102-620: The Houtman Abrolhos (often called the Abrolhos Islands ) is a chain of 122 islands and associated coral reefs in the Indian Ocean off the west coast of Australia about 80 kilometres (50 mi) west of Geraldton, Western Australia . It is the southernmost true coral reef in the Indian Ocean, and one of the highest latitude reef systems in the world. It is one of the world's most important seabird breeding sites, and
204-749: A Darwin Point is reached. One such point exists at the northwestern end of the Hawaiian Archipelago; see Evolution of Hawaiian volcanoes#Coral atoll stage . However, reefs in the Persian Gulf have adapted to temperatures of 13 °C (55 °F) in winter and 38 °C (100 °F) in summer. 37 species of scleractinian corals inhabit such an environment around Larak Island . Deep-water coral inhabits greater depths and colder temperatures at much higher latitudes, as far north as Norway. Although deep water corals can form reefs, little
306-553: A topographical trigger for the forming of eddies , so the Abrolhos can experience currents from any direction, even when the Leeuwin Current is flowing strongly. Unlike most other major ocean currents, there is no large-scale coastal upwelling associated with the Leeuwin Current. There is limited evidence for some sporadic, localised upwelling in the vicinity of the Abrolhos, but if so it appears to have little effect on
408-644: A continental shelf, and not around a volcanic island, Darwin's principles apply. Development stopped at the barrier reef stage, since Australia is not about to submerge. It formed the world's largest barrier reef, 300–1,000 m (980–3,280 ft) from shore, stretching for 2,000 km (1,200 mi). Healthy tropical coral reefs grow horizontally from 1 to 3 cm (0.39 to 1.18 in) per year, and grow vertically anywhere from 1 to 25 cm (0.39 to 9.84 in) per year; however, they grow only at depths shallower than 150 m (490 ft) because of their need for sunlight, and cannot grow above sea level. As
510-525: A little less than 4 °C (7 °F) over the year, peaking at nearly 24 °C (75 °F) in March, and falling to around 20 °C (68 °F) in September. This variability is much less than in nearby coastal waters, which reaches a similar peak in summer but drops as low as 18 °C (64 °F) in winter. The relatively low variability in sea temperatures at the Abrolhos is largely attributable to
612-563: A number of buildings on Rat, Gun and Pelsaert islands and had over 40 Malay workers mining and processing the phosphate for export. Over 48,000 tonnes (110 million pounds) was shipped between 1884 and 1896. The islands are a part of the Division of Durack in the federal House of Representatives and the electoral district of Geraldton in the Western Australian Legislative Council . Management
714-512: A picture of the Abrolhos' climate. The islands have a Mediterranean climate ( semi-arid climate , or BSh, under Köppen climate classification ) with warm dry summers and cooler, wet winters. Mean temperatures range from 9.3 to 19.5 °C (48.7 to 67.1 °F) in July, and from 19.1 to 32.4 °C (66.4 to 90.3 °F) in February. These temperatures have a substantially smaller range than on
816-412: A pinhead to 12 inches (30 cm) across. Reef-building or hermatypic corals live only in the photic zone (above 70 m), the depth to which sufficient sunlight penetrates the water. Coral polyps do not photosynthesize, but have a symbiotic relationship with microscopic algae ( dinoflagellates ) of the genus Symbiodinium , commonly referred to as zooxanthellae . These organisms live within
918-496: A pseudo-atoll. These can be distinguished from real atolls only by detailed investigation, possibly including core drilling. Some platform reefs of the Laccadives are U-shaped, due to wind and water flow. Atolls or atoll reefs are a more or less circular or continuous barrier reef that extends all the way around a lagoon without a central island. They are usually formed from fringing reefs around volcanic islands. Over time,
1020-433: A rapidly subsiding bottom can overwhelm coral growth, killing the coral and the reef, due to what is called coral drowning . Corals that rely on zooxanthellae can die when the water becomes too deep for their symbionts to adequately photosynthesize , due to decreased light exposure. The two main variables determining the geomorphology , or shape, of coral reefs are the nature of the substrate on which they rest, and
1122-445: A ship's crew had survived in the islands for a considerable time. John Lort Stokes , captain of HMS Beagle , also saw these artifacts in 1840. It is now widely thought that these artifacts were from Ridderschap van Holland , although it is possible that they came from Fortuyn , which disappeared in 1724 and is now thought to have been wrecked near Cocos Islands . Any archaeological remains were destroyed by guano mining on
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#17327809691921224-515: A single carbonate platform , so the waters within an island group are mostly shallow. The channels between groups are 40 to 50 metres (130 to 160 ft) deep, so no impediment to the exchange of offshore and inshore waters. The Houtman Abrolhos lies almost directly in the path of the Leeuwin Current , which draws warm, low-saline water of tropical origin southwards along the coast of Western Australia. This current flows all year round, but
1326-423: A zone extending from approximately 30° N to 30° S of the equator. Tropical corals do not grow at depths of over 50 meters (160 ft). The optimum temperature for most coral reefs is 26–27 °C (79–81 °F), and few reefs exist in waters below 18 °C (64 °F). When the net production by reef building corals no longer keeps pace with relative sea level and the reef structure permanently drowns
1428-512: Is a contraction of the Portuguese expression abre os olhos ("open your eyes", i.e. to protect yourself). As a technical term this phytonym was later applied to caltrops ( chevaux de frise ), i.e. "spiked obstructions", and Portuguese sailors used the word to refer to offshore reefs. Houtman thus named the islands using a Portuguese loanword that was current in the Dutch marine terminology of
1530-578: Is constantly changing. Each reef is made up of irregular patches of algae, sessile invertebrates, and bare rock and sand. The size, shape and relative abundance of these patches change from year to year in response to the various factors that favor one type of patch over another. Growing coral, for example, produces constant change in the fine structure of reefs. On a larger scale, tropical storms may knock out large sections of reef and cause boulders on sandy areas to move. Coral reefs are estimated to cover 284,300 km (109,800 sq mi), just under 0.1% of
1632-445: Is directly attached to a shore, or borders it with an intervening narrow, shallow channel or lagoon. It is the most common reef type. Fringing reefs follow coastlines and can extend for many kilometres. They are usually less than 100 metres wide, but some are hundreds of metres wide. Fringing reefs are initially formed on the shore at the low water level and expand seawards as they grow in size. The final width depends on where
1734-515: Is generally well-mixed, with no evidence of a significant halocline or thermocline . Mean differences in water temperature between sea surface and sea bed range from only half a degree (Celsius; 1 °F) in summer to almost zero in winter, and differences in salinity are very small even when the Leeuwin Current is at its strongest. As with the rest of southwestern Australia, tides at the Houtman Abrolhos are small and irregular. There
1836-523: Is known about them. The northernmost coral reef on Earth is located near Eilat , Israel . Coral reefs are rare along the west coasts of the Americas and Africa , due primarily to upwelling and strong cold coastal currents that reduce water temperatures in these areas (the Humboldt , Benguela , and Canary Currents , respectively). Corals are seldom found along the coastline of South Asia —from
1938-417: Is little tidal data available for the islands, but what there is accords very closely with the data for Geraldton. Geraldton tides follow a diurnal pattern, with maximum tidal ranges of around 1 metre (3 ft). Mean sea levels at Geraldton show seasonal fluctuations, being higher in winter when the Leeuwin Current is at its peak. There are also variations from year to year, which are strongly associated with
2040-410: Is nearly always windy. During summer a high pressure ridge lies to the south, causing persistent winds from the southeast or southwest, at speeds exceeding 8.7 metres per second (17 kn) almost half the time. During autumn and winter, the ridge moves northwards, increasing the atmospheric pressure over the islands, resulting in highly variable winds. Winter tends to produce both the strongest gales and
2142-418: Is often the reef's most diverse area. Coral and calcareous algae provide complex habitats and areas that offer protection, such as cracks and crevices. Invertebrates and epiphytic algae provide much of the food for other organisms. A common feature on this forereef zone is spur and groove formations that serve to transport sediment downslope. The reef flat is the sandy-bottomed flat, which can be behind
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#17327809691922244-475: Is partly due to the low-salinity waters of the Leeuwin Current, but there are a number of other factors involved, including high evaporation of coastal waters in summer. Temperatures can also vary substantially from year to year. Annual mean temperatures vary by as much as 1 °C (2 °F); with cooler years usually cooler throughout the year. There is evidence that annual mean temperatures are related to El Niño-Southern Oscillation events. The water column
2346-548: Is probable that Ridderschap van Holland was actually wrecked in the Pelsaert Group of the Houtman Abrolhos islands off the coast of Western Australia . The crew of a later East Indiaman, Zeewijk , which was wrecked on Pelsaert Island in 1727, discovered the remains of a Dutch ship of approximately the right period on their island, together with numerous artifacts, such as bottles, that suggested some of
2448-432: Is strongest during southern hemisphere winter months. In general the Leeuwin Current runs along the shelf break, and thus passes close by the western edge of the Abrolhos. It meanders, sometimes passing well out to sea and sometimes directly through the island chain, bathing it in warm tropical water; Although the direction of the Leeuwin Current is predominantly southerly, Shark Bay and the Houtman Abrolhos together act as
2550-506: Is to be carefully avoided as very dangerous to ships that wish to touch at this coast. It is fully ten miles in length, lying in 28° 46. The word Abrolhos is Portuguese , making the Houtman Abrolhos one of only two Australian places with a Portuguese name, the other being Pedra Branca in Tasmania . The noun abrolhos means 'thorn', and originally designated the spiny fruit of the bindii plant ( Tribulus terrestris ). Etymologically it
2652-500: Is vested in the Department of Fisheries. Some of the islands are considered to be remnants of the mainland isolated by rising sea level during the last 8,000 to 10,000 years. Others were more recently formed from coral rubble. The Houtman Abrolhos lies in the Indian Ocean about 60 kilometres (40 mi) off the coast of Western Australia, near the edge of Australia's continental shelf . At less than 50 metres (160 ft) deep,
2754-1437: The Cambrian . Sometimes called rainforests of the sea , shallow coral reefs form some of Earth's most diverse ecosystems. They occupy less than 0.1% of the world's ocean area, about half the area of France, yet they provide a home for at least 25% of all marine species , including fish , mollusks , worms , crustaceans , echinoderms , sponges , tunicates and other cnidarians . Coral reefs flourish in ocean waters that provide few nutrients. They are most commonly found at shallow depths in tropical waters, but deep water and cold water coral reefs exist on smaller scales in other areas. Shallow tropical coral reefs have declined by 50% since 1950, partly because they are sensitive to water conditions. They are under threat from excess nutrients (nitrogen and phosphorus), rising ocean heat content and acidification , overfishing (e.g., from blast fishing , cyanide fishing , spearfishing on scuba ), sunscreen use, and harmful land-use practices, including runoff and seeps (e.g., from injection wells and cesspools). Coral reefs deliver ecosystem services for tourism, fisheries and shoreline protection . The annual global economic value of coral reefs has been estimated at anywhere from US$ 30–375 billion (1997 and 2003 estimates) to US$ 2.7 trillion (a 2020 estimate) to US$ 9.9 trillion (a 2014 estimate). Though
2856-551: The City of Greater Geraldton . In July 2019, the Houtman Abrolhos was declared a national park by the Western Australian Government . In early November 2022, the government published its three-part plan to manage the site in the following 10 years, based on research and community consultation carried out after the preceding two years. Some critics thought that the plan did not encourage land-based tourism to
2958-661: The El Niño-Southern Oscillation (ENSO) cycle. Apparently, ENSO events induce a weaker Leeuwin Current, which results in lower sea levels. There is no published information on wave heights in the Abrolhos. In the open ocean, waves are typically a little over two metres high all year round. Nearer the mainland, they are usually less than 1.2 metres (3 ft 11 in), with a calmer period in March and April, and another in October and November. As of 2012,
3060-591: The Government of Western Australia . In July 2019, the Houtman Abrolhos was declared a national park by the state government. The Houtman Abrolhos archipelago is in the Indian Ocean off the west coast of Australia, about 80 kilometres (50 mi) west of Geraldton in the state of Western Australia . Its 122 islands and associated coral reefs comprise three island groups, the Wallabi Group , Easter Group and Pelsaert Group . The most northerly group,
3162-475: The Late Cretaceous (100–66 Ma), when reefs formed by a group of bivalves called rudists existed; one of the valves formed the main conical structure and the other, much smaller valve acted as a cap. Measurements of the oxygen isotopic composition of the aragonitic skeleton of coral reefs, such as Porites , can indicate changes in sea surface temperature and sea surface salinity conditions during
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3264-739: The Mesoamerican Barrier Reef System and the New Caledonian Barrier Reef . Barrier reefs are also found on the coasts of Providencia , Mayotte , the Gambier Islands , on the southeast coast of Kalimantan , on parts of the coast of Sulawesi , southeastern New Guinea and the south coast of the Louisiade Archipelago . Platform reefs, variously called bank or table reefs, can form on the continental shelf , as well as in
3366-592: The Middle Cambrian (513–501 Ma ), Devonian (416–359 Ma) and Carboniferous (359–299 Ma), owing to extinct order Rugosa corals, and Late Cretaceous (100–66 Ma) and Neogene (23 Ma–present), owing to order Scleractinia corals. Not all reefs in the past were formed by corals: those in the Early Cambrian (542–513 Ma) resulted from calcareous algae and archaeocyathids (small animals with conical shape, probably related to sponges ) and in
3468-480: The class Anthozoa in the animal phylum Cnidaria , which includes sea anemones and jellyfish . Unlike sea anemones, corals secrete hard carbonate exoskeletons that support and protect the coral. Most reefs grow best in warm, shallow, clear, sunny and agitated water. Coral reefs first appeared 485 million years ago, at the dawn of the Early Ordovician , displacing the microbial and sponge reefs of
3570-417: The tropics where the water is agitated, but can only live within a limited depth range, starting just below low tide . Where the level of the underlying earth allows, the corals grow around the coast to form fringing reefs, and can eventually grow to become a barrier reef. Where the bottom is rising, fringing reefs can grow around the coast, but coral raised above sea level dies. If the land subsides slowly,
3672-463: The 16th century. Kenneth McIntyre , for example, claimed that Houtman was in possession of Portuguese maps of the west coast of Australia, and that he named the islands "abrolhos" in accordance with the name on those maps. Charting of the islands was previously credited to Jorge de Menezes , but the notion that Menezes visited Australia is now thoroughly discredited, and no other candidate has been proffered. The primary piece of evidence used to support
3774-485: The Australian Bureau of Meteorology has not published climatic data for the Houtman Abrolhos. An automatic weather station has been installed on North Island since 1990, and hourly measures of precipitation , air temperature , wind speed , wind direction , relative humidity and atmospheric pressure have been publicly available since then. Based on this and other data, researchers have put together
3876-455: The Cape of Good Hope on 9 January 1694, remaining there until 5 February. She sailed from the Cape with a crew of around 300, and two passengers, including Admiral Sir James Couper. She never reached her destination, and was never heard from again. Contemporary rumours suggested that she had sprung her mast rounding the Cape, limped north, and was captured by pirates based at Fort Dauphin , near
3978-400: The Houtman Abrolhos contains a mix of tropical, temperate and west coast endemic fauna, resulting in unusual associations such as the occurrence of tropical corals in close association with temperate macro-algae. The proportions of tropical, temperate and west coast endemic biota vary from taxonomic group to group, but in general the biota is dominated by tropical species. This is attributable to
4080-550: The Houtman Abrolhos is located within the Southwest Shelf Transition , an Integrated Marine and Coastal Regionalisation of Australia (IMCRA) biotone that takes in the continental shelf from Perth to Geraldton . This province is defined as the area of shelf where tropical waters intergrade into temperate, thus supporting both tropical and temperate biota. In addition, this area contains the highest concentration of west coast endemics. Consistent with this,
4182-677: The Indian Ocean, for example, in the Maldives , the Chagos Islands , the Seychelles and around Cocos Island . The entire Maldives consist of 26 atolls. Coral reef ecosystems contain distinct zones that host different kinds of habitats. Usually, three major zones are recognized: the fore reef, reef crest, and the back reef (frequently referred to as the reef lagoon). The three zones are physically and ecologically interconnected. Reef life and oceanic processes create opportunities for
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4284-425: The Leeuwin Current, which bathes the islands in warm tropical water during winter months. A similar annual pattern occurs in salinity . There is a clear seasonal variation, with values ranging from a summer high of around 35.7 ppt , to a winter low of around 35.4 ppt. As with water temperatures, the variability in salinity is much smaller than in coastal waters, where summer salinity reaches 36.4 ppt. This difference
4386-599: The UV radiation and allowing them to better adapt to warmer water temperatures. In the event of UV or thermal damage, if and when repair occurs, it will increase the likelihood of survival of the host and symbiont. This leads to the idea that, evolutionarily, clade A is more UV resistant and thermally resistant than the other clades. Clades B and C are found more frequently in deeper water, which may explain their higher vulnerability to increased temperatures. Terrestrial plants that receive less sunlight because they are found in
4488-492: The Wallabi Group, consists of an island clump about 17 kilometres (11 mi) by 10 kilometres (6 mi), and also takes in the outlying North Island 14 kilometres (9 mi) to the north-west. The main islands of the Wallabi Group are North Island, West Wallabi Island , East Wallabi Island and Beacon Island . The group is best known for the shipwreck of Batavia on Morning Reef near Beacon Island in 1629, and
4590-481: The archipelago enough. A spokesperson for Western Australia's Department of Primary Industries and Regional Development said that their top priority is preservation of the wildlife on the islands. Aboriginal people visited the islands during the Holocene , as indicated by the discovery on Beacon Island of a flaked stone artefact made from Eocene fossiliferous chert . According to the surviving historical record,
4692-492: The capital of the Dutch East Indies . Pelsaert and some of his crew set off in an open boat to Batavia (now Jakarta ) to get help. A group of the crew who remained on some of the islets terrorised and killed many of the other crew and passengers, including women and children. When Pelsaert returned, a number of the culprits were executed. The Zeewijk was the last of the four Dutch East Indiamen to be wrecked on
4794-600: The centre of Western Australia's largest single-species fishery, the western rock lobster fishery. It has a small seasonal population of fishermen, and a limited number of tourists are permitted for day trips, but most of the land area is off-limits as a conservation habitat. It is the site of numerous shipwrecks , the most famous being two Dutch ships: Batavia , which was wrecked in 1629 (followed by massacre of over 100 survivors by mutineers), and Zeewijk , wrecked in 1727. The islands are an unincorporated area with no municipal government, subject to direct administration of
4896-457: The claim of Portuguese priority is the 16th century Dieppe maps , some of which are alleged to show the west coast of Australia, including an island at the position of the Houtman Abrolhos. That island is unlabelled on most of the Dieppe maps but, on Pierre Desceliers ' 1550 map, it is labelled Arenes . In 1895, George Collingridge suggested that Arenes was a corruption of Abrolhos , but that
4998-502: The coast. The fringing reefs of the Red Sea are "some of the best developed in the world" and occur along all its shores except off sandy bays. Barrier reefs are separated from a mainland or island shore by a deep channel or lagoon . They resemble the later stages of a fringing reef with its lagoon but differ from the latter mainly in size and origin. Their lagoons can be several kilometres wide and 30 to 70 metres deep. Above all,
5100-485: The coastal plains had become continental islands . As sea level rise continued, water topped most of the continental islands. The corals could then overgrow the hills, forming cays and reefs. Sea level on the Great Barrier Reef has not changed significantly in the last 6,000 years. The age of living reef structure is estimated to be between 6,000 and 8,000 years. Although the Great Barrier Reef formed along
5202-451: The coral animals. Coral that loses a large fraction of its zooxanthellae becomes white (or sometimes pastel shades in corals that are pigmented with their own proteins) and is said to be bleached , a condition which, unless corrected, can kill the coral. There are eight clades of Symbiodinium phylotypes . Most research has been conducted on clades A–D. Each clade contributes their own benefits as well as less compatible attributes to
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#17327809691925304-415: The corals shelter the zooxanthellae, averaging one million for every cubic centimetre of coral, and provide a constant supply of the carbon dioxide they need for photosynthesis. The varying pigments in different species of zooxanthellae give them an overall brown or golden-brown appearance and give brown corals their colors. Other pigments such as reds, blues, greens, etc. come from colored proteins made by
5406-529: The deep sea away from continental shelves , around oceanic islands and atolls . The majority of these islands are volcanic in origin. Others have tectonic origins where plate movements lifted the deep ocean floor. In The Structure and Distribution of Coral Reefs , Charles Darwin set out his theory of the formation of atoll reefs, an idea he conceived during the voyage of the Beagle . He theorized that uplift and subsidence of Earth's crust under
5508-637: The eastern tip of India ( Chennai ) to the Bangladesh and Myanmar borders —as well as along the coasts of northeastern South America and Bangladesh, due to the freshwater release from the Amazon and Ganges Rivers respectively. Significant coral reefs include: When alive, corals are colonies of small animals embedded in calcium carbonate shells. Coral heads consist of accumulations of individual animals called polyps , arranged in diverse shapes. Polyps are usually tiny, but they can range in size from
5610-479: The exchange of seawater , sediments , nutrients and marine life. Most coral reefs exist in waters less than 50 m deep. Some inhabit tropical continental shelves where cool, nutrient-rich upwelling does not occur, such as the Great Barrier Reef . Others are found in the deep ocean surrounding islands or as atolls, such as in the Maldives . The reefs surrounding islands form when islands subside into
5712-418: The extremely low levels of nutrients in the water. Sea temperature at the islands varies according to a diurnal cycle, with the water at its coldest between six and eight in the morning, and at its warmest between three and four in the afternoon. In summer the daily temperature range is around 1 °C (2 °F); in winter it is about half that. There is also an annual cycle, with sea temperature varying by
5814-554: The first sighting of the Houtman Abrolhos by Europeans was in 1619, sailing the Dutch VOC ships Dordrecht and Amsterdam , only three years after Dirk Hartog made the first authenticated sighting of what is now Western Australia, and only 13 years after the first authenticated European voyage to Australia by the Duyfken , in 1606. Discovery of the islands was credited to Frederick de Houtman , Captain-General of Dordrecht , as it
5916-413: The first-known Dutch-Malay and Dutch-Malagasy dictionaries in 1603. He appears to have been fluent-enough in Portuguese that he might have used an evocative Portuguese word if it described the area better than any Dutch word did. It has been argued by proponents of the theory of Portuguese discovery of Australia that the Portuguese name is evidence that the islands were charted by Portuguese navigators in
6018-401: The floor often supports seagrass meadows which are important foraging areas for reef fish. The reef drop-off is, for its first 50 m, habitat for reef fish who find shelter on the cliff face and plankton in the water nearby. The drop-off zone applies mainly to the reefs surrounding oceanic islands and atolls. The reef face is the zone above the reef floor or the reef drop-off. This zone
6120-403: The fringing reef, barrier reef, atoll and table reef based on Stoddart, D.R. (1969). Spalding et al. list four main reef types that can be clearly illustrated – the fringing reef, barrier reef, atoll, and "bank or platform reef"—and notes that many other structures exist which do not conform easily to strict definitions, including the "patch reef". A fringing reef, also called a shore reef,
6222-463: The fringing reefs keep pace by growing upwards on a base of older, dead coral, forming a barrier reef enclosing a lagoon between the reef and the land. A barrier reef can encircle an island, and once the island sinks below sea level a roughly circular atoll of growing coral continues to keep up with the sea level, forming a central lagoon. Barrier reefs and atolls do not usually form complete circles but are broken in places by storms. Like sea level rise ,
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#17327809691926324-419: The growth of the coral. This technique is often used by climate scientists to infer a region's paleoclimate . Since Darwin's identification of the three classical reef formations – the fringing reef around a volcanic island becoming a barrier reef and then an atoll – scientists have identified further reef types. While some sources find only three, Thomas lists "Four major forms of large-scale coral reefs" –
6426-548: The history of the change in sea level relative to that substrate. The approximately 20,000-year-old Great Barrier Reef offers an example of how coral reefs formed on continental shelves. Sea level was then 120 m (390 ft) lower than in the 21st century. As sea level rose, the water and the corals encroached on what had been hills of the Australian coastal plain. By 13,000 years ago, sea level had risen to 60 m (200 ft) lower than at present, and many hills of
6528-666: The island erodes away and sinks below sea level. Atolls may also be formed by the sinking of the seabed or rising of the sea level. A ring of reefs results, which enclose a lagoon. Atolls are numerous in the South Pacific, where they usually occur in mid-ocean, for example, in the Caroline Islands , the Cook Islands , French Polynesia , the Marshall Islands and Micronesia . Atolls are found in
6630-562: The islands, including two boys who were accused of sodomy and marooned on separate islands of the Mangrove Group. On 10 July the longboat, fitted for a voyage and crewed with eleven men, was sent to Batavia to obtain help. It never arrived there, and nothing is known of its fate. Four months later the castaways began building a 18-metre (60 ft) boat, sufficient to carry all the men and the money chests. Completed in March 1728 and affectionately named Sloepie ("little sloop "), it
6732-415: The label Fr. Houtman's abrolhos . On a map produced by Gerritsz the following year, they are labelled Houtman's Abrolhos . On British Admiralty charts, the islands are labelled Houtman's Rocks . In 1629, some of the islands were the scene of an infamous shipwreck and mutiny. The Dutch ship Batavia , under the command of Francisco Pelsaert , ran aground on her maiden voyage to the port of Batavia ,
6834-563: The largest island in the Pelsart Group. The commander, John Clements Wickham , named the place Gun Island and the passage between the Easter and Pelsart Groups, Zeewijk Channel. Later during the 19th century many islets were used by men collecting guano . A lease to mine rock phosphate on the islands was obtained by Charles Broadhurst in 1884. After surveying the area he constructed a plant, stone jetty and tramways on Rat Island,
6936-529: The location of the Houtman Abrolhos at the northern limit of the Southwest Shelf Transition, together with the warming influence of the Leeuwin Current. Under IMCRA, the Southwest Shelf Transition divides into two meso-scale bioregions. One is named Abrolhos Islands , and covers the shelf waters surrounding the Houtman Abrolhos, with an area of 6,645 square kilometres (2,566 sq mi). The other bioregion, Central West Coast , covers
7038-418: The main reef, containing chunks of coral. This zone may border a lagoon and serve as a protective area, or it may lie between the reef and the shore, and in this case is a flat, rocky area. Fish tend to prefer it when it is present. The reef lagoon is an entirely enclosed region, which creates an area less affected by wave action and often contains small reef patches. However, the topography of coral reefs
7140-484: The mainland. Unlike fringing and barrier reefs which extend only seaward, platform reefs grow in all directions. They are variable in size, ranging from a few hundred metres to many kilometres across. Their usual shape is oval to elongated. Parts of these reefs can reach the surface and form sandbanks and small islands around which may form fringing reefs. A lagoon may form In the middle of a platform reef. Platform reefs are typically situated within atolls, where they adopt
7242-520: The mainland: the summer temperature is typically a degree cooler, while winter temperatures are a good deal warmer. This is due to the influence of the ocean, and to the Leeuwin Current. 86% of the rain falls between April and September; on average there are 89 raindays, resulting in 469 millimetres (18.5 in) of rain. The wettest month is June, when over 100 millimetres (3.9 in) typically falls. In contrast, only about 70 millimetres (2.8 in) can be expected to fall between October and March. It
7344-497: The men landed ten chests of money, containing 315,836 guilders and weighing a total of three tonnes (6,600 lb). The crew of Zeewijk were stranded in the Pelsaert Group for ten months, during which time they lived off seals, seabirds, eggs, and victuals salvaged from the wreck. They obtained some water from rainfall, but were forced to explore throughout the group in search of further supplies. A great many men died on
7446-414: The most frequent periods of calm. In addition to these winds, there is daily pattern of land breezes in the morning, followed by the onset of south-westerly sea breezes in the afternoon. This pattern is caused by temperature differences between the land and the ocean, and is not as strong in the Houtman Abrolhos as on the mainland, but is present nonetheless. Three classes of storm have been identified for
7548-400: The name "patch reefs" and often span a diameter of just a few dozen meters. In instances where platform reefs develop along elongated structures, such as old and weathered barrier reefs, they tend to arrange themselves in a linear formation. This is the case, for example, on the east coast of the Red Sea near Jeddah . In old platform reefs, the inner part can be so heavily eroded that it forms
7650-528: The name implies, coral reefs are made up of coral skeletons from mostly intact coral colonies. As other chemical elements present in corals become incorporated into the calcium carbonate deposits, aragonite is formed. However, shell fragments and the remains of coralline algae such as the green-segmented genus Halimeda can add to the reef's ability to withstand damage from storms and other threats. Such mixtures are visible in structures such as Eniwetok Atoll . The times of maximum reef development were in
7752-757: The northwest initially, then gradually moving around to southerly. Under the Interim Biogeographic Regionalisation for Australia (IBRA), the islands of the Houtman Abrolhos fall within the Geraldton Hills subregion of the Geraldton Sandplains region. The main biogeographic significance of the islands is their isolation, allowing them to provide refugia for such threatened fauna as tammar wallabies ( Macropus eugenii ), Australian sea lions ( Neophoca cinerea ) and rare breeding seabirds. In marine terms,
7854-415: The ocean, and atolls form when an island subsides below the surface of the sea. Alternatively, Moyle and Cech distinguish six zones, though most reefs possess only some of the zones. The reef surface is the shallowest part of the reef. It is subject to surge and tides . When waves pass over shallow areas, they shoal , as shown in the adjacent diagram. This means the water is often agitated. These are
7956-528: The oceans formed the atolls. Darwin set out a sequence of three stages in atoll formation. A fringing reef forms around an extinct volcanic island as the island and ocean floor subside. As the subsidence continues, the fringing reef becomes a barrier reef and ultimately an atoll reef. Darwin predicted that underneath each lagoon would be a bedrock base, the remains of the original volcano. Subsequent research supported this hypothesis. Darwin's theory followed from his understanding that coral polyps thrive in
8058-535: The oceans' surface area. The Indo-Pacific region (including the Red Sea , Indian Ocean , Southeast Asia and the Pacific ) account for 91.9% of this total. Southeast Asia accounts for 32.3% of that figure, while the Pacific including Australia accounts for 40.8%. Atlantic and Caribbean coral reefs account for 7.6%. Although corals exist both in temperate and tropical waters, shallow-water reefs form only in
8160-490: The offshore outer reef edge formed in open water rather than next to a shoreline. Like an atoll, it is thought that these reefs are formed either as the seabed lowered or sea level rose. Formation takes considerably longer than for a fringing reef, thus barrier reefs are much rarer. The best known and largest example of a barrier reef is the Australian Great Barrier Reef . Other major examples are
8262-451: The open ocean, in fact anywhere where the seabed rises close enough to the surface of the ocean to enable the growth of zooxanthemic, reef-forming corals. Platform reefs are found in the southern Great Barrier Reef, the Swain and Capricorn Group on the continental shelf, about 100–200 km from the coast. Some platform reefs of the northern Mascarenes are several thousand kilometres from
8364-429: The polyps' tissues and provide organic nutrients that nourish the polyp in the form of glucose , glycerol and amino acids . Because of this relationship, coral reefs grow much faster in clear water, which admits more sunlight. Without their symbionts, coral growth would be too slow to form significant reef structures. Corals get up to 90% of their nutrients from their symbionts. In return, as an example of mutualism ,
8466-420: The precise condition under which corals flourish. The light is sufficient for photosynthesis by the symbiotic zooxanthellae, and agitated water brings plankton to feed the coral. The off-reef floor is the shallow sea floor surrounding a reef. This zone occurs next to reefs on continental shelves. Reefs around tropical islands and atolls drop abruptly to great depths and do not have such a floor. Usually sandy,
8568-403: The region. Brief squalls may occur between December and April. A tropical cyclone occurs in the area about once in three years, between January and April; these may generate extremely high wind speeds that are potentially destructive. During winter, extra-tropical cyclones sometimes pass south of Geraldton, generating winter gales with gusts of up to 35 metres per second, the wind direction from
8670-559: The remaining area. The vegetation of the Houtman Abrolhos islands is dominated by chenopod shrubs. The flora is generally the same as the coastal flora of the adjacent mainland, with the exception of the islands' mangrove, saltbush and salt lake vegetation. Coral reef A coral reef is an underwater ecosystem characterized by reef-building corals . Reefs are formed of colonies of coral polyps held together by calcium carbonate . Most coral reefs are built from stony corals , whose polyps cluster in groups. Coral belongs to
8772-412: The sea bed begins to drop steeply. The surface of the fringe reef generally remains at the same height: just below the waterline. In older fringing reefs, whose outer regions pushed far out into the sea, the inner part is deepened by erosion and eventually forms a lagoon . Fringing reef lagoons can become over 100 metres wide and several metres deep. Like the fringing reef itself, they run parallel to
8874-609: The shallow water tropical coral reefs are best known, there are also deeper water reef-forming corals, which live in colder water and in temperate seas. Most coral reefs were formed after the Last Glacial Period when melting ice caused sea level to rise and flood continental shelves . Most coral reefs are less than 10,000 years old. As communities established themselves, the reefs grew upwards, pacing rising sea levels . Reefs that rose too slowly could become drowned, without sufficient light. Coral reefs are also found in
8976-409: The shelf is quite shallow. It is also fairly level, the depth increasing to the west at a modest gradient of about 1.3 metres per kilometre (7 ft/mi). About five kilometres (3 mi) to the west of the Houtman Abrolhos lies the shelf break , beyond which the seabed falls away much more steeply, averaging around 50 metres per kilometre (260 ft/mi). The islands of each island group arise from
9078-529: The southeast, across Zeewijk Channel , lies the Pelsaert Group, the most southerly true coral reef in the Indian Ocean. The main islands of this group are Middle Island , Square Island , Long Island , Pelsaert Island , Basile Island and the Mangrove Group . A great many ships have been wrecked in the Pelsaert Group, most notably Zeewijk , which was wrecked on the Half Moon Reef in 1727,
9180-443: The southeastern corner of Madagascar . However, Abraham Samuel , the pirate supposedly responsible, did not arrive in the area until 1697. In 1697, Willem de Vlamingh was sent with three ships to search for Ridderschap van Holland at Île Saint-Paul and Île Amsterdam , and then along the west coast of Australia. Nothing was found. Two years later, two ships made investigations while visiting Madagascar, but without success. It
9282-445: The subsequent mutiny and massacre that took place among the survivors. The Easter Group lies to the southeast of the Wallabi Group, from which it is separated by a 9-kilometre-wide (5.6 mi) channel named Middle Channel . It is about 20 by 12 kilometres (12.4 mi × 7.5 mi), and consists of a number of islands, including Rat Island , Wooded Island , Morley Island , Suomi Island and Alexander Island . Further to
9384-424: The survival of their coral hosts. Each photosynthetic organism has a specific level of sensitivity to photodamage to compounds needed for survival, such as proteins. Rates of regeneration and replication determine the organism's ability to survive. Phylotype A is found more in the shallow waters. It is able to produce mycosporine-like amino acids that are UV resistant , using a derivative of glycerin to absorb
9486-546: The survivors staying on Gun Island for some time afterwards. Other wrecks include Ocean Queen , wrecked on the Half Moon Reef in 1842; Ben Ledi , wrecked off Pelsaert Island in 1879; Windsor , wrecked on the Half Moon Reef in 1908.; and possibly Ridderschap van Holland in 1694 and Aagtekerke in 1727. The islands are a part of Western Australia. They are part of the Electoral district of Geraldton , and
9588-590: The time. John Forsyth states that the islands are named after the Abrolhos Archipelago off the east coast of Brazil , which was discovered and named by Portuguese navigators early in the 16th century. That position is supported by the fact that Houtman was familiar with the Abrolhos Archipelago, having sailed through it in 1598. Others assert that abrolhos was a Portuguese lookout 's cry which, like many other Portuguese maritime terms,
9690-485: The undergrowth are analogous to clades B, C, and D. Since clades B through D are found at deeper depths, they require an elevated light absorption rate to be able to synthesize as much energy. With elevated absorption rates at UV wavelengths, these phylotypes are more prone to coral bleaching versus the shallow clade A. Ridderschap van Holland (1681) Ridderschap van Holland ( Dutch pronunciation: [ˈrɪdərsxɑp fɑn ˈɦɔlɑnt] ; 'Knighthood of Holland')
9792-526: The west coast of Australia. Due to the Ship's Council having made the inexplicable decision to disregard sailing orders and actually seek out the west coast of Australia, the ship ran onto the Half Moon Reef at about 7:30 pm on 9 June 1727. It did not break up immediately, and the heavy swell made evacuation impossible until 18 June. The longboat was launched on that day, and the crew and stores were thereafter gradually transferred to nearby Gun Island . Later,
9894-513: Was Houtman who later wrote about the discovery in a letter to the directors of the Dutch East India Company : On the 29th do. deeming ourselves to be in an open sea, we shaped our course north-by-east. At noon we were in 29° 32' S. Lat.; at night about three hours before daybreak, we again unexpectedly came upon a low-lying coast, a level, broken country with reefs all round it. We saw no high land or mainland, so that this shoal
9996-487: Was a large retourschip ('return ship'), the largest class of merchantmen built by the Dutch East India Company ( Dutch : Vereenigde Oostindische Compagnie , commonly abbreviated to VOC) to trade with the East Indies . In 1694 the ship sailed for Batavia (now Jakarta , Indonesia ) on her fifth voyage, but was never heard from again. She is now thought to have been shipwrecked off the west coast of Australia. She
10098-572: Was built in Amsterdam , Dutch Republic in 1682 by the VOC, and registered at Vlissingen . She was 50 metres (164 ft) long and 12 metres (39.5 ft) wide, with a gross tonnage of about 520 tons. Ridderschap van Holland completed four voyages before her final voyage. On 11 July 1693, Ridderschap van Holland departed Wielingen on a voyage to Batavia under Captain Dirk de Lange. She arrived at
10200-583: Was mocked by J. E. Heeres in 1898, and, according to J. S. Battye , "this suggestion can scarcely be regarded seriously. It certainly does not in any way add to the merit of the Portuguese claim". Setting aside the Portuguese claims, the Houtman Abrolhos first appear on a map in 1622, on a little-known portolan by Hessel Gerritsz . They are unlabelled, marked merely as a group of small circles. They are first named in print in Gerritsz' 1627 map Caert van't Landt van d'Eendracht , where they bear
10302-472: Was taken up by sailors of other nationalities, becoming, by Houtman's time, a Dutch loan word for offshore reefs. Additionally, Frederick De Houtman had at least some grasp of Portuguese, having been sent by Amsterdam merchants to Lisbon from 1592 – 1594 with his brother Cornelis to learn about the Portuguese route to the Indies. Frederick also appears to have been a keen linguist, having published
10404-493: Was the first ocean-going vessel built in Australian history. On 26 March 1728, the surviving men set sail for Batavia, arriving in Sunda Strait late the following month. During Admiralty surveys of the north west coast in 1840, crew from HMS Beagle discovered a brass gun of about three pounds calibre, an iron swivel on which paint still adhered as well as numerous other artefacts signifying European occupation, on
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