Demersal slope and associated fish communities of the Central Western Province

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 western continental slope provides important habitat for demersal fish communities. In particular, the continental slope of the Central Western provincial bioregion supports demersal fish communities characterised by high diversity compared with other, more intensively sampled, oceanic regions of the world. Its diversity is attributed to the overlap of ancient and extensive Indo-west Pacific and temperate Australasian fauna (Williams et al. 2001). Scientists have described 480 species of demersal fish that inhabit the slope of this bioregion, and 31 of these are considered endemic to the bioregion.

National and/or regional importance

The Demersal slope and associated fish communities of the Central Western Province are recognised as a key ecological feature for their high levels of biodiversity and endemism.

Location

This KEF extends from the edge of the shelf to the limit of the exclusive economic zone, between Perth and the northern boundary of the South-west Marine Region; IMCRA v4.

The spatial boundary of this KEF, as defined on the Conservation Values Atlas, includes the continental slope in the Geoscience Australia geomorphic features dataset within the IMCRA 4.0 central western provincial bioregion.

Description

A diverse assemblage of demersal fish species below a depth of 400 m is dominated by relatively small benthic species such as grenadiers, dogfish and cucumber fish. Unlike other slope fish communities in Australia, many of these species display unique physical adaptations to feed on the seafloor (such as a mouth position adapted to bottom feeding), and many do not appear to migrate vertically in their daily feeding habits (Williams et al. 2001).

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.

No pressures were assessed as of concern for the Demersal slope and associated fish communities of the Central Western Province.

Pressures assessed as of potential concern are changes in sea temperature, change in oceanography and ocean acidification as a result of climate change, noise pollution, physical habitat modification, extraction of living resources and bycatch. 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 implications of changes to the region’s sea temperatures and oceanography for deeper ocean environments are uncertain, although sea-water warming at depth is expected to have significant implications for benthic and demersal fish.

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). Numerous water bodies and currents converge at the Naturaliste Plateau, including the subtropical convergence front. These complex water dynamics are likely to experience considerable changes if there are significant changes to ocean circulation in the region, with flow-on effects for the entire ecosystem.

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).

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.

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). Inshore habitats, such as the seagrass beds and limestone reefs of the west coast and Geographe Bay, are important breeding and nursing areas for a range of fish and shark species and for western rock lobster. 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 tuna, 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. They 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.