Tristan Da Cunha Tristan Da Cunha Cape Town
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AC3 Doc/Inf 28 Agenda Item No. 11.3 Agreement on the Conservation of Albatrosses and Petrels Third Meeting of Advisory Committee Valdivia, Chile, 19 – 22 June 2007 ________________________________________________________________ UKOT IBAs Tristan de Cuhna Author: AFRICA AC3 Doc/Inf 28 Agenda Item No. 11.3 Tr istan da Cunha (including Gough Island) Beau W. Rowlands (updated b y Geoff Hilton, 2004) Alison R othwell Tr istran from Nightingale, with a pair of Y ellow -nosed Albatrosses General introduction The Dependency of T ristan da Cunha, which covers both 217 miles (350 km) to the south-south-east. L ying the Tr istan group (Tristan, Inaccessible, Nightingale, somewhat east of the crest of the mid-Atlantic Ridge, near Middle and Stoltenhoff Islands) and Gough Island, has a its junction with the aseismic W alvis Ridge, the islands rise total land area of 69 square miles (179 square km). The from a sea depth of about 3,500 m. At least 212 plant taxa islands are of volcanic origin, of varying geological age and have been r ecorded, including 35 native ferns and 58 native stage of erosion, the oldest r ocks dating back 18 million flowering plants. Of these, 20 fern and 34 flowering plant years. However, the three largest islands all show evidence taxa are considered to be endemic. There are no r eptiles, of r ecent activity, and therefore cannot be r egarded as amphibians or freshwater fish, and there are no r ecords, volcanically extinct. Isolated, the five islands of the T ristan other than poultry, of introduced birds. An invertebrate group lie within 25 miles (40 km) of each other in the mid- fauna includes weevils and snails of particular interest, but South Atlantic Ocean, on similar latitudes 1,729 miles (2,782 with a r elatively low number of native species. The only km) from South Africa (Cape T own) and 2,453 miles (3,947 native breeding mammals are seals, which have been km) from South America (Mar del Plata). Gough Island lies exploited in the past. Five whales – Eubalaena glacialis 227 Important Bird Areas in the United Kingdom Overseas T erritories Angola Namibia 0 500 1,000 kilometres SH005 Tristan da Cunha Tristan da Cunha Cape Town SH006 SH008 0 10 20 SH007 kilometres Gough Island 228 Tristan da Cunha australis, Physeter macrocephalus, Megaptera novaeangliae, Gough, where the mean monthly rainfall in summer is 230 Globicephala melas and Tasmacetus shepherdi – occur r elatively mm; in winter it is 289 mm. The mean annual temperature frequently, the last with r egular strandings. V arious species at the T ristan settlement is 14.5ºC while, near sea level on of dolphin, including Lagenorhynchus obscurus, are common. the south-east coast of Gough, it is 1 1.3ºC. T ristan (settlement) has a r ecorded mean annual rainfall of 1,676 The islands are a dependency of St Helena, itself a UK mm, with rain on 250 days of the year, and Gough 3,397 Overseas T erritory, 1,513 miles (2,435 km) to the north- mm with rain on 296 days. Rainfall on the uplands of north-east. There is no air link. None is on any r egular Tristan and Gough may be at least twice as heavy as that at shipping lane, but there is a shipping link with Cape T own sea level. Snow lies intermittently during the winter above and cruise ships occasionally call. The islands are governed about 600 m on T ristan and above about 300 m on Gough, by an administrator appointed by the UK Foreign & but frosts at sea level are almost unknown on either island. Commonwealth Office, who is advised by an Island The cloud base at T ristan usually lies above 600 m, but Council of eight elected and thre e appointed members, of frequently descends below the Base plateau. Nightingale whom one is chosen as Chief Islander by popular ballot. Island is usually cloud free, being warmer and drier than The administrator is accountable to the Governor of St Tristan. On Inaccessible Island, orographic cloud is a Helena. One of the most isolated communities in the world, common feature. At Gough the cloud base is typically the human population in 2006 was 273 (seven family between 300 m and 500 m. At all the islands it occasionally names), all on the largest and main island of T ristan but for descends virtually to sea level. the six non-resident members of a South African meteorological team on Gough. The Tr istan settlement, The islands lie within the W est W ind Drift, where the Edinburgh, in the north, has some 75 homes (100 families). prevailing winds in the Southern Ocean impart an easterly The community is self-sufficient in food due to the set to the surface waters of some 8 miles (13 km) per day. surrounding seas and to the management of the grasslands The Subtropical Convergence usually lies between the around the village and the Potato Patches (about 1.8 Tr istan group and Gough, but occasionally south of Gough. miles/3 km south-west) where the staple potato crop is The dependency is therefore affected by both sub-Antarctic grown. The economy is based on crayfish Jasus tristani (the and cold temperate mixed water types. Mean sea frozen product is exported), philately (worldwide sales) temperatures are around 18ºC and 13ºC in summer and and handicrafts (mainly woollen goods). The other islands 13ºC and 1 1ºC in winter at T ristan and Gough Island, are uninhabited, but there are traditional longboat trips to respectively. Nightingale Island to collect penguin and shearwater eggs, The six islands of the dependency each have distinct shearwater chicks and guano. About 30 wooden huts and topographical, floral and faunistic characteristics. shacks, as well as pathways, have been constructed on Nightingale for this purpose. Inaccessible Island has been Tr istan Island visited less often since 1938 but, before then, was much Roughly circular in plan, covering 37 square miles (96 more visited than Nightingale. A single r esearch hut built square km), with an average diameter of some 7.5 miles (12 on Inaccessible by the Denstone Expedition in 1982 was km), T ristan is a strato-volcano made up of interbedded both demolished and r eplaced in January 2000. The only lavas (mainly basaltic) and pyroclastic deposits, with a settlement on Gough is the meteorological station, about 10 central cone, the Peak, rising to 2,060 m. It is geologically buildings, at T ransvaal Bay in the south-east of the island. the youngest in the group, about 500,000 years old. At the summit it is an unbreached crater, containing a shallow The climate of the dependency is cool temperate oceanic, lake that is frozen in winter. The flanks are steepest near but can vary locally from island to island. L ying on the the summit, and gradients slacken to a more gently edge of the W est W ind Belt, the islands are under the inclined area known as the Base plateau, lying between 600 influence of both maritime tropical (mT) and maritime m and 900 m. T o seaward, the Base is truncated by polar (mP) air masses from the western South Atlantic. The precipices and cliffs, but there are discontinuous lowlands pattern is dominated by the passage of often severe between the cliffs and the sea, the three most prominent cyclonic storms, generated by outbursts of mP air on the being the Settlement Plain (5 miles by 0.6 miles/8 km by 1 Polar Front. (A storm in May 2001, with 1 18 mph/190 km) in the north-west, Sandy Point in the east, and Stony kmph winds, caused considerable disruption and damage Hill and Cave Point in the south. Numerous parasitic in the T ristan settlement.) At T ristan, the prevailing winds cones, r esulting from secondary eruptions, protrude from are north-westerly to south-westerly, occasionally from the the flanks of the Peak, the Base plateau and coastal strips. north and south, and only rarely from the east. Since the At least eight on the Base plateau are considered less than mP air is cooler than the ocean, and many fronts are 25,000 years old. The last eruption was that of October occluded by the time they arrive, the weather is mostly 1961, at the eastern end of the Settlement Plain, when about cloudy with frontal rain. The islands also induce much 0.3 square miles (0.5 square km) of land, mostly as lava orographic rainfall. In summer (October to March), the flows, was added. Seaward erosion of both the main flows Tr istan group may be influenced by the sub-tropical high- and the coastal strips has produced a precipitous cliffed pressure cell bringing mT air with orographic cloud, fewer coastline fringed by narrow boulder beaches and r ocky storms and less rain than in the winter (April to headlands. T wo stacks – The Hardies, the higher 37m – lie September), though summer droughts seldom exceed a few about 1.8 miles (3 km) south-west of Herald Point in the weeks. Frontal rainfall occurs throughout the year at 229 Important Bird Areas in the United Kingdom Overseas T erritories north-west. The island r etains a youthful drainage system, comprises three main types. The lower slopes are blanketed with radially arranged flat-bottomed gorges, or ‘gulches’, with dense, uniform Spartina tussock-grassland, up to 2.5 m deeply incised into the main sequence of lava flows on the high. The western part of the plateau comprises largely Base plateau.