Located 800 m offshore, the precipitous Mercury Island lies within Spencer Bay, about 110 km north of Lüderitz. The island is within a zone of intense oceanic upwelling that is responsible for the elevated nutrient levels and high fish biomass around these near-shore islands. Somewhat elongate, this steep-sided island reaches 40 m, is 500 m long and 100 m wide. It is the smallest of the three guano islands at 3 ha. Known as the island that shakes (hence the name Mercury), the interior of the island is hollow, and large swells, common in this region, thunder inside the coves under the island, causing it to reverberate ominously. The island is unvegetated and was first exploited for guano in the 1840s when thousands of tons of ‘white gold’ were stripped from its flanks. It is the northernmost of the 18 near-shore islands of the Diamond Coast used by breeding seabirds.
See Box for key species. Mercury Island is one of three very important coastal seabird-breeding islands along the Diamond Coast of south-western Namibia; the other two are Ichaboe (IBA NA016) and Possession (NA018). Mercury regularly supports over 15,000 seabirds, including Spheniscus demersus, Morus capensis, Phalacrocorax neglectus, and small numbers of P. coronatus. The island’s breeding population of Phalacrocorax neglectus has decreased by about 50% in the last 15 years. The seabirds cover virtually the entire surface area of the island, leaving no space for other species.
Non-bird biodiversity: Several whale species migrate through these waters, including Megaptera novaeangliae (VU) and Eubalaena australis (LR/cd). The cetaceans Cephalorhynchus heavisidii (DD), Lagenorhynchos obscurus (DD) and Tursiops truncatus (DD) occur in these waters.
Pressure/threats to key biodiversity
The island has one stone building for permanent staff, whose job is to prevent seals Arctocephalus pusillus from settling on the island’s shores. The staff also undertake regular counts of breeding birds and monitor their success. All of the near-shore islands on the Namibian coast were managed by Cape Nature Conservation as nature reserves when they were under South African rule. Now under Namibian law, they are not reserves but are protected by staff of the Ministry of Fisheries and Marine Resources. Three seabird species, Spheniscus demersus, Morus capensis and Phalacrocorax capensis, have suffered serious population declines in the last 30 years, mostly because of overfishing of surface-shoaling fish, such as Sardinops sagax, their main food source. From an already reduced population of 70,000 Spheniscus demersus in the 1950s, only 5,300 pairs remained 30 years later. However, since humans displaced seals from the island in 1986, penguin numbers have increased.Penguins and cormorants are renowned for their guano, which has been harvested for hundreds of years for the fertilizer trade, resulting in disturbance during breeding, which compounds their population declines. This has affected Spheniscus demersus most severely, as it prefers to burrow in the guano. Large-scale guano harvesting removed the penguins’ cover and forced them to breed in the open, exposing their chicks and eggs to increased predation by gulls and seals, and excessive daytime heat and storms. Conservation measures currently ensure that these coastal seabirds are not disturbed during breeding and no guano is harvested.Potential threats to the seabirds include the steadily increasing population of Arctocephalus pusillus along the Namib coast. Seals compete for space on some islands and eat young birds as they leave the islands. A programme has been initiated in which seals are discouraged from breeding where they dramatically affect sensitive seabird species. Furthermore, stocks of the fish Sufflogobius bibarbatus have declined and those of Jasus lalandi have been severely over-exploited in the last 30 years. Phalacrocorax neglectus, which forage primarily on gobies and lobster, are particularly numerous on this island because the rocky nature of both the island and the adjacent shoreline is favoured by their prey-species. Phalacrocorax neglectus has declined since 1956 on this island, possibly because of a reduction of food stocks. Appropriate management, permitting stocks of Sufflogobius bibarbatus and Jasus lalandi to recover, may lead to a concurrent improvement in population levels of Phalacrocorax neglectus. Offshore diamond mining, in which sections of the seabed are vacuumed up and then re-deposited from small vessels, may disturb prime lobster habitat.