Commonwealth marine environment surrounding the Houtman Abrolhos Islands (and adjacent shelf break)

Legal description

The Commonwealth marine area is a matter of national environmental significance under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). As such, actions that have, will have, or are likely to have, a significant impact on it require approval.

In February 2014, the National Offshore Petroleum Safety and Environmental Management Authority (NOPSEMA) became the sole designated assessor of petroleum and greenhouse gas activities in Commonwealth waters in accordance with the Minister for the Environment’s endorsement of NOPSEMA’s environmental authorisation process under Part 10, section 146 of the EPBC Act. Under the streamlined arrangements, impacts on the Commonwealth marine area by petroleum and greenhouse gas activities will be assessed solely through NOPSEMA.

Key ecological features are not matters of national environmental significance and have no legal status in their own right. However, they may be considered as components of the Commonwealth marine area.

This KEF was identified in the South-west Marine Bioregional Plan. Under section 176 of the EPBC Act, once a bioregional plan has been made the minister responsible for the environment must have regard to it when making any decision under the Act to which the plan is relevant.

Values

The Houtman Abrolhos Islands and surrounding reefs support a unique mix of temperate and tropical species, resulting from the southward transport of species by the Leeuwin Current over thousands of years. The area represents the southern limit in Western Australia of many widespread Indo-Pacific tropical fish. The islands are the largest seabird breeding station in the eastern Indian Ocean. They support more than one million pairs of breeding seabirds, including sedentary and migratory species. Many of the islands’ biodiversity features rely on the benthic and pelagic ecosystems in deeper, offshore waters; most notably, seabirds and rock lobster.

National and/or regional importance

The Commonwealth marine environment surrounding the Houtman Abrolhos Islands (and adjacent shelf break) is defined as a key ecological feature for its high levels of biodiversity and endemism in benthic and pelagic habitats.

Location

The Houtman Abrolhos Islands are a complex of 122 islands and reefs located at the edge of the continental shelf between 28°15’ S to 29° S, approximately 60 km offshore from the mid-west coast of Western Australia. The Houtman Abrolhos waters and reefs have been relatively well studied and are noted for their high biodiversity and mix of temperate and tropical species, resulting from the southward transport of species by the Leeuwin Current over thousands of years (e.g. Phillips 2001, Roberts et al. 2002, Williams et al. 2001).

The spatial boundary of this KEF, as defined on the Conservation Values Atlas, is based on the 12nm territorial boundary around the Houtman Abrolhos Island group from the Geoscience Australia, Australian Maritime Boundary (AMB) dataset.

Description

By global standards, the marine environment of the South-west Marine Region has high biodiversity and large numbers of species native to the region (known as endemism). Particular hotspots for biodiversity are the Houtman Abrolhos Islands, the overlap between tropical and temperate fauna along the west coast, the Recherche Archipelago and the soft sediment ecosystems in the Great Australian Bight.

The Houtman Abrolhos Islands lie in a transitional zone between major marine biogeographic provinces, caused by the juxtaposition of the warm, tropical water of the Leeuwin Current and colder water more typical of the islands’ latitude (Hayes et al. 2008). The Leeuwin Current allows the tropical to subtropical transition to occur at 28 - 29.5° S, whereas on the east coast of Australia this transition occurs at 24° S (Collins et al. 1991, in Richardson et al. 2005). The Leeuwin Current allows the Houtman Abrolhos Islands to support the highest-latitude coral reefs in the Indian Ocean (Richardson et al. 2005). The islands also represent the southern limit in Western Australia of many widespread Indo-Pacific tropical fish (Baker 2006). The reefs are composed of 184 known species of coral that support approximately 400 species of demersal fish, 492 species of molluscs, 110 species of sponges, 172 species of echinoderms and 234 species of benthic algae (Wells & McDonald 2010). In addition, the area provides important habitat for rock lobsters (Panulirus cygnus) (MacArthur et al. 2007). The Houtman Abrolhos Islands are the largest seabird breeding area in the eastern Indian Ocean (Hayes et al. 2008), supporting over one million pairs of breeding seabirds (predominantly terns), including sedentary and migratory species (Surman & Wooller 2003). Many of the islands’ biodiversity features, most notably seabirds and rock lobster, rely on the benthic and pelagic ecosystems in the immediate surrounding Commonwealth marine environment, which is also recognised as an important resting area for migrating humpback whales (DEH 2005b). The islands are the northernmost breeding site of the Australian sea lion (DEWHA 2008a), although sea lions are not thought to be an important component of this ecosystem because of their low population numbers.

Calcium carbonate production and associated reef growth in the Houtman Abrolhos Islands is not significantly different from tropical reefs, and depends on the strength or occurrence of the Leeuwin Current over time (Collins et al. 1991, cited in Richardson et al. 2005). The islands and reefs may act as a refuge for corals when sea surface temperatures become too hot in more northern latitudes. As such, the Houtman Abrolhos Islands can provide insight into coral responses to climate change, which may be important for managing coral reefs in the face of rising global temperatures (Richardson et al. 2005).

Pressure

During the development of marine bioregional plans, a regional pressure analysis was conducted to assess present and emerging pressures affecting conservation values in the Marine Regions and the effectiveness of mitigation and management arrangements that are currently in place to address these pressures. The analysis enabled pressures to be categorised in terms of their relative importance or concern and has informed the identification of regional conservation priorities. For the purpose of the plans, pressures are defined broadly as human-driven processes and events that do or can detrimentally affect the region's conservation values.

Pressures assessed as of concern for the Commonwealth marine environment surrounding the Houtman Abrolhos Islands are sea level rise, changes in sea temperature and change in oceanography as a result of climate change.

Pressures assessed as of potential concern are ocean acidification as a result of climate change; changes to water quality due to chemical and nutrient pollution; physical habitat modification; extraction of living resources; bycatch; and oil pollution. These pressures are described in more detail below.

Sea level rise (climate change)
Global sea levels have risen by 20 cm between 1870 and 2004 and predictions estimate a further rise of 5–15 cm by 2030, relative to 1990 levels (Church et al. 2009). Longer term predictions estimate increases of 0.5 m to 1 m by 2100, relative to 2000 levels (Climate Commission 2011). The Houtman Abrolhos Islands are low-lying islands with an average altitude of 3 m above sea level. Many species of seabirds breed on the islands, which are considered of national and global significance as habitat for many migratory seabird species (Surman & Nicholson 2006).

As many seabirds are ground nesting, the loss of habitat associated with sea level rise could result in reduced reproductive success, as well as displacement of populations from their breeding area. Because of the diversity and numbers breeding on the island, seabirds are key components of the marine ecosystem surrounding the Houtman Abrolhos Islands, including the Commonwealth marine environment, where most foraging grounds occur. The broader ecological implications for this key ecological feature of the effects of sea level rise on seabirds are uncertain.

Changes in sea temperature (climate change)
Sea temperatures have warmed by 0.7 °C between 1910–1929 and 1989–2008, and current projections estimate ocean temperatures will be 1 °C warmer by 2030 (Lough 2009). The south-west of Western Australia in particular is one of three hotspots in the Indian Ocean where rising temperature trends exceed the Indian Ocean basin average (Feng et al. 2009). All key ecological features considered important for the region’s primary productivity and biological production are at risk from climate change. Changes in sea temperature and oceanographic processes have been implicated in the region in shifts in distribution of marine species, changes to prey variability (including positive changes for some species, such as bridled tern) and effects on reproductive time and success (Dunlop 2009; Gaughan et al. 2002; Surman & Nicholson 2009). The overall implications for ecosystem processes in the region are not known.

Change in oceanography (climate change)
The strength of the Leeuwin Current has decreased slightly since the 1970s. This weakening is expected to continue, although this prediction currently has low confidence (Feng et al. 2009). The Leeuwin Current is the basis of much of the region’s biological productivity, and its strength and seasonal/climatic variability are a primary driver for the intensity, timing and locations of productivity events in the region, from the waters off Geraldton to the Great Australian Bight (Pattiaratchi 2007). The long-term implications for the region’s ecosystems and its key ecological features are uncertain.

Ocean acidification (climate change)
Driven by increasing levels of atmospheric CO₂ and subsequent chemical changes in the ocean, acidification is already underway and detectible. Since pre-industrial times, acidification has lowered ocean pH by 0.1 units (Howard et al. 2009). Furthermore, climate models predict this trend will continue, with a further 0.2–0.3 unit decline by 2100 (Howard et al. 2009). Projected changes in Australian waters by 2070 include a decline in pH of 0.2 units (Lawrence et al. 2007). There is a high level of uncertainty about the effects of ocean acidification on marine life. While some organisms might be able to adapt (Orr et al. 2009), anticipated changes to phytoplankton and zooplankton have the potential to detrimentally affect ecosystem processes and the structure of ecological communities. Research on the impact of ocean acidification on Antarctic krill has found that increased levels of CO₂ kill their embryos (Kawaguchi et al. 2011). Krill are an important part of the food chain because they feed on phytoplankton and zooplankton, and are a key food source for many species that occur in Australian waters. Consequently, acidification impacts have the potential to affect species further up the food chain. Ocean acidification will also reduce coral growth rates (Anthony & Marshall 2009), making reefs, including those occurring in waters surrounding the Abrolhos Islands, more susceptible to erosion and disturbance from storms. The potential effects of increased acidity on the region’s biodiversity also include changes to growth and population dynamics of some shell-forming organisms, impacts on the reproductive and metabolic functions of a number of fish and invertebrate species, and sensitivity of some early-life stages to acidification (Orr et al. 2009).

Chemical pollution/contaminants
Introduction of chemical pollution into the marine environment is usually localised and associated with shipping and ports operations, particularly dredging; industrial development in the coastal zone; and proximity to urban centres and estuaries in areas with intense industrial and agricultural development. This pressure is of potential concern for key ecological features that are close to coastal areas experiencing industrial developments, intense land use and/or large ports. There are potential contamination impacts from historical sea dumping in the Perth Canyon, although the likelihood is low.

Nutrient pollution
The South-west Region waters are generally low in nutrients; any significant increase in nutrient loads in the water column has potential detrimental effects for biodiversity. Aquaculture development in the Houtman Abrolhos Islands might result in significant although highly localised increase in nutrients, with potential impacts on adjacent rich ecological communities.

Physical habitat modification
Coastal, urban and industrial development and port activities can result in loss or significant degradation of areas of important habitat. Of potential concern are those features occurring on the west-coast continental shelf, where the rate of development is higher (Clifton et al. 2007). Loss and disturbance of mangrove habitat in the Houtman Abrolhos Islands due to climatic changes and/or further future development would have significant impacts on the year-round resident seabird populations of Australian lesser noddy. In the region, disturbance and physical changes to benthic habitats result from bottom trawling, although the extent of trawling varies broadly among the fisheries in the region. Sponges, bryozoans and sea squirts are particularly vulnerable to trawling impacts (Currie et al. 2009). Bottom trawling occurs on the west-coast shelf edge and slope, and loss or modification of the benthic community is likely (20 out of 48 habitat types occurring within the fishery area are at risk; Wayte et al. 2007). Other sources of benthic habitat loss or modification at more localised scales are dredging, construction of infrastructure, and laying of underwater pipelines and cables. The implications of benthic habitat loss on the region’s species and ecosystems are not adequately understood.

Extraction of living resources
Understanding of ecosystem effects of fishing is very limited. Evidence of reliance of species on targeted stocks include research on the feeding ecology of seabirds foraging around the Houtman Abrolhos Islands, which indicates that a number of species, including the threatened Australian lesser noddy, are reliant on the availability of specific species of small pelagic fish (Gaughan et al. 2002). Some seabird species that inhabit the region are also known to depend on large pelagic predators, such as tunas, for driving prey fish to the surface, and some species, such as sooty tern, are thought to be totally reliant on this method of feeding (Dunlop 2008; Jaquemet et al. 2007). There are also some concerns, underpinned by high levels of uncertainty, about competition for resources between Australian sea lion and fisheries (DEWHA 2010).

Based on the latest fisheries status reports from the Australian Government, Western Australia and South Australia (respectively, considering the 2009, 2008–09 and 2006 stock status reports), 10 stocks in the region are assessed as ‘overfished.’ 16 stocks have an uncertain status, including dhufish, for which substantial declines have been reported (Fletcher & Santoro 2009; Linnane et al. 2010, 2010a; Wilson et al. 2010). Recent management changes have been implemented to facilitate stock recovery for dhufish and other west coast demersal finfish (DoF 2011, 2011a). In light of limited understanding about the ecosystem effects of extraction of living resources for the key ecological features of the region, the pressure is considered of potential concern.

Bycatch
There is a general lack of evidence to demonstrate the performance of bycatch mitigation measures (Bensley et al. 2010). In relation to bycatch of non-protected species, Phillips et al. (2010) indicate that current levels of independent observers preclude a cumulative assessment of the catch of non-target species, but recommend that such assessment is important to understand more broadly the environmental effects of fishing and to underpin an ecosystem approach to fisheries management (Phillips et al. 2010). In the absence of an integrated assessment of the cumulative ecosystem effects of current catch rates for a number of non-target species, this pressure is of potential concern.

Oil pollution
A number of the key ecological features, including the Commonwealth marine environment surrounding the Houtman Abrolhos Islands, have characteristics that make their ecosystems and communities vulnerable to the effects of an oil spill, for example, features that include localised areas of high productivity, which attract large aggregations of marine life.

The intensity and distribution of activities implicated in oil spills, such as oil production and transport, are likely to increase in the region.

Australia has a strong system for regulating industry activity that is the potential source of oil spills and this system has been strengthened further in response to the Montara oil spill. While oil spills are unpredictable events and their likelihood is low based on past experience, their consequences, especially for threatened species at important areas, could be severe.