Gun Island is one of the larger islands in the Pelsaert Group of the Houtman Abrolhos, in the Indian Ocean off the west coast of Australia. It is nominally located at Coordinates: , about 4 km north and east of Half Moon Reef and is a flat limestone outcrop of about 800 metres (2,600 ft) by 420 metres (1,380 ft) in size. The island is part of the Houtman Abrolhos Important Bird Area, identified as such by BirdLife International because of its importance for supporting large numbers of breeding seabirds.
Between June 1727 and March 1728, crew of the Dutch VOC ship Zeewyk were stranded on the island after it struck Half Moon Reef. After a longboat with 11 seamen which had been despatched to go for help had failed to return, the remaining survivors constructed a 20 metres (66 ft) sloop from the wreckage, which they named the Sloepie. Of the 88 who survived the ordeal, 82 arrived in Batavia on 30 April 1728.
During admiralty surveys of the north-west coast in 1840, crew from HMS Beagle discovered a brass gun of about three pounds calibre and an iron swivel on which paint was still adhered. Captain Stokes with Commander John Clements Wickham named the place Gun Island. They also discovered several coins, including one dated about 1707 and another dated 1720. Also seen was what appeared to be the beam of a ship with an iron bolt through it, and glass bottles and clay pipes. The material was presumed to have been left there by the Zeewyk castaways, 112 years earlier.
In 1883, Charles Edward Broadhurst, who had been granted a lease for guano export, discovered several campsites and seal bones there, which had evidentially been killed for sustenance by the Zeewyk crew.
Gun Island was one of the most heavily worked islands of the Abrolhos Islands for guano. Guano workings continued on a commercial scale from the 1880s to the 1920s, and again in the mid-1940s. A 100 metres (330 ft) stone jetty built for loading guano on the south-eastern corner is still intact.
A yacht, the Nautilus was wrecked at Gun Island in 1897.
A stratigraphic test well was drilled on Gun Island (28º53'30.11"S, 113º51'27.0"E) by BP Petroleum Development Australia in June–November 1968. It was drilled within the Perth Basin and reached a total depth (TD) of 3725 m (12,220 ft). The extent of the drilling platform near the wharf can still be seen on satellite images today.
Gun Island is classified as having 'High' conservation significance and is one of the seven protected zones in the Abrolhos Islands. Protected zone restrictions mean that "visitors shall not carry out any digging or major earthworks within the zones around declared maritime archaeological sites unless permitted to do so", and that they shall also not take metal detecting devices onto the island.
A 20 metres (66 ft) wide rock called "Gun Islet" is situated about 30 metres (98 ft) off the southern tip of the island.
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