Houtman Abrolhos Islands
The Houtman Abrolhos Islands (the Abrolhos) is an amazing archipelago consisting of 122 islands which network over an expansive 100 kms of ocean. The Abrolhos is the most southern true coral reef in the Indian Ocean and boasts an extensive range of both of tropical and temperature marine species. The Abrolhos is under the management of the Department of Fisheries and is now part of the City of Greater Geraldton. However, due to the complexity of geology and environment, the Abrolhos has been considered separately.
|Islands||The Abrolhos is comprised of three major island groups including the Wallabi-North Island Group, Easter Group and Pelsaert which are separated by the Middle and Zeewijk Channels.|
|Temperature||Average maximum temperature of 28.5ºC in February
Average minimum temperature of 15.3°C in July and August.
|Rainfall||Annual rainfall of 277 mm (DoF, 2012).|
The Abrolhos has a small seasonal population of fishermen who inhabit their island homes from March to June each year during the Abrolhos western rock lobster fishing season. A limited number of tourists are permitted for day trips.
The Abrolhos attracts significant economic and tourist activity, providing substantial benefits to the Western Australian community. These activities include commercial fisheries for rock lobster, scallops and finfish, as well as aquaculture for pearls, recreational fisheries, diving and bird watching and a developing tourism. According to the Department of Fisheries (2012), the total value of marine species landed was $34 million in 2008/09 equating to 18% of the total value of marine species in Australia.
The islands of the Abrolhos are geologically diverse, with North Island, the Wallabis’, Rat Island and Gun Island being classified as ‘mainland remnant’ type 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. In contrast, the newly created adjacent islands, such as Long, Suomi and Pelsaert, consist of coral rubble of more recent origin.
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 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 of the State down the Western Australian coast. 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.
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.
Groundwater on the Abrolhos is sourced by freshwater wells located on the East and West Wallabi Island, Rat Island and Middle Island. In these areas rainwater trickle into small, shallow limestone caverns (DoF, 2012). Some of the key issues which pose a threat to water quality at the Abrolhos are:
- Wastewater discharge from land based infrastructure and marine vessels
- Chemical spills and discharges from marine vessels, jetties and land-based storage areas