Animal Demography Unit
Photo L.G. Underhill, 2000
Dassen Island from the east. The lighthouse is on the left hand side. Boom Point, where the pelicans breed, is in the right background
Dassen Island lies 9 km off the coast, almost directly opposite the village of Yzerfontein. It is 55 km north of Cape Town, about halfway between Table Bay and Saldanha Bay. With an area of 273 ha, it is the second largest of the offshore islands. It is generally flat and sandy, apart from patches of exposed bedrock, and some areas strewn with huge granite boulders.
At the start of the 20th century, Dassen Island was home to well over a million African Penguins. At this time, about 600 000 penguin eggs were harvested each year. This rate of utilization was not sustainable. As the population size declined, the number of eggs decreased steadily, and egg harvests from Dassen Island finally ended in 1967.
Another problem for the penguins on Dassen Island are domestic cats that have become feral. Cats are estimated to have killed about 2000 penguin chicks in 1983. At this time, the penguin population was probably about 7000-8000 pairs. The cat population has been greatly decreased; by 2000 there were very few left. It has proved extremely difficult to eradicate these last animals. Unless this is done, the cat population, and the predation on seabirds, will increase again. Over the past few years, with reduced cat numbers, the number of breeding penguins has increased. The nest count on Dassen Island in May 2000, a month before the bulk-ore carrier Treasure sank, was 15 000; this made this penguin colony top of the league table in size.
Egg harvesting and cats were threats that were especially prevalent on Dassen Island. These birds were also impacted by most of the other factors affecting penguins throughout the range: reduced availability of food through over-fishing, destruction of breeding habitat by the collection of guano during the 19th century, and oil pollution. A tiny oil slick that came ashore at one of the main penguin landing beaches on Dassen Island in 1972 oiled about 4000 penguins. This was the largest number of African Penguins oiled in one incident between 1968, when SANCCOB started, and 1994, when the Apollo Sea sank off Dassen Island; in this event, 10 000 birds were oiled, of which about half came from Dassen Island.
White Pelicans have bred on Dassen Island since 1956. In contrast to population trends for most large birds, the size of the pelican population here has increased steadily, and currently about 700 pairs breed, mostly on Boom Point in the northwest on the island. They fly to the mainland to feed. However, in the past few years, they have taken to eating the chicks of Kelp Gulls, Cape Cormorants and Swift Terns.
Photo L.G. Underhill
An adult Leach's Storm Petrel Oceanodroma leucorhoa, mistnetted for ringing on Dassen Island
Dassen Island is one of three South African offshore islands where Leach's Storm Petrel is known to breed. Until 1995, this species was regarded exclusively as a nonbreeding migrant from the northern hemisphere. In this year it was found breeding on Dyer Island. The following year it was demonstrated to be breeding on Dassen Island too. However, Leach's Storm Petrel has probably bred on these islands for many years; for example, one was mistnetted near the centre of Dassen Island in December 1971.
Dassen Island has vast numbers of European Rabbits, introduced by early seafarers as a precaution against being shipwrecked. There is also an impressive population of Angulate Tortoises.
Dassen Island is a reserve of the Western Cape Nature Conservation Board. Because of the sensitivity of the island to disturbance, and a variety of logistic problems such as a lack of freshwater, and the difficulties of landing on the island when the sea is rough, there are no visitor facilities. However, the possibility of opening the island to ecotourism, at least on small-scale experimental basis, is currently being explored.
Phil Whittington and Anton Wolfaardt have written a description of the avifauna of Dassen Island. It was published in the December 1999 issue of the ADU journal Bird Numbers.
Dassen Island on the old ADU web site...
- Susie Moloney worked on the island as a volunteer in January 2001. Read her account of her experiences.
- A joint MCM/ADU had a pelican ringing expedition to Dassen Island in January 2002.
- One of the islands visited by Marienne de Villiers, Janine le Roux and Michelle de Villiers in February 2002 was Dassen Island. They describe their visit as A trip in time.