Houtman Abrolhos

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Houtman Abrolhos
Houtman Abrolhos is located in Indian Ocean
Houtman Abrolhos
Houtman Abrolhos
Geography
LocationIndian Ocean
Coordinates28°43′S 113°47′E / 28.717°S 113.783°E / -28.717; 113.783Coordinates: 28°43′S 113°47′E / 28.717°S 113.783°E / -28.717; 113.783
ArchipelagoHoutman Abrolhos
Total islands122
Area16.4 km2 (6.3 sq mi)
Administration
Australia
StateWestern Australia
LGACity of Greater Geraldton[1]
Demographics
Population6[2] (2016 census)
Additional information
Southernmost true coral reef in the Indian Ocean
Aerial photograph of Rat Island
(Easter Group)

The Houtman Abrolhos (often called the Abrolhos Islands) is a chain of 122 islands and associated

Batavia, which was wrecked in 1629, and Zeewijk
, wrecked in 1727.

The islands are an

unincorporated area with no municipal government, subject to direct administration of the Government of Western Australia. In July 2019, the Houtman Abrolhos was declared a national park
by the state government.

Geography

The Houtman Abrolhos archipelago is located in the Indian Ocean off the west coast of Australia, about 80 km (50 mi) west of Geraldton in the state of Western Australia. It comprises 122 Islands and associated coral reefs,[3] made up of three island groups, the Wallabi Group, Easter Group and Pelsaert Group.

The most northerly group, 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, located 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.[4] The group is best known for the shipwreck of Batavia on Morning Reef near Beacon Island in 1629,[5] and the subsequent mutiny and massacre that took place among the marooned survivors.[6]

The Easter Group lies to the south-east of the Wallabi Group, from which it is separated by a 9-kilometre (5.6 mi)-wide channel named Middle Channel. It is about 20 kilometres (12 mi) by 12 kilometres (7.5 mi), and consists of a number of islands, including Rat Island, Wooded Island, Morley Island, Suomi Island and Alexander Island.

Further to the south-east, across

Zeewijk, which was wrecked on the Half Moon Reef in 1727, the survivors staying on Gun Island for some time afterwards.[7] Other wrecks include Ocean Queen, wrecked on the Half Moon Reef in 1842; Ben Ledi, wrecked off Pelsaert Island in 1879; and Windsor, wrecked on the Half Moon Reef in 1908.[8]

Governance

The islands are an

unincorporated area with no municipal government, subject to direct administration of the Government of Western Australia.[citation needed
]

In July 2019, the Houtman Abrolhos was declared a

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 the archipelago enough. A spokesperson for WA's Department of Primary Industries and Regional Development said that their top priority is preservation of the wildlife on the islands.[10][11][12]

History

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.[13]

Discovery by Europeans

According to the surviving historical record, the first sighting of the Houtman Abrolhos by Europeans was in 1619, by 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 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 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."[14]

The word

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 the time. John Forsyth states that the islands are named after the

loan word for offshore reefs.[6] 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 the first-known Dutch-Malay and Dutch-Malagasy dictionaries in 1603.[18]
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 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.[15] 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.[citation needed
]

The primary piece of evidence used to support the claim of Portuguese priority is the 16th century

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".[21]

Setting aside the Portuguese claims, the Houtman Abrolhos first appear on a map in 1622, on a little-known

portolan by Hessel Gerritsz.[16] They are unlabelled, marked merely as a group of small circles.[22] 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 Gerritsz the following year, they are labelled Houtman's Abrolhos.[23]

On British Admiralty charts, the islands are labelled Houtman's Rocks.[24]

Wreck of Batavia

A 1647 engraving showing the Beacon Island massacre of survivors of the Batavia
shipwreck

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, 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.

Wreck of Zeewijk

The

Zeewijk was the last of the four Dutch East Indiamen to be wrecked on the west coast of Australia. 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, the men landed ten chests of money, containing 315,836 guilders and weighing a total of three tons.[7]

The crew of 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.[7]

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 60-foot (18 m) boat, sufficient to carry all the men and the money chests. Completed in March 1728 and affectionately named 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.[7]

Gun Island and phosphate mining

British Admiralty published a chart of "The Houtman Rocks", based on the 1840 survey of John Clements Wickham and John Lort Stokes
.

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.[25] Later during the 19th century many islets were used by men collecting guano
.

A

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, a number of buildings on Rat, Gun and Pelsaert islands[26] and had over 40 Malay workers mining and processing the phosphate for export.[27]
Over 48,000 tonnes was shipped between 1884 and 1896.

Tenure, governance and management

The Houtman Abrolhos is Australian territory. There is no dispute about this, although it has been suggested[

UNCLOS by extending their continental baseline to encompass it, a breach of Article 7 of the convention.[citation needed
]

The islands are a part of Western Australia. They are part of the Electoral district of Geraldton, and the City of Greater Geraldton. Management is vested in the Department of Fisheries.

Geology

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.

Oceanography

Bathymetry

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

shelf break, beyond which the seabed falls away much more steeply, averaging around 50 metres per kilometre (260 ft/mi).[28]

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 are 40 to 50 metres deep, so no impediment to the exchange of offshore and inshore waters.[28]

Currents

The Houtman Abrolhos lies almost directly in the path of the

Shark Bay and the Houtman Abrolhos together act as a topographical trigger for the forming of eddies,[29] so the Abrolhos can experience currents from any direction, even when the Leeuwin Current is flowing strongly.[28]

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.[28]

Temperature and salinity

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.[28]

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.[28]

A similar annual pattern occurs in

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.[28]

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

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.[28]

Sea levels

As with the rest of southwestern Australia, tides at the Houtman Abrolhos are small and irregular. There 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).[28]

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.[28]

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.[28]

Climate

As of 2012, the Australian

relative humidity and atmospheric pressure
have been publicly available since then. Based on this and other data, researchers have put together 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 °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 (31 km/h) 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

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.

Biogeography

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.[30][31] 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.[32]

In marine terms, the Houtman Abrolhos is located within the

Geraldton. This province is defined as the area of shelf where tropical waters intergrade into temperate, thus supporting both tropical and temperate biota.[33][34] In addition, this area contains the highest concentration of west coast endemics.[35]

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.[35]

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.[33][34]

Terrestrial flora

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.[36]

Flora

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 totalled 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.[36]

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.[36]

As of 2001, five species of priority flora occurred on the islands.[36] One, Acacia didyma, is no longer considered a priority species.[37] 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;[38] Calocephalus aervoides and Galium migrans, both Priority Three;[39][40] and Lepidium puberulum, Priority Four.[41]

Ninety-five 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;[36] there was a lull in eradication in the late 1990s, but the program was later reinstated,[42] 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.[43] Other noxious weeds include Opuntia stricta (prickly pear),[36] Verbesina encelioides and Echium plantagineum (Paterson's curse).[42]

Terrestrial fauna

Birds

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.[44]

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 and have been identified by

crested tern, roseate tern and fairy tern. In addition, they contain important breeding areas for the Pacific reef heron, Pacific gull, bridled tern, white-bellied sea eagle and osprey.[44][47]

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.[47]

Mammals

Only two species of land mammal are indigenous to the Houtman Abrolhos, the

Francisco Pelsart in his 1629 Ongeluckige Voyagie. This represents the first recorded sighting of a macropod by Europeans,[50] and probably also the first sighting of an Australian mammal.[51] Tammar wallabies were introduced to North Island from East Wallabi Island by fishermen, probably in the 1950s, but failed to establish themselves. 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,[52] but in July 2007 the research was discontinued and the population culled instead.[53]

Two introduced mammals are established on the islands. The

domestic goat (Capra aegagrus hircus) is also reported to have been present on East Wallabi Island, but is no longer.[44]

Reptiles

Abrolhos dwarf bearded dragon
(Pogona minor minima) on West Wallabi Island

23 terrestrial reptile species are known to occur on the islands of the Houtman Abrolhos.

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.[47] For a full list of species, see list of reptiles of the Houtman Abrolhos
.

Amphibians

Specimens of the pobblebonk (

Myobatrachus gouldii) were collected from the Houtman Abrolhos during the Percy Sladen Trust Expedition of 1913 and 1915, but no amphibians have been recorded on the islands since that time.[55]

Marine flora

Algae

260 species of

benthic algae have been recorded at the Houtman Abrolhos. This figure comprises 178 species of red algae (Rhodophyta), 50 species of brown algae (Phaeophyta) and 32 species of green algae (Chlorophyta). Both temperate and tropical species are present, in many cases near the northern or southern extent of their range.[57] For a full list, see list of algae of the Houtman Abrolhos
.

Seagrass

Only ten species of seagrass have been recorded at the Houtman Abrolhos. Seven of these are temperate species at or near the northern limit of their range; the other three have a tropical distribution. That there are so few tropical species may be due to periods of low sea temperatures, or the small areas of suitable habitat at the Abrolhos; alternatively it may be that more collecting effort is needed in habitats that suit tropical species. The seagrass species recorded at the Houtman Abrolhos islands are:[58]

Marine fauna

Fish

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:[59]

Commercially important species include

Plectropomus leopardus (coral trout).[60] 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.[59] 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.[61]

Marine mammals

The Houtman Abrolhos maintains a breeding population of

trypots, and this may have affected the survival of young pups. Populations are thought to have been fairly stable for the last fifty years, although the lack of genetic diversity in the smaller population remains of concern, as does climate change.[63]

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.[47]

Little information is available on other marine mammals at the Abrolhos, as no direct research on this subject has been undertaken. Sightings of the

pygmy Bryde's whale (Balaenoptera edeni), orca (Orcinus orca), bottlenose dolphin (Tursiops aduncus), common dolphin (Delphinus delphis),[64] and striped dolphin (Stenella coeruleoalba).[60]

Marine reptiles

The

loggerhead turtle (Caretta caretta) both live in the waters off the Houtman Abrolhos, albeit in low numbers. Neither species breeds in the area, as water temperatures are too low.[60]

Coral

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,[65] 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.[65] 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.[66]

Crustaceans

Perth