|Avian Demography Unit
Department of Statistical Sciences
University of Cape Town
Mercury Island, Namibia
Set in the picturesque Spencer Bay, 100km north of Lüderitz, Mercury Island (25 43 S, 14 50 E) must surely rate as one of the most spectacular seabird islands in the world. It is a small island (3 ha), 1.5km offshore, and resembles an elongated, steep pyramid. Its highest point, Jupiter Peak, rises 35 m above the sea. North Islet, a rocky outcrop about 30 m to the north, is connected to Mercury Island by a rickety wooden bridge, 5 m above the surging sea. Once used by guano harvesters to access North Islet, the bridge is now densely covered in African Penguin and Bank Cormorant nests, and it is a rather unnerving balancing act to access North Islet without being pecked to shreds in the process.
Mercury Island is hollow, and altogether seven cave entrances (with exciting names like "Savage Gash", "Gut Cut" and "Glory Hole") meet in a large central cave below the centre of the island where waves crash into each other from all directions. On some days, when the sea is particularly rough, the force of the colliding waves makes the whole island shake and rumble. Mercury Island is managed by the Ministry of Fisheries and Marine Resources, Namibia, and is permanently staffed. Because of the lack of level space on the island, the only house is built on concrete pillars against the northeast slope. Being separated from the nearest town by a seven hour boat-ride makes life on Mercury Island quite a challenge. Fresh water and food are only supplied twice a year. Until very recently there was no electricity - only candle light. Since February 2001, the house is powered by solar panels.
After sealing ceased on Mercury Island, numbers of breeding seabirds increased and the island was subsequently managed for guano production. Seals recolonised Mercury Island in 1983 and it became a fully-fledged seal breeding colony by 1985. This in turn meant a displacement of seabirds. In the early 1990s, it was decided to chase the seals off the island to allow the seabird population to recover. This was successfully done, and the seals have now settled on the mainland near the Otavi, a guano boat which got stranded at the southern tip of Spencer Bay in 1945.
Despite its small size, Mercury Island currently supports by far the largest breeding colony of penguins in Namibia, numbering roughly 16 500 adult individuals. With only a few exceptions, most penguins breed on the surface as the guano layer is too shallow to burrow into. This leaves chicks, particularly large, fat, downy chicks, vulnerable to heat exhaustion. Egg-rolling, and sometimes even chick-rolling, where eggs or small chicks roll out of the nest, irretrievably down the slope and sometimes into a nest below, occurs fairly commonly on the island.
Mercury Island also sports the largest Bank Cormorant colony in Namibia, with 1074 active nests at breeding peak recorded in 2001, but down from the 1986 nests counted in 1979. Other seabirds breeding on Mercury Island include Cape Cormorants, a few Crowned Cormorants (34 in 2001) as well as a number of small colonies of Cape Gannets. There seems to be a general trend for the penguin distribution to shift northwards, probably related to fish distribution and abundance, but it is uncertain how many more birds this small island can accommodate. Owing to its sensitivity and limited accommodation, Mercury Island cannot be visited by the public.