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).

Pelseart Island

Photo: L Strumpher - Beagle Island Australian Sealion

Juvenile Pacific Gull with Plastic Bottle at East Wallabi

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People

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.

Economy

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.

Landscape, Geology & Environment

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).

Coastal & Marine

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.

Marine Assets

Seagrass Meadows

The Central West Coast limestone reef system extends from Kalbarri to Perth and supports an extremely high species diversity of seagrasses. Extensive seagrass meadows in protected, near-shore areas of the NAR, provide shelter and nursery habitat for many fish and other marine creatures including Western Rock Lobster. Seagrasses also provides services as primary biomass producers, sources of dissolved oxygen, sediment traps and nutrient cyclers.
Houtman Abrolhos Islands

The Houtman Abrolhos Islands, one of the largest temperate limestone reef systems in Australia, comprises 122 coral cays located off the coast of Geraldton. The islands are clustered into the Wallabi, Easter and Pelsaert groups and were declared a national park in 2019. They are home to a mixture of temperate and tropical coastal and marine flora and fauna, including several threatened species.

Water

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.