Covering c. 20 ha, Dyer Island is one of two low-lying islands situated 4.7 km south-east of Danger Point; the nearest harbour is at Kleinbaai/Franskraal, just south of Gansbaai. Its coastline is rugged, with some low rocky areas spreading inland. This flat, low-lying island reaches 7 m a.s.l. at its highest point and is covered by a pebble surface with little sand. The mixed vegetation consists primarily of species of the Mesembryanthemaceae family and exotic weeds. In the south-eastern section of the island are several buildings that house the island staff, boats and stores. The smaller Geyser Island also forms part of the IBA and Dyer Island Nature Reserve.
Dyer Island is approximately 20 hectares and is one of two low-lying islands situated 4.7 km south-east of Danger Point; the nearest harbour is at Kleinbaai/Franskraal, just south of Gansbaai. Its coastline is rugged, with some low rocky areas spreading inland. This flat, low-lying island reaches 7 m a.s.l. at its highest point and is covered by a pebble surface with little sand. The mixed vegetation consists primarily of species of the Mesembryanthemaceae family and exotic weeds. In the south-eastern section of the island are several buildings that house the island staff, boats and stores. The smaller Geyser Rock also forms part of the IBA and Dyer Island Nature Reserve.
CapeNature has recorded 12 seabird and five terrestrial species breeding on the island (Birss et al. 2012). Unfortunately, as in the cases of other island IBAs in the Western Cape that have been gazetted specifically for their seabirds, these species have shown major declines from the numbers recorded in the original IBA description (Barnes 1998).
The African Penguin Spheniscus demersus population at Dyer Island is in rapid decline. Although the island supported some 72 500 birds in 1976, it is thought that the population may have fallen below 3 000 pairs by 1997. It now numbers approximately 1 000 breeding pairs.
The discovery of Leach’s Storm Petrel Oceanodroma leucorhoa in several of the stone walls on the island in October 1995 was the first evidence of a procellariiform breeding in southern Africa. In November 1996 at least 19 birds were heard calling from within the walls and one was found to be incubating an egg. It was estimated that 8–9 pairs were breeding. In early February 1997 the nest contained a month-old chick and chicks were heard calling from two other nests, confirming breeding for this species.
In 1996, after an absence of some 25 years, Roseate Tern Sterna dougallii, which breeds regularly only at two other islands in southern Africa (Bird and St Croix Islands, part of the Algoa Bay Islands IBA, SA095), attempted to breed again at Dyer Island. The attempt failed, it is suspected as a result of human disturbance. However, in subsequent years the species remained on the island and a small breeding colony persists. Caspian Tern Sterna caspia has also been recorded breeding on the island and has been listed as an additional IBA species.
Many other species breed on this island, including large numbers of Cape Cormorant Phalacrocorax capensis (approximately 12% of the species’ population) and small numbers of White-breasted Cormorant P. lucidus, Bank Cormorant P. neglectus and Crowned Cormorant P. coronatus. Numbers of Bank and Cape Cormorants have declined in recent years, while numbers of Crowned Cormorant seem to have remained stable. Several large colonies of Hartlaub’s Gull Chroicocephalus hartlaubii and Swift Tern Thalasseus bergii breed on the island. Sandwich Tern T. sandvicensis, Common Tern Sterna hirundo, Arctic Tern S. paradisaea and Antarctic Tern S. vittata all form large mixed tern roosts with the breeding Swift Terns.
Dyer Island is also estimated to hold more than 1% of the world’s African Black Oystercatcher Haematopus moquini population.
Globally threatened trigger species are African Penguin (975 breeding pairs; Crawford et al. 2012), Bank Cormorant (nine breeding pairs; Crawford et al. 2012), Cape Cormorant (24 000 breeding pairs; Crawford et al. 2012) Crowned Cormorant (266 breeding pairs; Crawford et al. 2012) and African Black Oystercatcher (21 breeding pairs; Birss et al. 2012). Regionally threatened species are Roseate Tern (15–30 breeding pairs; Birss et al. 2012) and Caspian Tern (four breeding pairs; Birss et al. 2012).
Congregatory species with more than 1% of the population represented are Cape Cormorant and African Black Oystercatcher. More than 0.5% of the population of Hartlaub's Gull (160 breeding pairs; Birss et al. 2012) and Swift Tern (500 breeding pairs; Birss et al. 2012) are present on the island.
The waters around Dyer Island hold a large population of great white shark Carcharodon carcharias.
The main focus of conservation on this island nature reserve is the seabird colonies, which are the reason for the island being given IBA status (Barnes 1998). The threats facing the seabirds on Dyer Island are not dissimilar to those impacting other colonies around the east and west coasts of South Africa. In particular, the seabirds are impacted by an increase in competition for food resources. Competition with commercial fisheries, especially purse-seining for surface-shoaling fish such as anchovy Engraulis capensis and pilchard Sardinops sagax, has been implicated as one of the most significant factors causing seabird population declines in the region. Anchovy recruitment was impaired and stocks were greatly reduced between 1989 and 1990, which may have triggered mass penguin emigration. It is thought that birds that used to breed at Dyer Island may have relocated to Stony Point, Boulders Beach, Robben Island and as far east as St Croix in the Algoa Bay Islands IBA, where the populations are growing steadily. It has been recommended 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. Regulations of this type may prevent the local depletion of food resources that contributes to low breeding success and may precipitate mass desertion of these colonies.
Loss of suitable breeding habitat for seabirds and degradation of habitat by pollution are also threats. Pollution incudes both chronic and catastrophic oil pollution by crude oil or other pollutants that spill into the ocean when tankers run aground and break open, wash their tanks, dump cargo or pump bilge. Chronic plastic pollution also occurs and can result in birds either becoming entangled in plastic that washes up on the breeding islands or ingesting plastic when they are foraging in the open ocean.
Another threat is disturbance due to uncontrolled tourism activities, in conjunction with disturbance that can be generated through monitoring activities. CapeNature is very aware of the latter issue and ensures that negative impacts are minimal. Tourism to the reefs around the island to see great white shark is increasing and the industry needs to be carefully monitored. Landings on the island are currently not permitted. Several introduced species also occur on the treeless Dyer Island, including 15 exotic plants, which are mostly small weeds and forbs.
The Cape fur seal Arctocephalus pusillus population on nearby Geyser Island has been growing steadily. There has been an increase in the number of cases of seals mauling and killing penguins and Cape Cormorant fledglings. Certain ‘rogue’ individuals seem to be the principal agents of this mortality and it is important that management action be introduced to eliminate the animals responsible. Furthermore, increased seal numbers, coupled with the animals’ ability to outcompete and displace birds at breeding islands, pose a major threat to all breeding seabirds.
The poaching of abalone Haliotis asinina and other marine resources, potential outbreaks of disease and the impacts of climate change are also noted as threats to the seabirds. An outbreak of an epizootic occurred in 2002, affecting five offshore islands in the Western Cape. Since then, outbreaks of avian cholera Pasteurella multocida have occurred annually on Dyer Island. However, these outbreaks are carefully monitored and managed by CapeNature staff in order to reduce seabird mortalities.
Dyer Island is State-owned and administered and managed by the Walker Bay office of CapeNature based in Hermanus. This status affords the island the necessary protection, management planning and associated resources required for long-term conservation. A number of actions deal with all the aspects of conservation management at this site, as laid out in detail in the Dyer Island Nature Reserve Complex Protected Area Management Plan of 2012. These actions include (but are not limited to) finalising the legal status and reserve expansion as far as possible; co-operative governance with all relevant government departments and other agencies; ecosystem and biodiversity monitoring and management; law enforcement and compliance; infrastructure management and maintenance; and disaster management.
Seabird populations on specific islands are seldom stable over long periods and they go through spells of flux. The most important island at any one time may be of minimal significance a few decades later. It is imperative that the entire network, including other IBAs of the Western Cape and Eastern Cape where threatened birds breed or could breed, is suitably protected.
Dyer Island Conservation Trust: http://www.dict.org.za/index.php
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