The Houtman Abrolhos (often informally called the Abrolhos Islands) is a chain of 122 islands, and associated coral reefs, in the Indian Ocean off the west coast of Australia. Nominally located at , it lies about eighty 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 is the centre of Western Australia's largest single species fishery, the Western Rock Lobster fishery. It is also well known as the site of numerous shipwrecks, the most famous being the Dutch ships Batavia, which was wrecked in 1629, and Zeewijk, wrecked in 1727.
The most northerly group, the Wallabi Group, consists of an island clump about 17 kilometres (10 mi) by 10 kilometres (6 mi), and also takes in the outlying North Island, located 14 kilometres to the northwest. 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 the Batavia on Morning Reef near Beacon Island in 1629, and the subsequent mutiny and massacre that took place among the marooned survivors.
The Easter Group lies to the southeast of the Wallabi Group, from which it is separated by a 9 kilometres (5 mi) wide channel named Middle Channel. It is about 20 kilometres by 12 kilometres, and consists of a number of islands including Rat Island, Wooded Island, Morley Island, Suomi Island and Alexander Island.
Further to 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 the Zeewijk, which was wrecked on the Half Moon Reef in 1727, the survivors staying on Gun Island for some time afterwards. Other wrecks include the Ocean Queen, wrecked on the Half Moon Reef in 1842; the Ben Ledi, wrecked off Pelsaert Island in 1879; and the Windsor, wrecked on the Half Moon Reef in 1908.
According to the surviving historical record, the first sighting of the Houtman Abrolhos was by the Dutch VOC ships Dordrecht and Amsterdam in 1619, 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 voyage to Australia, that of the Duyfken in 1606. Discovery of the islands was credited to Frederick de Houtman, Captain-General of the Dordrecht, as it was Houtman who later wrote of the discovery in a letter to the directors of the Dutch East India Company:
The word abrolhos is of Portuguese origin, making the Houtman Abrolhos one of only two Australian places with a Portuguese name, the other being Pedra Branca in Tasmania. The term is commonly reported to be a contraction of the Portuguese expression abro olhos ("open the eyes") or abri vossos olhos ("keep your eyes open"), but this has been shown to be a false etymology. In the Portuguese language of the time, abrolhos meant "spiked obstructions", and served not only as a word for offshore reefs, but also as their word for caltrops and chevaux de frise. To Spanish ears, however, the word sounded like abre ojos ("open your eyes"), so they imputed this meaning, and this false etymology was later borrowed by English, Dutch and French writers.
Another explanation has been proferred by Philippe Godard. He reports, without citation, that some authors have claimed that the word abrolhos comes from a 15th century Portuguese nobleman named Frederico de Abrolhos. Godard notes, however, that there is no trace of such a person in Lusitanian archives, and that Abrolhos is not known as a family name in Portugal, making this an unconvincing explanation.
Why Houtman named the islands using a Portuguese word remains a subject of debate. 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. This 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, was taken up by sailors of other nationalities, becoming by Houtman's time a Dutch loan word for offshore reefs. Finally, 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 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 his map. 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 for the claim of Portuguese priority is the 16th century Dieppe maps, some of which are said to show the west coast of Australia, including an island at the position of the Houtman Abrolhos. This 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 this was mocked by 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 appears on a published map in 1622, on a little-known portolan by Hessel Gerritsz. They are unlabelled, however, being 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 the label Fr. Houtman's abrolhos. On a map produced by him the following year, they are labelled Houtman's Abrolhos.
On British Admiralty charts, the islands are labelled Houtman's Rocks.
In 1629 and 1630 some of the islands were the scene of a spectacular shipwreck and mutiny. The Dutch ship Batavia under the command of Francisco Pelsaert ran aground and Pelsaert and some men went in an open boat to the town of Batavia (now Jakarta) in order to get help. A group of the men who stayed on some of the islets decided to terrorise and massacre many others, including women. When Pelsaert came back many culprits were executed.
The crew of the Zeewijk would be marooned 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 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 boat, sufficient to carry all the men and the money chests. Completed in March 1728 and affectionately named the Sloepie ("little sloop"), it 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 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.
Management is vested in the Department of Fisheries.
The islands of each island group arise from a single carbonate platform, so the waters within an island group are mostly shallow. The channels between groups, however, are 40 to 50 metres deep, and therefore no impediment to the exchange of offshore and inshore waters.
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 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 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 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 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 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.
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 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, with a calmer period in March and April, and another in October and November.
The islands have a Mediterranean climate, with warm dry summers and cooler, wet winters. Mean temperatures range from 9.3°C to 19.5°C in July, and from 19.1°C to 32.4°C in February. These temperatures have a substantially smaller range than on 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 of rain. The wettest month is June, when over 100 millimetres typically falls. In contrast, only about 70 millimetres can be expected to fall between October and March.
It 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 17 knots 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 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 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 the northwest initially, then gradually moving around to southerly.
In marine terms, 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, 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 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. The other bioregion, Central West Coast, covers the remaining area.
The vascular flora of the Houtman Abrolhos has been thoroughly surveyed, and species lists have been published for 119 islands. As of 2001, these lists totaled 239 species from 68 families. A further six species have been collected in the Houtman Abrolhos, but cannot be allocated to islands because insufficient location information was recorded. There have also been collections of mosses, liverworts and lichens, but no information has been published on these non-vascular groups.
The islands with the greatest floristic diversity are East and West Wallabi Islands, with 124 and 97 species respectively. 54 species occur in all three island groups. The most widely distributed species are Nitraria billardierei (Nitre Bush), which has been recorded on 106 islands; the exotic Mesembryanthemum crystallinum (Iceplant), on 88 islands; Threlkeldia diffusa (Coast Bonefruit), on 72 islands; and Atriplex cinerea (Grey Saltbush), on 70 islands. On the other hand, Eucalyptus oraria (Ooragmandee) and Acacia didyma occur only on East Wallabi Island.
As of 2001, 5 species of priority flora occurred on the islands. One, Acacia didyma, is no longer considered a priority species. The remaining priority species are Chthonocephalus tomentellus, which is rated Priority Two under the Department of Environment and Conservation's Declared Rare and Priority Flora List; Calocephalus aervoides and Galium migrans, both Priority Three; and Lepidium puberulum, Priority Four.
95 exotic species from 29 families have been recorded. In general, islands that have or had human settlements are the weediest. Of greatest concern is the noxious weed Lycium ferocissimum (African Boxthorn), which has long spines that can trap birds. This weed was recorded on the islands as early as 1970. Efforts to eradicate it began in 1990,; there was a lull in eradication in the late 1990s, but the program was later reinstated, and in July 2007, the Department of Environment and Conservation reported that the species had been eradicated from 14 of the 18 islands on which it had been recorded. Other noxious weeds include Opuntia stricta (Prickly Pear), Verbesina encelioides and Echium plantagineum (Paterson's Curse).
The Houtman Abrolhos is home to around 100 species of bird; for a complete list, see list of birds of the Houtman Abrolhos. Six species are land birds, and three are shore birds. The remainder, the vast majority, are seabirds. Most seabird species have a tropical distribution, but some occur in both tropical and warm-temperate seas, and a small number are warm-temperate only.
When numbers of individuals are taken into account, the tropical birds overwhelmingly dominate. The islands are one of the most important breeding sites for tropical seabirds in Australia. They contain by far the largest colonies of Wedge-tailed Shearwater in the eastern Indian Ocean, with over a million breeding pairs recorded there in 1994. They also contain Western Australia's only breeding colonies of the Lesser Noddy, and the largest colonies in Western Australia of the Little Shearwater, White-faced Storm Petrel, Common Noddy, Caspian Tern, Crested Tern, Roseate Tern and Fairy Tern. In addition, they contain important breeding areas for the Eastern Reef Heron, Pacific Gull, Bridled Tern, White-bellied Sea Eagle and Osprey.
There are two subspecies of bird endemic to the islands. The Abrolhos Painted Button-quail occurs only on five islands in the Wallabi Group, and is protected as rare under the Wildlife Conservation Act 1950. Also gazetted as rare, the Australasian subspecies of the Lesser Noddy, Anous tenuirostris melanops, breeds only on Wooded Island, Morley Island and Pelsaert Island.
Only two species of land mammal are indigenous to the Houtman Abrolhos, the Tammar Wallaby (Macropus eugenii) and the Bush Rat (Rattus fuscipes). Both are native only to West and East Wallabi Islands, although R. fuscipes has not been collected on East Wallabi Island since August 1967, and is probably extinct there. The Tammar Wallabi was seen on West Wallabi Island by survivors of the 1628 Batavia shipwreck, and recorded by Francisco Pelsart in his 1629 Ongeluckige Voyagie. This represents the first recorded sighting of a macropod by Europeans, and probably also the first sighting of an Australian mammal. Tammar Wallabies were introduced to North Island from East Wallabi Island by fishermen, probably in the 1950s, but failed to establish. In 1987 they were reintroduced again, this time successfully. By the 2000s, there were over 400 wallabies on the island, resulting in overgrazing of native vegetation and increased erosion. Research into the effectiveness of controlling population levels by the use of implanted contraceptives was begun in 2005, but in July 2007 the research was discontinued and the population culled instead.
Two introduced mammals are established on the islands. The Domestic Cat (Felis silvestris catus) was introduced to Rat Island around 1900, and the House Mouse (Mus musculus) was introduced onto North Island in the 1970s, presumably with food. In 1995 the House Mouse was reported as also present on Rat Island for many years before 1987, but a recent report makes no mention of this. In addition, three introduced mammals were previously established in the Houtman Abrolhos, but have since been eradicated. The Black Rat (Rattus rattus) was established on Pigeon and Rat Islands, but has been eradicated by poisoning. The European Rabbit (Oryctolagus cuniculus) has been introduced at various times onto Leo Island, Middle Island, Morley Island, Pelsaert Island and Wooded Island. In the case of Pelsaert Island, it is not clear whether it ever established; in all other cases, established populations have been eradicated by poisoning. The Domestic Goat (Capra aegagrus hircus) is also reported to have been present on East Wallabi Island, but is no longer.
23 terrestrial reptile species are known to occur on the islands of the Houtman Abrolhos. This relatively low biodiversity is apparently due to the homogeneity of habitat on the islands, which provide few distinct ecological niches. The most significant terrestrial reptile species are the Spiny-tailed Skink (Egernia stokesii stokesii) and the Carpet Python (Morelia spilota imbricata), both of which are listed as rare and therefore afforded special protection under the Wildlife Conservation Act 1950; and the Abrolhos Dwarf Bearded Dragon (Pogona minor minima), a Houtman Abrolhos endemic that is listed as a Priority 4 species by the Department of Environment and Conservation. For a full list of species, see list of reptiles of the Houtman Abrolhos.
At last count, a total of 389 species of fish have been recorded from the Houtman Abrolhos. 16 species occur in very large numbers; in decreasing order of abundance, these are:
Commercially important species include Pagrus auratus (Pink Snapper), Choerodon rubescens (Baldchin Groper), Glaucosoma hebraicum (Westralian Dhufish) and Plectropomus leopardus (Coral Trout). For a complete list of fish species recorded at the Houtman Abrolhos, see list of fishes of the Houtman Abrolhos.
About two thirds of the total number of species are tropical in distribution, the remainder being subtropical or warm-temperate. This ratio also holds for the most abundant species, eleven of the sixteen species being tropical. On the other hand, over 70% of tropical species occur in extremely low numbers, so low in fact that they are thought not to maintain breeding populations at the Abrolhos; rather, populations are maintained by larvae carried to the islands by the Leeuwin Current from populations further north.
Sea lions come ashore to rest on leeward beaches throughout the island chain, but only a small number of these "haulout sites" are used for breeding. Breeding has been observed on Serventy Island, Gilbert Island, Alexander Island, Suomi Island, Keru Island, Square Island, Stick Island, Gibson Island, Gun Island, Morley Island and Wooded Island. All but the last three of these are considered current breeding sites, and are therefore considered by the Department of Fisheries to have a high conservation value.
Little information is available on other marine mammals at the Abrolhos, as no direct research on this subject has been undertaken. Sightings of the Humpback Whale (Megaptera novaeangliae) are common between April and October, when the whales are migrating. Other marine mammals sometimes sighted at the islands include Pygmy Bryde's Whale (Balaenoptera edeni), Bottlenose Dolphin (Tursiops aduncus) and Striped Dolphin (Stenella coeruleoalba).
The Houtman Abrolhos is unusual in having a luxuriant and diverse living coral reef at such a high latitude. 194 species in 50 genera have been recorded there, all but two of which are tropical. This is a surprisingly high coral diversity, considering the high latitude of the reef, and the relatively low diversity of other biota. For a full list, see list of corals of the Houtman Abrolhos.
The coral reef community at the Houtman Abrolhos is unusual in having tropical coral growing alongside and in direct competition with, temperate seaweed. As a result of this competition for light, space and nutrients, coral at the Houtman Abrolhos tends to grow more slowly and die younger than is usual. Reef production is to a large extent due to the production of carbon by coralline algae rather than by coral.
The most notable species of crustacean at the Houtman Abrolhos is the Western Rock Lobster (Panulirus cygnus). 44 species of crab and 9 species of amphipod were recorded there by the Percy Sladen Trust Expedition of 1916.
The Southern Saucer Scallop (Amusium balloti) is the only commercially important species. This occurs in sheltered areas of medium-fine sand in deep water to the north-east of the reefs; it is usually the dominant species there.
The marine worms identified at the Houtman Abrolhos include 22 species of the polychaete family Terebellidae, and 16 species of the family oligochaete family Tubificidae. For a list of species, see list of worms of the Houtman Abrolhos.
A total of 38 hydroid species have been collected at the Houtman Abrolhos. 34 of these are leptothecates, the remainder being anthoathecates. 92% of the species attach to temperate algae, the others to coral rubble. For a list of species, see list of hydroids of the Houtman Abrolhos.
Scallop fishers mainly operate east of the Houtman Abrolhos and between the island groups, in waters deeper than 30 metres. Activity is targeted at sheltered areas of bare sand, where scallops tend to settle. Catches vary greatly from year to year; from 2001 to 2003, for example, the total annual catch totalled 1182 tons, 195 tons and 5840 tons (whole weight) respectively. This variability is apparently related to the strength of the Leeuwin Current, as strong current is correlated with low scallop recruitment.
The total value of the fishery in 2003 was AU$19.6 million, although this figure includes a small prawn fishery operating out of Port Gregory. Most of the catch is frozen and exported to Asia.
95% of the pearl aquaculture is carried out in the Pelsaert Group. Most licenses are over areas of sand, but some areas contain small amounts of coral reef. The colour of the pearls produced is quite different to that of Pacific black pearls, and this is considered a potential marketing tool.
In addition to pearl aquaculture, a pilot sea cage finfish farm was licensed in 2004, although as of 2007 the license had not been exercised. Interest has also been expressed in the culture of live rock and coral for the aquarium industry. The Department of Fisheries has identified a number of species as having potential for aquaculture in the Abrolhos, including the Shark Bay pearl oyster (Pinctata albina), the maxima clam (Tridacna maxima), rock oysters (Saccostrea sp.), the saucer scallop (Amusium balloti), the western rock lobster, and a number of species of finfish, most of which are filter feeders.
Nowadays, many of the islets are used by fishermen. Although an important tourist destination, the tourists are not allowed to stay overnight.
Other shipwrecks, notably the Zeewijk, have also become the subject of books and other works. Shipwrecks aside, however, cultural references to the Houtman Abrolhos are rare. By far the best known book on the Houtman Abrolhos itself is Malcolm Uren's Sailormen's ghosts: The Abrolhos islands in three hundred years of romance, history, and adventure. First published in 1940, this book saw numerous editions published in the 1940s, and was even republished in 1980 as a "West Australian classic". In it, Uren tells both the history of the islands and the story of his own visit to the islands.
Other books include William Bede Christie's 1909 Christmas on the briny: the innocents abroad, or, a holiday trip to the Abrolhos islands, and Alison Louise Wright's 1998 Abrolhos Islands Conversations. The latter, a book of interviews and portraits of the people of the Abrolhos, won the Special Award in the 1999 Western Australian Premier's Book Awards.
The islands featured in the first episode of Surfing the Menu, an eight-part food and travel series produced for the ABC in 2003., and the following year were featured on Getaway, Australia's longest-running and most popular holiday and travel television programme. They were the subject of a motion picture entitled Eye opener, published by The Film Centre WA in 1981, and of a piece of classical music entitled Abrolhos: A ceremonial overture, written by William Stewart in 1988 under commission to the Geraldton Town Council.