and protected mainland sites) such as Dassen provide a relatively safe breeding refuge for this species and generally have a high breeding productivity compared to mainland sites (Loewenthal 1998).
Other waders and shorebirds
Other waders that breed on the island include the White-fronted Plover Charadrius marginatus, Kittlitzís Plover Charadrius pecuarius, Blacksmith Plover Vanellus armatus, and Crowned Plover Vanellus coronatus. The island is also frequented, mostly in the summer months, by a number of Palearctic migrants.
There are four resident terrestrial birds on the island. These are the Rock Pigeon Columba guinea, the European Starling Sturnus vulgaris, the Cape Wagtail Motacilla capensis and the House Sparrow Passer domesticus. The European Starling and the House Sparrow are aliens, which reached the island by natural dispersion after introduction to the South African mainland (Cooper and Brooke 1986).
Early European sailors noted the presence of a "rabbit", which was, in fact, the Rock Hyrax or Dassie Procavia capensis. One of the islandís early names (Coney Island) and its present name are based on the presence of this species on the island. From the first arrival of Europeans to the island dassies were hunted for meat. Such large numbers dassies were killed that in 1654 Jan van Riebeeck ordered that no "rock-rabbits" were to be caught or hunted so that they could multiply. Dassies later (the precise date is unknown) became extinct on the island.
The first introduced mammal on the island was a domestic hunting dog brought ashore by hunters on the 3rd of December 1652 (Cooper et al. 1985). The European Rabbit Oryctolagus cuniculus was introduced as food in about 1665 and is still present in large numbers (Cooper and Brooke 1982). House Mice Mus musculus were introduced, presumably accidentally, probably in the 19th Century (Cooper et al. 1985), and are still present. It is not known exactly when domestic cats were first introduced, but it was probably towards the end of the 19th Century (Cooper et al. 1985), presumably to control the mice. The cats became a nuisance, and their numbers were controlled intermittently until 1979 when a study of the cat population on the Island was initiated (Apps 1983, Cooper et al. 1985). Although Apps (1983) recommended that the cats on the Island should not be eradicated, numerous other researchers have strongly urged for their eradication, and this has been the recent -and is the present- management policy (see section 4.1.1.a). There is presently a small feral cat Felis catus population (estimated to be less than 10 individuals) on the Island.
Residents on the island have kept a number of domestic animals, which include Swiss Goats Capra hircus, sheep, pigs, donkeys Asinus africanus, different kinds of "buck" and tame Chacma baboons Papio ursinus (Cooper et al. 1985). Appendix 5 lists the terrestrial mammals still present on the island.
The Cape Fur Seal Arctocephalus pusillus used to breed on the island in large numbers, but is now only rarely seen. The Southern Elephant Seal Mirounga leonina and the Sub-Antarctic Fur Seal Arctocephalus tropicalus have been recorded as vagrants to the island. A number of whale and dolphin species occur offshore of the island. The most commonly seen are the Humpback Whale Megaptera novaengliae, the Southern Right Whale Balaena glacialis, the Dusky Dolphin Lagenorynchus obscurus, the Heavisideís Dolphin Cephalorhynchus heavisidii and the Common Dolphin Delphinus delphis.
2.5.3 Marine and intertidal communities
The West Coast is influenced by the cold, upwelling Benguela Current. Nutrient-rich upwelled waters fertilize microscopic floating phytoplankton. Both phytoplankton and seaweeds are far more productive on the West Coast than on the South and East Coasts, and fuel more productive food-chains, culminating in the lucrative fisheries that are concentrated in this region. Although productivity is high on the West Coast, there are far fewer species than the East Coast. The West Coast, including the waters offshore of Dassen Island, is characterised by prolific kelp forests. The large plankton populations feed large offshore stocks of pelagic fish such as pilchard Sardinops sagax and anchovy, which are in turn preyed upon by marine predators, including numerous seabirds and other fish such as snoek Thyrsites atun.
The Intertidal communities on the island are typical of the West Coast, being made up of five zones. These include:
a) The Littorina highest and most barren zone on the shore, is inhabited only by the small, air breathing Littorina snails and the very hardy seaweed, Porphyra
b) The Upper Balanoid few algae, except for the bright green sea lettuce Ulva; small barnacles are more visible
c) The Lower Balanoid supports thick beds of algae, particularly Gigartina species
d) The Cochlear relatively barren, except for dense populations of the limpet, Patella cochlear.
The Infratidal lowest region on the shore; also the richest in plant and animal life
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