Houtman Abrolhos

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 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 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 conservation habitat. It is well known as the site of numerous shipwrecks, the most famous being the Dutch ships , which was wrecked in 1629, and , 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 west of Geraldton in the state of Western Australia. It comprises 122 Islands and associated coral reefs, 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 by , and also takes in the outlying North Island, located 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 the subsequent mutiny and massacre that took place among the marooned survivors.

The Easter Group lies to the south-east of the Wallabi Group, from which it is separated by a -wide channel named Middle Channel. It is about by , 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 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, 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; and ''Windsor'', wrecked on the Half Moon Reef in 1908.Australia 1:100000 Topographic Survey, Map sheet 1640 (Edition 1): Abrolhos Special

Governance

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 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 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.

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.

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:

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 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 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, 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 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 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 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'',Project Gutenberg of Australia edition
but that 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 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''.

Wreck of ''Batavia''

In 1629, some of the islands were the scene of an infamous shipwreck and mutiny. The Dutch ship , 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 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.

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.

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 ''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.

Gun Island and phosphate mining

During Admiralty surveys of the north west coast in 1840, crew from 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.

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, 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 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 that Australia claims the island chain under the UNCLOS by extending their continental baseline to encompass it, a breach of Article 7 of the convention.

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 continental shelf. At less than 50 metres (160 ft) deep, 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 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.

Currents

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 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 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 the extremely low levels of nutrients in the water.

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.

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.

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).

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.

Climate

, 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 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 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.

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.IBRA Version 6.1
data 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, 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.IMCRA 4.0 data 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.

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.

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 lichen

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