The North-west Marine Region

Packed full of natural wonders, the North-west Marine Region will be the apple of your eye

The North-west Marine Region comprises all Commonwealth waters offshore from Kalbarri, WA, to the Western Australia/Northern Territory border, and seaward 200 nautical miles (370km) to deep ocean at the edge of Australia’s Exclusive Economic Zone (EEZ). 

Within the Region are 13 marine parks that collectively cover more than 335,437 square kilometres — Kimberley, Cartier Island, Ashmore Reef, Argo-Rowley, Mermaid Reef, Roebuck, Eighty Mile Beach, Dampier, Montebello Island, Gascoyne, Carnarvon Canyon, Shark Bay, and Ningaloo. 

THE PHYSICAL ENVIRONMENT

The physical structure of the North-west seafloor is the product of a complex geological history dating back 300 million years. Its dominant feature is a broad continental shelf, widest (400km) off the Kimberley-Pilbara coastline and tapering to Australia’s narrowest margin (10km) off North West Cape near Exmouth. Overall, the shelf is relatively shallow, with depths of less than 200m, but the Argo and Cuvier abyssal plains lie beyond the shelf in depths of almost 6000m.

The seafloor also contains many topographic features important for determining the different ecosystems — canyons (Cape Range, Cloates, Carnarvon, and Swan), plateaux (Exmouth and Scott), coral reefs (Ashmore, Cartier, Rowley, Dampier and Ningaloo), terraces (such as the ‘ancient coastline’), ridges, banks and shoals.

Major surface currents exert a strong influence in these waters. The Indonesian Throughflow dominates, delivering warm low-salinity water from the western Pacific Ocean through the Indonesian archipelago to the Indian Ocean. As well as providing a key link in the exchange of water and heat between the oceans — an essential element in the global climate system — it is the primary driver of oceanographic and ecological processes in the region. The North-west is also subject to extreme tides, especially in the north where semi-diurnal tides range from 3m at neap to over 10m at spring.

A common feature of the region’s monsoonal climate is the high incidence of cyclones between December and March. These both produce large influxes of freshwater run-off from the Kimberley and Pilbara coasts, and enhance the upwelling and dispersal of plankton. At their extreme they can also have a major effect on habitats and species distribution.

BIOLOGICAL DIVERSITY

The Region is home to many different species, but the number of endemic species is small compared to Australia’s cooler and more temperate southern waters. The majority of species are tropical and typical of those found in other parts of the Indian Ocean and western Pacific. 

While the Region generally has low productivity, with boom and bust cycles driven by the monsoons, some locations have consistently higher productivity — Ningaloo Reef and associated canyons, Ashmore and Scott coral reefs, the Rowley Shoals along the shelf edge, and the carbonate banks of the Sahul Shelf. 

The high species richness and diversity reflect the interaction between currents and the variety of habitats, which range from hard limestone pavements, coral reefs and seagrass meadows to sandy seabeds and muddy, soft-sediment deep-water plains.

Each year thousands of humpback whales migrate to reproduce off the Kimberley coast, especially around Camden Sound. Whale sharks congregate in prolific numbers at Ningaloo Reef and the dugong population in Shark Bay is the largest in the world. Five of the world’s seven species of marine turtles, including the endangered hawksbill, have large breeding rookeries on coastal beaches and offshore islands. The Region also lies astride international flyways for millions of shorebirds that migrate every year from breeding grounds in the Northern Hemisphere. 

A wide variety of crustaceans occur in habitats ranging from intertidal zones to deep slope waters. Small plankton-eating fish comprise a third of the Region’s total fish biomass, and are preyed upon by large pelagic fish, seabirds and marine mammals. Many pelagic species are highly mobile, migrating between the southern oceans and spawning grounds in tropical waters off Java. One hundred and fifty-seven species of high order predators, like sharks, skates, and rays occupy a range of habitats, with some species patrolling pelagic waters, while others prefer the seafloor. Sharks in the Region represent about 19 per cent of the world’s species. 

KEY ECOLOGICAL FEATURES

In the North-west Marine Region, thirteen ecological features have been identified as being of key importance in promoting biodiversity. Eight of these are represented in the Commonwealth marine parks.

ANCIENT COASTLINE

The Northwest Shelf off the Pilbara coast contains several terraces that reflect gradual changes in sea levels during the last 100,000 years. The most prominent of these is a rocky escarpment cut by wave action about 12,000 years ago, now submerged at a depth of 125m, which transects the Montebello Marine Park. 

Its complex topography enables vertical mixing of the water to produce a nutrient-rich environment for sponges, corals, crinoids, and molluscs. The abundant marine life associated with these communities attracts large pelagic fish and high order predators. It has been suggested that humpback whales, whale sharks and other migratory species use this escarpment as a guide. 

ASHMORE REEF AND CARTIER ISLAND 

Ashmore Reef and Cartier Island form part of a series of submerged reef platforms along the shallow outer edge of the Region’s continental slope, 350km off the Kimberley coast. Rising from a depth of more than 100m and covering 239 square kilometres, Ashmore is the largest of the three emergent oceanic reefs in the north-eastern Indian Ocean and is the only one with vegetated islands. 

Forty-five kilometres south-east of Ashmore, Cartier Island Marine Park covers 167 square kilometres, with one unvegetated sand cay and a flat reef enclosing two shallow pools.

With 225 kinds of corals, the reefs are richer in such species than anywhere else on the WA coast, and support a wide range of molluscs, crustaceans and echinoderms. More than 700 species of fish inhabit the reefs, along with high numbers of sea snakes across 13 species. Ashmore and Cartier also provide important seasonal nesting sites for large colonies of migratory seabirds and wetland waders, in numbers exceeding 50,000 over 56 species. A genetically distinct dugong population shares seagrass meadows on the reef flats with green and hawksbill turtles.

MERMAID AND THE ROWLEY SHOALS 

The Rowley Shoals are a collection of three atoll reefs — Mermaid, Clerke and Impereuse — that rise steeply from depths of about 400m on the edge of the Argo-Rowley Terrace, 300km north-west of Broome. Mermaid is the only reef entirely within Commonwealth waters and is enclosed by its own marine park. Lying south-west of Mermaid, Clerke and Impereuse are in State waters and constitute the Western Australian Rowley Shoals Marine Park.

Despite sharing the same undersea platform with the Ashmore, Scott and Seringapatam Reefs, more than 400km north-east, the Rowley Shoals are different from the other reefs on the outer shelf, both in structure and biological diversity. Although large areas of the atolls fall dry at low tide, Clerke and Impereuse have small, bare sand cays above high water when Mermaid is totally submerged. The coral rim of each atoll encloses a lagoon of 80–90 square kilometres.

The reefs of the Rowley Shoals are noted for their high species diversity. Surveys of the reefs have identified 214 species of corals, 264 molluscs, 82 echinoderms and 530 species of finfish — unique sponge communities are also found on each of the reefs. Productivity is enhanced by internal (subsurface) waves generated by tides and currents that raise and mix nutrients from depths of 500–700m into the water circulating throughout the reefs. 

Steep changes in slope around the reefs also attract a range of migratory pelagic species such as dolphins, tuna, billfish and considerable numbers of sharks. Significantly, sea snakes do not occur here.

DEMERSAL FISH COMMUNITIES OF THE CONTINENTAL SLOPE 

The continental slope provides important habitats for demersal (bottom-dwelling) fish, in two distinct communities — the upper slope in depths of 225–500m and the mid-slope at 750–1000m. These communities are comprised of ‘infauna’, such as clams and burrowing worms that live within the seabed, and ‘epifauna’ such as starfish, urchins, and sea cucumbers that live on top of it. Both kinds of organisms rely on bacteria and detritus for food and, in turn become prey for crustaceans and a range of fish like flounder, cod, herring, salmon and perches. Predators include carnivorous fish, deepwater sharks, large squid and toothed whales. Demersal communities on the continental slope of the North-west Region contain more than 1000 species of fish, of which 15 per cent are found nowhere else.

CANYONS

Within the North-west Marine Region there are two areas of seafloor canyons that are ecologically significant. Two hundred and seventy kilometres north-west of Broome, the Argo-Rowley Terrace Marine Park spans 146,000 square kilometres of the continental shelf to depths from 220–6000m. 

Within its vast reaches, 50 million-year-old canyons link the Argo Abyssal Plain and Scott Plateau, cutting deeply into the south-west margin of the plateau. The largest of these are the Bowers and Oates Canyons, in whose inky ridges live communities of invertebrates and micro-organisms that feed on particles falling from the ocean above. The canyons channel upwellings of cold, nutrient-rich water from the abyssal plain to the plateau, attracting large aggregations of fish and larger predators like sharks, dolphins and toothed whales. 

In the south closer to the coast, canyons link the Cuvier Abyssal Plain with the continental slope along the edge of Exmouth Plateau, adjacent to Ningaloo Reef. The canyons collect particles drifting down from the plateau to form soft-sediment habitats on their floors and sides for fish like deepwater snapper. Strong internal tides create upwellings to the canyon heads, where the Leeuwin Current mixes the cool, higher-nutrient waters with the warmer ocean above the plateau. In these eddies congregate many different kinds of marine life, enhancing the biodiversity of the Ningaloo Reef.

EXMOUTH PLATEAU

The Exmouth Plateau occupies a vast area of the North West Shelf, west of the Cape Range Peninsula. Much of it is protected by the Gascoyne Marine Park, which covers 82,000 square kilometres to the limit of the EEZ. 

The plateau’s surface is rough and undulates at depths of 500–5000m. Its northern margin is steep and intersected by deep canyons, such as the Montebello and Swan canyons, while the western part is moderately steep and smooth, and the south slopes gently with no canyons. It is bounded by the Argo, Cuvier and Gascoyne abyssal plains.

Biological productivity of the ocean above the plateau is generally low because of the lack of nutrients in the water. However, the plateau’s bulk constitutes a physical obstacle that modifies the flow of deep-ocean currents, generating internal waves and upwellings of nutrient-rich water through the canyons onto the plateau. This enhanced productivity is seasonal and occurs in sporadic bursts. 

The plateau’s surface is covered by sediments and detritus that create a soft-bottom environment supporting communities of filter-feeders, soft corals, sponges, starfish, urchins and sea cucumbers. The canyons also act as conduits for delivering sediments and other material downwards to sustain deep-sea communities on the slope and abyss at the base of the plateau.

Across the plateau, molluscs and crustaceans scavenge the seabed, while above them masses of small plankton-eating pelagic fish are preyed upon by larger fish and predators such as tuna, billfish, sharks and dolphins. The deeper waters of the Montebello Trough at the edge of the plateau are an important feeding site for sperm whales. Migrating whale sharks, marine turtles and several species of whales traverse the region and feed on and around the adjacent Ningaloo Reef.

NINGALOO MARINE PARKS

Between Exmouth and Coral Bay, two Ningaloo Marine Parks jointly conserve the World Heritage-listed Ningaloo Reef, which stretches for almost 300km along the west coast of the Cape Range Peninsula. It is Australia’s longest fringing barrier reef and the only extensive coral reef in the world on the western side of a continent. 

Declared separately under Commonwealth and Western Australian laws, the two marine parks cover a total of 5070 square kilometres and are managed cooperatively as a single unit for both governments by the WA Department of Conservation and Land Management (CALM) and the Department of Fisheries.

From its crest, the reef drops gently through depths of 8–10m to an outer edge heavily dissected by spurs of coral formations projecting into the ocean, interspersed with sand channels. Beyond the reef edge, the northern part of the shelf is narrow, less than 10km from the coast, and drops rapidly from 30m to over 500m. At its southern end, the shelf extends more than 30km offshore. The shelf margin is riven by a series of canyons that channel upwellings of colder, nutrient-rich waters from the abyssal plain onto the shelf. These internal waves are generated by the interaction of the Leeuwin and Ningaloo Currents that vie for dominance in different directions throughout the year, producing seasonal ‘pulses’ of biological productivity on and around the reef.

The reef lies entirely within State waters, which extend from the shoreline to approximately 3 nautical miles seaward of the edge of the reef, and the Commonwealth reserve continues a further 3–9 nautical miles, widening south of Point Cloates. Together, the marine parks encompass the entire range of marine habitats in the region, ranging from rocky shores, sandy beaches and intertidal mangroves through the coral reef and its large inshore lagoon to open ocean and deep-water environments at the edge of the continental shelf. 

Ningaloo Reef is internationally renowned for its high diversity of corals (over 200 species) and abundant marine life that includes 500 species of fish. It is one of the few locations in the world where whale sharks gather regularly in significant numbers. Reaching lengths of more than 12m and weighing more than 11 tonnes, as many as 500 of these gigantic filter feeders gather here between March and June when spawning coral and plankton blooms provide a rich source of food.

Humpback whales transit the outer reef waters twice a year on their annual migration between calving grounds off the Kimberley and feeding grounds in Antarctica, and several other species of whale are often seen further offshore. The reef system also supports congregations of manta rays during autumn and winter, as well as resident populations of dolphins, dugongs, and marine turtles.

In the open ocean off the Ningaloo coast are found many kinds of pelagic fish including trevally, tuna, mackerel, marlin, and sailfish, often occurring much closer to shore of the Ningaloo coast than in other parts of Australia due to the narrow continental shelf. These abundant fish populations attract large predators such as the oceanic white-tipped shark, tiger shark, and blue shark. The seabed of the continental slope and shelf provides soft sediment environments for gardens of sponges, soft corals and gorgonian fans inhabited by gold band snapper, sea perch, red emperor, ornate rock lobster, comet grouper and potato rock cod.

Ningaloo lies on the migration route of many trans-equatorial shorebirds and waders, and the offshore waters provide valuable feeding grounds and resting places for about 30 species of migratory seabirds of international significance.

SHARK BAY MARINE PARK

Shark Bay Marine Park spans 7500 square kilometres of Commonwealth waters, about 60km offshore from Carnarvon, and connects the Shark Bay World Heritage Area with the deeper ocean to the west. The park lies over the Dirk Hartog Shelf and varies in depth from 15–220m. 

Its warm, shallow waters are an important resting site for humpback whales and their calves on their southern migration, and Dirk Hartog Island at the mouth of the bay is a critical habitat for the largest breeding population of loggerhead turtles in Australia. The seagrass meadows of Shark Bay support as many as 10,000 dugongs in what is thought to be their largest population in the world. There is also a large resident community of bottlenose dolphins which are a feature of eco-tourism at Monkey Mia. The marine park is also culturally significant for protecting some 20 historic shipwrecks that lie in its waters.

BIOREGIONAL TRANSITION

The Ningaloo and Shark Bay Marine Parks occupy a significant transition zone between tropical northern and temperate southern bioregions, with a mix of marine plants and animals common to both and several tropical species reaching the southern limit of their distribution. The composition of species is greatly influenced by the south-flowing Leeuwin Current that brings in tropical Indo-Pacific species, and the north-flowing Ningaloo Current that transports temperate species from southern waters, providing a strong link with the South-west Marine Region.

For more information on the North-west Marine Region visit: parksaustralia.gov.au/marine/parks/north-west.