Houtman Abrolhos Islands
The Houtman Abrolhos Islands (the Abrolhos) is an island archipelago 60 km off the coast of Geraldton (DBCA 2020). The islands form the most southern true coral reef in the Indian Ocean and are clustered into three groups – Easter, Pelsaert and Wallabi – spread out from north to south over 100 km of ocean. The islands and their surrounding reef communities are a meeting place for tropical and temperate sea life and one of the region’s most interesting and diverse marine areas. The Abrolhos islands were declared a National Park in 2019 (DBCA 2020) and the surrounding waters are a Fish Habitat Protection Area for the conservation of marine flora and fauna, for tourism, and to support sustainable fishing and aquaculture (DPIRD 2015). The islands fall under the City of Greater Geraldton local government authority but have been considered separately here due to their unique ecology and economy.
|Islands||There are three major island groups – Easter, Pelsaert and Wallabi – separated by the Middle and Zeewijk Channels.|
|Average Temperature||Mean daily maximum temperature over ~20 years 2001 – 2019 27.6°C (Jan) and 21.1°C (Jul).|
|Annual Rainfall||Average annual rainfall over ~20 years 2001 – 2019 270 mm (BoM 2020).|
Inhabited islands are private property, managed by DPIRD and used by western rock lobster and aquaculture industries (DBCA 2020). Around 120 licensed fishers, their families and deckhands take up temporary residence on 22 designated islands to fish commercially for western rock lobster from March to June each year (DPIRD 2015). A limited number of tourists are permitted for day trips. There is no commercial overnight accommodation on any of the islands, although there are moorings for private boats.
The waters around the Abrolhos are important habitat for lobster-breeding and western rock lobster is the most valuable commercial fishery (DPIRD 2015). Other commercial fisheries include scallop and finfish. Highly-prized black pearls are produced from hatchery-raised black lipped pearl oysters at eight aquaculture sites in the archipelago. Boating, fishing, diving and snorkelling are popular recreational activities around the islands and several tourism operators runs tours out of Geraldton. The Abrolhos Islands’ marine and terrestrial environments are fragile and it is important that visitors and the fishers who temporarily reside there protect them. The Abrolhos recreational fishing regulations can be viewed here.
The islands of the Abrolhos are geologically diverse. North Island, the Wallabis, Rat Island and Gun Island are classified as mainland remnant islands, made up of limestone, siltstone and marls of continental origin that have been isolated by rising sea levels over the last 8,000 – 10,000 years (DPIRD 2015). In contrast, the newly created adjacent islands, such as Long, Suomi and Pelsaert, consist of coral rubble of more recent origin.
The Abrolhos Islands are home to an array of protected flora and fauna that have adapted uniquely to the diverse range of island ecosystems (DPIRD 2015). Most of the islands have important bird nesting and breeding sites and the Abrolhos is amongst Australia’s most important sites for breeding seabirds (DBCA 2020). These include the Endangered Lesser Noddy Anous tenuirostri, which only breeds on the Abrolhos islands (DAWE 2005). Around 20 birds species are found on the Abrolhos islands and the Abrolhos Painted Button-quail Turnix vaius scintillans is found nowhere else (DBCA 2018). Two terrestrial mammal species – Tammar wallaby Notamacropus eugenii and Bush Rat Rattus fuscipes – and 26 terrestrial reptile species occur on the islands, at least one of which is endemic. Australian Sealion are commonly seen at the Abrolhos, which is the northern breeding limit for the species.
The islands’ vegetation is similar to that on the nearby coast however there are several communities of special conservation interest including mangroves, saltbush flats and dwarf Atriplex shrubland(DBCA 2020). There are over 140 species of native flora at the Abrolhos Islands and all are classified as protected (DPIRD 2015).
Priority Fauna Species
Geraldton Hills subregion (Geraldton Sandplains 1)
The Geraldton Hills is located in the LGAs in the middle portion of the region. This subregion is characterised by sand heaths of emergent Banksia and Cypresses, York Gum woodlands on alluvial plains. Areas of limestone are dominated by proteaceous heath and Acacia scrubs. Low closed Acacia shrublands occupies much of the alluvial plains associated with the Greenough and Irwin Rivers (Desmond and Chant, 2001b).
The Abrolhos islands lie in the stream of the southward-flowing Leeuwin Current, which funnels warm, low-nutrient, tropical water along the edge of the continental shelf, from the north down the Western Australian coast (DPIRD 2015). The current carries a cargo of larvae, eggs and juveniles of many species of corals and other marine life far south of their usual range. Water temperatures in the current are maintained throughout the winter at around 20 to 22 ºC, enabling corals and tropical species of fish and invertebrates to thrive in latitudes where they normally wouldn’t survive.
Aboriginal people visited the islands during the Holocene, as indicated by the discovery of a flaked stone artefact made from Eocene fossiliferous limestone on Beacon Island chert. Shipwrecks at the Abrolhos represent the earliest European archaeological sites in Australia (DPIRD 2012). The most famous wreck is the Batavia which hit a reef in the Wallabi Group in 1629.
Seagrasses are flowering plants that complete their life cycle submerged in seawater. Western Australia has the world’s highest diversity of seagrasses, with 27 species occurring in shallow waters off the coast. Seagrasses form a vital component of marine ecosystems through their services as primary biomass producers, sources of habitat (including breeding and nursery areas) and dissolved oxygen, sediment traps, and nutrient cycling. Seagrass distribution is determined by a combination of shelter, sediment, turbidity, nutrient, temperature, current and tidal influences.
Extensive seagrass meadows occur in protected near-shore areas of the NAR, where clear water, low nutrients and sandy sea floors prevail, and are dominated by the long strap-like Ribbonweed or Strapweed (Posidonia spp) and the thin-stemmed Wireweed (Amphibolis spp).
Seagrass habitats are fragile and susceptible to damage and can take many years to recover from disturbance, such as physical damage/removal and shading due to algal blooms (as a result of increased nutrients), and sedimentation (due to dredging activities and erosion in catchment areas).
More information on seagrasses in Western Australia can be found in the following publications: Flowers of the Ocean: WA’s Expansive Seagrass Meadows; The Wonders of Weed Information Sheet; Fisheries Fact Sheet: Seagrasses; Establishing Reference and Monitoring Sites to Assess a Key Indicator of Ecosystem Health (Seagrass Health) on the central west Coast of Western Australia (see references).
Houtman Abrolhos Islands
An archipelago of 122 islands that lie between 60 and 80 km off the mid-west coast of WA. The Abrolhos Islands stretches for around 100 km and consists of three groups of islands, and accompanying coral reefs – the Wallabi-North Island Group; Easter Group; and Pelsaert (or Southern) Group.
The Houtman Abrolhos Nature Reserve has been an A-Class Reserve since 1929 for the purposes of: ‘Conservation of flora and fauna, tourism, and for the purposes associated with the fishing and aquaculture industries.’
Along with being host to a diverse and unique range of both terrestrial and marine plants and animals, the Abrolhos Islands are significant from a historical perspective in relation to numerous shipwrecks as well as the whaling, guano mining, and fishing industries.
The islands provide the northern-most habitat of the Australian sea lion (Neophoca cinerea), which are classified as Vulnerable.
Areas of the Abrolhos below the high water mark and including State territorial waters form part of a Fish Habitat Protection Area (FHPA) that was declared in 1999.
More information can be found in the Department of Fisheries WA, The Abrolhos Islands Information Guide, 2012.
There is very limited fresh water on the islands, all of which is sourced from rainfall (DPIRD 2012). There are freshwater wells on East and West Wallabi Islands, Rat Island and Middle Island, where rainwater drains and percolates into small, shallow limestone caverns. Some of the key issues which pose a threat to water quality at the Abrolhos are wastewater discharge and chemical spills from land based infrastructure and storage areas, marine vessels and jetties.