Continental slope demersal fish communities

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 (KEFs) 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 North-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 diversity of demersal fish assemblages on the continental slope in the Timor Province, the Northwest Transition and the Northwest Province is high compared to elsewhere along the Australian continental slope. The continental slope between North West Cape and the Montebello Trough has more than 500 fish species, 76 of which are endemic, which makes it the most diverse slope bioregion in Australia (Last et al. 2005). The slope of the Timor Province and the Northwest Transition also contains more than 500 species of demersal fish of which 64 are considered endemic (Last et al. 2005). The Timor Province and Northwest Transition bioregions are the second-richest areas for demersal fish across the entire continental slope.

National and/or regional importance

This species assemblage is recognised as a key ecological feature because of its biodiversity values, including high levels of endemism.

Location

The spatial boundary of this KEF, as defined in the Conservation Values Atlas, is defined as the area of slope, as defined in the Geoscience Australia geomorphic features dataset, found in the Northwest Province and Timor Province provincial bioregions as defined in IMCRA 4.0, at the depth ranges of 220-500 metres and 750-1000 metres.

Description

The demersal fish species occupy two distinct demersal community types (biomes) associated with the upper slope (water depth of 225–500 metres) and the mid-slope (750–1000 metres). Although poorly known, it is suggested that the demersal-slope communities rely on bacteria and detritus-based systems comprised of infauna and epifauna, which in turn become prey for a range of teleost fish, molluscs and crustaceans (Brewer et al. 2007). Higher-order consumers may include carnivorous fish, deepwater sharks, large squid and toothed whales (Brewer et al. 2007). Pelagic production is phytoplankton based, with hot spots around oceanic reefs and islands (Brewer et al. 2007).

Although the reasons for the high levels of endemism are not fully understood, the presence of such a diversity of fish and high numbers of endemic species in these bioregions suggests there are important interactions occurring between the physical processes and trophic structures (P Last & A Williams, pers. comm., 2009). The data to support high endemism is scarce and the assumption of high endemism could reflect the small sample size.

Bacteria and fauna present on the continental slope are the basis of the food web for demersal fish and higher-order consumers in this system. Loss of benthic habitat along the continental slope at depths known to support demersal fish communities may lead to a decline in species richness, diversity and endemism associated with this feature.

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 Continental slope demersal fish communities.

Pressures assessed as of potential concern are changes in sea temperature and ocean acidification as a result of climate change; physical habitat modification; 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). Key ecological features supporting coral reef communities are vulnerable to bleaching and mortality from elevated sea temperatures. Projected temperature changes in Australian seas exceed the threshold for inducing bleaching events on an annual basis (Hobday et al. 2006). A decrease in coral abundance would lead to changes in ecosystem structure, processes, and connectivity between reefs and adjacent waters. Nutrients and organic matter sourced from dynamic reef complexes may be disrupted.

While the effects of increased sea temperatures are likely to vary greatly across communities and ecosystems, there is a high level of agreement from different datasets that warming is affecting distributional ranges and larval phase of tropical marine fishes (Munday et al. 2009). Changes in sea temperature may also result in changes to phytoplankton and zooplankton communities, with implications for trophic dynamics (Richardson et al. 2009) and fish larval supply and survival (Lo-Yat et al. 2011).

Increases in ocean water temperature may also affect deeper water fish species, such as those within the demersal slope fish key ecological feature. Climate change modelling predicts that by 2070 ocean water temperatures at a depth of 500 m will warm by 0.5–1 °C, which could adversely impact larval fish development and survival (Hobday et al. 2006). Benthic and demersal fish species may shift south and some populations may decline where ranges are bounded to the south (Hobday et al. 2006). Structurally complex epifauna (i.e. sponges, algae and coralline algae) may also suffer mortality from elevated water temperatures (Lawrence et al. 2007).

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).

Ocean acidification will compromise carbon accretion and, together with increasing ocean temperatures, may result in loss of ecosystems based on geologic features formed from coral or coralline algae (Hoegh-Guldberg 2011; Hoegh-Guldberg et al. 2007; Kleypas & Yates 2009; Kuffner et al. 2008). Increasing acidity impairs the ability of species with calcareous shells, such as echinoderms, crustaceans and molluscs, to maintain shell integrity resulting in reductions of the overall abundance and biodiversity of these species (see review by Kleypas & Yates 2009). A decrease in the abundance of fauna with carbonate-based skeletons and coral and coralline algal abundance, and the complex structural habitats they create, could lead to changes in ecosystem structures, processes, and connectivity between the reef complex and the adjacent deeper waters.

Physical habitat modification (fishing gear)
Trawling is potentially damaging to benthic habitats which can adversely affect demersal fish and other fauna dependent on these habitats. The continental slope provides a habitat for a rich and diverse range of demersal fish species, many of which are endemic to the North-west Marine Region (Last et al. 2005). Loss of benthic habitat along the continental slope at depth ranges known to support demersal fish communities (225–500 m and 750–1000 m) could lead to a decline in species richness and diversity associated with this feature.

Evidence exists for physical habitat modification as a result of North-west Slope Trawl fishing on demersal slope fish communities. According to logbook data for 2001–04, between a third and a half of the total catch was discarded (Wayte et al. 2007). The full composition of bycatch was unknown. However, in 1998–2000, benthic taxa were the dominant (23.1 %) bycatch category by weight of exploratory trawls in the North-west Slope Trawl Fishery (Wayte et al. 2007). Fewer hexactinellid sponges have been recorded from heavily trawled areas in the North-west Slope Trawl Fishery. Concern has also been raised about the impacts of trawling on bryozoan-rich substrates that appear from a depth of 120 m and progressively dissipate until a depth of 300 m. In addition, distribution patterns of female giant crabs (Pseudocarcinus gigas) may be correlated with bryozoan-rich substrates and giant crabs form a major part of catches taken in the West Coast Deep Sea Crab Fishery (Wayte et al. 2007).

Bycatch (commercial fishing)
The North West Slope Trawl Fishery operates in waters 250–800 m deep on muddy substrates. Target species include scampi and deepwater prawns (Wilson et al. 2010). There are currently seven permits to fish and the fishery operates over the entire continental slope of the region. The fishery is considered to be sustainable as far as the harvest of target species (Wilson et al. 2010). However, there is little information available on the composition and volume of the rest of the biomass removed by the North West Slope Trawl Fishery. Bycatch diversity is reputedly high and, according to logbook data for 2001–04, between a third and a half of the total catch is discarded (Wayte et al. 2007). It is not known what impact trawling has on the demersal slope communities and whether it has the potential to diminish the species richness and diversity of these communities.