The South-West Marine Region

Teeming with a broad spectrum of wildlife, the South-west Marine Region is essential in keeping Australia's marine ecosystems thriving

The South-west Marine Region is comprised of Commonwealth waters and seabed from Cape Inscription on the northern tip of Dirk Hartog Island, Western Australia, to the eastern end of Kangaroo Island, South Australia. It extends 200 nautical miles (370km) offshore to the edge of Australia’s Exclusive Economic Zone (EEZ) and includes areas of ‘extended continental shelf’ beyond the EEZ that Australia claims under the United Nations Convention on the Law of the Sea (UNCLOS).

The Region embraces 14 marine parks — Abrolhos, Jurien, Two Rocks, Perth Canyon, Geographe, Bremer, South-west Corner, Eastern Recherche, Twilight, Great Australian Bight, Murat, Western Eyre, Western Kangaroo Island and Southern Kangaroo Island — that collectively span more than 508,000 square kilometres.

The Region is popular for a host of recreational activities including charter fishing, diving and boating, especially in parks that are noted for congregations of whales such as Geographe Bay and Bremer Bay. Local seafood is a tourism drawcard for many coastal towns. Perth and Adelaide dominate the population and economic geography of the Region, while Geraldton, Fremantle, Albany, Esperance and Port Lincoln are major regional centres for industries of national significance including commercial fishing, aquaculture, shipping, petroleum exploration and production, boat-building, and defence. 

Twenty ports provide vital services to these industries, and marine-based businesses like marinas, chandleries, and repair yards are important contributors to local economies.

KEY ECOLOGICAL FEATURES

In the South-west Marine Region, sixteen ecological features have been identified as crucial in promoting biodiversity, most of which are seen in each of the Commonwealth marine parks in their embodiment of a mix of physical features, groups of species, and oceanographic phenomena occurring in different locations. Here are some that provide a sample of the marine marvels found in these remote and far-flung waters.

Meso-scale eddies

Meso-scale eddies are common oceanic features: energetic circulations in counter-rotating pairs or triplets, typically 50–200km wide and lasting several months. They perform a vital role in global climate transporting heat, salt, and carbon around the oceans. Generally, cyclonic (clockwise) eddies have cores that are cold from lifting deep, nutrient-rich water toward the surface where they enhance plankton production and attract other marine life, unlike anticyclonic warm-core eddies that are usually unproductive. Eddies form at predictable locations throughout the pelagic waters of the Region, near Shark Bay, the Houtman Abrolhos Islands and Perth Canyon, south-west of the Naturaliste-Leeuwin Peninsula, and off the south coast between Albany and the Eyre Peninsula. 

These eddy systems have a profound effect on the Region’s biological productivity. They carry plankton into the low-nutrient tropical waters carried south by the Leeuwin Current, providing nutrients for marine organisms that attract and sustain higher order animals such as marine mammals, seabirds, tuna, and billfish. They also transport coastal phytoplankton far offshore into the Indian Ocean where they are consumed by oceanic communities. The eddies also play a critical role in species distribution by influencing the range boundaries of tropical and temperate species and moving fish populations between coastal zones and pelagic waters.

Houtman Abrolhos Islands 

The Houtman Abrolhos Islands comprise 122 islands and reefs at the edge of the continental shelf, 60km west of Geraldton. They are clustered into three main groups — Wallabi, Easter, and Pelsaert — spread across 100km of ocean. Some of the islands are ‘mainland remnants’ isolated by rising sea levels, while others were more recently created from coral rubble. 

The island group forms a nature reserve managed by the WA Department of Fisheries for conservation, tourism and aquaculture, and the waters surrounding the islands are a classified Fish Habitat Protection Area. 

The Commonwealth Abrolhos Marine Park lies to the north and west of the state reserve, covering 88,000 square kilometres to the edge of the EEZ at depths of 15–6000 metres. The seafloor is characterised by a narrow continental slope fringed by submarine canyons providing habitats for deep-water communities and numerous species of bottom-dwelling fish. Meso-scale eddies regularly (three to nine times a year) spin off the Leeuwin Current and combine with seasonal upwellings to transport nutrients and plankton around the islands.

The islands lie in a biogeographic transition zone between 28–29 degrees south, where the Leeuwin Current brings warm, tropical water south along the edge of the continental shelf to mix with colder water from lower latitudes. On Australia’s east coast this transition occurs at 24 degrees south. Water temperatures of around 20–22 degrees throughout the winter enable corals and tropical marine life to thrive far south of their usual range, while Australian sea lions have their northernmost breeding sites here. 

Swept along by the currents, tropical and temperate marine species come together in the islands’ reefs and surrounding waters, resulting in rich biodiversity — 184 species of corals that support 400 kinds of fish, 492 molluscs, 110 sponges, 172 echinoderms, abundant sharks and western rock lobsters, one of the Region’s flagship species.

The islands are the largest seabird breeding area in the eastern Indian Ocean and are among the most important in Australian waters. More than two million birds from 35 resident and migratory species roost in the sand dunes, foreshores, and rocky atolls of the islands. White-bellied sea eagles are the natural predators of smaller seabirds and hunt in unusually large numbers. 

West Coast Canyons 

The edge of the continental slope offshore from Kalbarri to south of Perth is deeply incised by 200 submarine canyons covering almost 9000sqkm. The canyons are pathways along which sediments and nutrients are transported between the continental shelf and the abyssal plain. 

Perth Canyon, 20km west of Rottnest Island, is Australia’s largest undersea canyon and the most prominent in the west coast system for its sheer size and ecological importance. The head of the canyon starts on the shelf at 200m depth and plunges to 5000m along its 6.5km length to the abyssal plain. The canyon spans 1505sqkm with a volume that would fill more than 800 Sydney Harbours. Most of the canyon and several smaller ones are enclosed within the Perth Canyon Marine Park.

The Canyon’s topography includes towering near-vertical cliffs, large mass movement debris, jagged spurs and seabed composition ranging from exposed bedrock to sandy mud. Sediment and organic material drifting down from productive shelf waters collect in the Canyon to support deep-water habitats for a great variety of species, such as deep-sea corals, sponges, starfish, sea cucumbers, sea urchins and anemones, some of which are found only in the Canyon. 

The complex submarine landscape also interacts with deep ocean currents to produce meso-scale eddies and upwellings of cold, nutrient-rich water, sparking the production of tiny plant-like organisms, phytoplankton — the foundation of the marine food web. The plankton blooms nourish masses of krill, squid, and small fish that attract larger fish and predators. Several species of whale congregate here during summer to feast on the abundant food concentrated in and above the canyon, including humpbacks, southern rights and blue whales (the largest animal in the world) which can consume around 40 million krill in a day. 

Geographe Bay

Geographe Bay encompasses a wide north-facing curve of the Western Australian coastline from Cape Naturaliste to Bunbury. Much of it is covered by the Commonwealth Geographe Bay Marine Park (977sqkm), which lies adjacent to the near-shore WA Ngari Capes Marine Park. Although exposed to winter storms from the north-west, the bay is well protected from prevailing south-westerly winds and swells by Cape Naturaliste, making a sheltered waypoint for migratory humpbacks and blue whales. Lying inshore of the Leeuwin Current, the bay more affected by the north-flowing Capes Current, which delivers cooler waters more favourable to temperate fish species, such as salmon, pilchards, herring, and tailor. 

The bay’s relatively shallow seabed (15–70m) is a mix of submerged linear reefs, remnants of the ancient coastline, and partly exposed rocky reefs interspersed with sandy patches that extend into deeper water. The hard, rocky substrates support sponges and seaweed communities, while dense seagrass covers 60 per cent of the bay in some of the largest continuous meadows in Australia. 

One hundred and forty-eight species of fish have been recorded here, while the extensive seagrass beds are foraging grounds for migrating turtles and important nurseries for invertebrates and western rock lobsters. Many species of fish spend their juvenile years among these leafy, sunlit habitats before ranging out to adult feeding grounds along the shelf break.

Naturaliste Plateau

The Naturaliste Plateau is a submerged fragment of the continent — a ‘biogeographical island’ — located 260km west of Cape Leeuwin and Cape Naturaliste. Much of it, and many other ecologically significant features, lies within the Commonwealth South-west Corner Marine Park, which encloses a whopping 271,833sqkm, extending as far east as Esperance. 

Lying in depths between 2000m and 5000m, it is the deepest submarine plateau on Australia’s continental margin. The plateau is roughly rectangular in shape, extending 400km east-west and 250km north-south, covering about 90,000sqkm. Less than one-third of it lies within Australia's EEZ, with its western part within the ‘extended continental shelf’ proclaimed under UNCLOS. It is separated from the Australian continental plate to the east by the Naturaliste Trough, and is bordered by abyssal plains in the south, west and north. It is relatively flat, with large canyons incising its northern slope and steep cliffs on its southern and western sides. This remote feature occurs in an area where several major oceanic currents converge and vie for dominance, interacting with them to create deep meso-scale eddies. There has been little scientific investigation of the plateau due to its remoteness and rough seas, but experts believe many unique and highly diverse species are likely to inhabit its deep-water ecosystems.

Diamantina Fracture Zone

South of the Naturaliste Plateau lies the Diamantina Fracture Zone, Australia’s longest and deepest underwater mountain chain. Diamantina is not a true fracture zone caused by plate tectonics, rather a ridge separating two oceanic plateaus. Its complex, rugged topography covers 100,000 square kilometres in a long series of closely spaced peaks and troughs, with seamounts rising up to 4000m from trenches measured as deep as 7019m, the deepest known point in Australian waters. This dynamic bathymetry creates obstacles to current flow, causing eddies and turbulent mixing of deep-water nutrients and organisms, as well as highly variable environmental conditions, such as depth, temperature and productivity. While little is known about the ecology of this remarkable seascape, scientists suggest its size and physical complexity would support a rich diversity of marine life uniquely adapted to these distinctive deep-water habitats.

Albany Canyons Group

The long continental slope off Western Australia’s south coast is serrated by numerous submarine canyons, of which the Albany Group is the most dramatic. The Albany system comprises 32 closely spaced canyons which cut deeply into 700km of steep slope at regular intervals from Cape Leeuwin to east of Esperance. Some canyons are up to 2000m deep and extend uninterrupted through the shelf break to the abyssal plain 90km offshore.

The canyons receive a high load of sediment and organic material from the shelf that nurtures deep-sea habitats within them and close to their mouths on the abyssal plain. They also funnel nutrient-rich water up to the continental shelf, driven by eddies created by the interaction of the warm east-flowing Leeuwin Current and the deeper, colder Flinders Current flowing towards the west. These periodic upwellings of plankton support schools of fish that attract larger predatory fish, sharks, orcas and toothed whales. The canyons are known to be sites of orange roughy communities and feeding areas for sperm whales.

The Bremer Canyon, half-way between Albany and Esperance, is a known biodiversity hotspot. It is protected by the Commonwealth Bremer Marine Park, which covers 4500sqkm, from shallow coastal waters of 15–5000m at the edge of the abyssal plain. Marine life in its waters includes masses of mackerel, herrings, sardines, and southern bluefin tuna, which in turn attract blue and bronze whaler sharks and orca whales. Humpback whales are regular visitors to the park, as are the southern right whales that come here to calve and nurse their young.

Recherche Archipelago

The Recherche Archipelago stretches about 300km along the south coast of WA, from Esperance east to Israelite Bay at the edge of the Great Australian Bight. It is the most extensive rocky reef system in the South-west Marine Region, comprising 105 islands and 1500 ‘obstacles to shipping’ (reefs, rocks and islets) dotted across 4000sqkm. The cluster of islands at the eastern end of the archipelago are protected by the Commonwealth Eastern Recherche Marine Park, which reaches to the limit of the EEZ.

The islands are dome-like granite outcrops projecting from the Recherche Shelf, a 75km-wide plain that slopes gently to a shelf break at 150m before plunging rapidly to 6000m on the abyssal plain. Fronting the Southern Ocean, the Archipelago is subject to some of the most extreme wave energy on the Australian coast, with unrestrained swells battering the steep-sided islands and abrading the shelf as far down as 100 metres during storms. Submerged reefs provide shelter on the islands’ leeward (north-eastern) side, enabling the accumulation of sediment and nutrients important for commercial fisheries such as abalone and southern rock lobster.

The warming influence of the Leeuwin Current extends as far east as the archipelago where it meets the Cresswell Current flowing in the opposite direction. This interaction generates eddies and upwellings of cold, unusually clear water that allows light to penetrate to depths of 50m for the production of extensive seaweed, kelp and seagrass communities. These habitats support a rich diversity of temperate marine life including 263 species of fish, 347 molluscs, 300 sponges, and numerous soft corals. The islands provide haul-out (resting areas) and breeding sites for Australian sea lions and New Zealand fur seals and protect inshore waters that are important calving areas for southern right whales. 

Western Eyre Marine Park

The Commonwealth Western Eyre Marine Park is located off the South Australian coast, south of Ceduna. It covers almost 58,000 square kilometres from South Australian state coastal waters to the edge of Australia’s EEZ, where the ocean floor lies at more than 6000m. It is adjacent to three SA marine parks — Nuyts Archipelago, West Coast Bays and Investigator — which collectively protect inshore waters from the Western Australian border to Elliston, mid-way down the Eyre Peninsula. The Commonwealth marine park spans three bioregions and includes the Kangaroo Island ‘Pool’ and the Eyre Peninsula upwelling, two significant ecological features that emerge from the region’s complex seabed topography.

The seafloor at this eastern end of the Great Australian Bight is characterised by a long, shallow continental shelf interrupted mid-slope by two extensive terraces. The Ceduna Terrace is 700km long and 200km wide, falling from 200m to 3000m in depth. The Eyre Terrace is smaller and narrower, reaching down to 1600m. Both are intruded deeply at the shelf break by numerous, steep-sided canyons that are most dramatic south of the Eyre Peninsula and west of Kangaroo Island. 

Around these prominent canyons and the adjacent shelf break, undersea currents, winds and bathymetry interact to produce a mass of nutrient-rich water along the 100m depth contour offshore and to the west of Kangaroo Island, known as the ‘Kangaroo Island Pool’. Between December and April, south-easterly winds propagate the ‘Eyre Upwelling’ — a subsurface stream that is driven from the Pool past the mouth of the Spencer Gulf to surface and disburse along the rocky west coast of the Eyre Peninsula.

Both phenomena produce seasonally predictable ‘pulses’ of plankton attracting krill, squid, and small fish alongside a succession of predators up the food chain such as southern bluefin tuna, barracudas, sharks, dolphins, orcas, toothed whales, New Zealand fur seals, and seabirds. In deeper water from the mid-slope down to the shelf break are commercially important species, including blue grenadier, school shark, gemfish, leatherjacket, blue-eye trevalla, ling and warehou. The shelf break adjacent to the canyons yields giant crab and southern rock lobster. 

The West Eyre Marine Park straddles the boundary between the South-west and South-east Marine Regions, providing biological continuity with temperate waters to the east.

For more information about the marine parks in the South-west Marine Region visit: parksaustralia.gov.au/marine/parks/south-west.