Perth Canyon and adjacent shelf break, and other west-coast canyons

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 Perth Canyon is the largest canyon on the Australian margin and, together with numerous smaller submarine canyons that incise the continental slope of southern Western Australia (Potter et al. 2006), is expected to have high biodiversity values. Canyons can be characterised by higher productivity and species diversity than surrounding slope areas of similar depth or distance offshore (Richardson et al. 2005). They are pathways for transporting sediments, nutrients and biota off the continental shelf and slope and onto the abyssal plain, either acting as a sink for this relatively organic-rich material or directing it into deeper water (Richardson et al. 2005). Canyons are also conduits for upwelling and downwelling, processes that influence environmental variables such as nutrient availability and water temperature. Upwelling of water from the deep ocean supplies nutrients to the continental shelf and slope, which is important for phytoplankton blooms and production in local fisheries (Richardson et al. 2005). The west-coast canyons are believed to be associated with small periodic upwellings that locally increase productivity and attract aggregations of marine life. In the Perth Canyon, interactions between the canyon topography and the Leeuwin Current induce clockwise-rotating eddies that transport nutrients upwards in the water column from greater depths. Due to the canyon’s depth and the Leeuwin Current’s barrier effect, this remains a subsurface upwelling (depths greater than 400 m), which confers ecological complexity that is typically absent from canyon systems in other areas (Pattiaratchi 2007). The Perth Canyon also marks the southern boundary for numerous tropical species groups on the shelf, including sponges, corals, decapods and xanthid crabs.

National and/or regional importance

The Perth Canyon forms a major biogeographical boundary and it is defined as a key ecological feature because it is an area of higher productivity that attracts feeding aggregations of deep-diving mammals and large predatory fish. It is also recognised as a unique seafloor feature with ecological properties of regional significance.

Location

The west coast system of canyons spans an extensive area (8744 km²) of continental slope offshore from Kalbarri to south of Perth. It includes the Geographe, Busselton, Pelsaert, Geraldton, Wallaby, Houtman and Murchison canyons and, most notably, the Perth Canyon, which is Australia’s largest ocean canyon. The Perth Canyon (offshore from Rottnest Island, at 32° S) is prominent among the west coast canyons because of its magnitude and ecological importance; however, the sheer abundance of canyons spread over a broad latitudinal range makes this feature important as a whole.

The spatial boundary of this KEF, as defined on the Conservation Values Atlas, includes all canyons in the Geoscience Australia geomorphic features dataset offshore from Kalbarri to south of Perth. A separate dataset was created for the Perth canyon and is based on the specific canyon, along with additional areas of the shelf break between 200 m and 500 m depth which are biologically important areas for blue whales.

Description

The biological productivity of the South-west Marine Region is low by global standards and in comparison with other Australian marine regions because of the low-nutrient tropical waters carried south by the Leeuwin Current and its effect in suppressing upwelling of nutrients from deeper cold waters and the absence of significant rivers contributing nutrients into the marine environment through run-off. Small seasonal upwellings occur regularly at known locations and, because of the overall nutrient-poor nature of the region’s waters, these hotspots of productivity have a disproportionate influence on the region’s ecosystems, which is the reason that they are identified as key ecological features. The Perth canyon group is one of the main areas of relatively higher seasonal productivity in the region.

The Perth Canyon is a major cutting canyon in the region, that is, it cuts into the continental shelf. Cutting canyons experience high sediment load from offshore transport and receive organic material from productive shelf waters. The Perth Canyon is long, deep, narrow and steep-sided, cutting 4 km into the continental shelf (Pattiaratchi 2007). The head of the canyon starts at the 200 m depth contour on the continental shelf and drops to a depth of 1000 m over a 6.5 km distance before doglegging down onto the abyssal plain (at about 4000 m) (Rennie et al. 2006). The canyon head transports shelf material into the deep ocean and is an important link between continental shelf habitats and deepwater habitats (Richardson et al. 2005). The Perth Canyon marks the southern boundary of the Central Western Province (IMCRA v.4). Deep ocean currents upwelling in the canyon create a nutrient-rich, cold-water habitat that attracts deep-diving mammals and large predatory fish, which feed on small fish, krill and squid (Pattiaratchi 2007). A number of cetaceans, predominantly pygmy blue whales (Balaenoptera musculus brevicauda), aggregate in the canyon during summer to feed on the prey aggregations (Pattiaratchi 2007). Arriving from November onwards, their numbers peak in March to May. The topographical complexity of the canyon is also believed to provide more varied habitat that supports higher levels of epibenthic biodiversity than adjacent shelf areas (Hayes et al. 2008).

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 Perth canyon and adjacent shelf break, and other west coast canyons are 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; chemical pollution; noise pollution; extraction of living resources; bycatch; oil pollution and collision with vessels. These pressures are described in more detail below.

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. This pressure is considered of concern for those key ecological features important for the region’s productivity and for which the Leeuwin Current is considered a primary ecological driver.

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. 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.

Noise pollution
All sources of man-made noise in the region are predicted to increase (Clifton et al. 2007). Sources include shipping, marine infrastructure construction and operation involving underwater blasting and pile driving, defence naval exercises and seismic surveys. Many marine organisms use sound for a number of biological functions, including navigation, social communication and location of prey. The effects of seismic activities on large whales have received the most attention in the region; however, there have also been experimental studies into the effects of seismic surveys on fish (Popper et al. 2002; McCauley et al. 2003). Although extrapolation from experimental studies to wild populations is problematic, the evidence available to date justifies a precautionary approach. The Royal Australian Navy’s Western Australia exercise area is located off Perth, and its operation involves noise-generating activities, including firing of live ammunition, shipping exercises and use of active sonar. Guidelines under the EPBC Act are in place to mitigate the effect of noise generated by seismic surveys on whales; similarly, the Royal Australian Navy implements operational procedures to minimise environmental impacts.

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 2009; 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 Perth canyon and adjacent shelf break, and other west coast canyons, 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.

Collisions with vessels
The Perth canyon is an important habitat for a number of large whale species; its proximity to the Fremantle and Kwinana ports and its overlap with the Royal Australian Navy’s Western Australia exercise area make the potential effects of ship strikes in this area of potential concern, particularly in light of anticipated increases in shipping traffic (Clifton et al. 2007). Although vessel collision across the region is unlikely to be as serious a problem as in some other parts of the world, there is the possibility that events that occur well offshore are not detected (Kemper 2008) or that ship-related deaths go unrecognised (Laist et al. 2001).