North Arm Westernport
North Arm Westernport is impressive in its different habitats right across the area. There are black and sulphurous mudflats, salt marsh habitat and mangroves, sea grass meadows, and deep underwater canyons and reefs. With each of these habitats providing a home or feeding ground to some diverse fauna, it is certainly an important marine region in Victoria.
What are the natural values of the area?
- Western Port to Melbourne’s south-east is Victoria’s second largest bay covering 680 square kilometres and containing two of Victoria’s largest islands, French and Phillip Islands.
- Western Port occupies part of a depression caused by faulting along the edges of the Bay, between the Mornington Peninsula to the West and the South Gippsland Highlands to the East. Erosion and deposition from runoff and rivers have altered the coastline over time. The depression has been submerged by marine waters, and the coastal and sea floor have been shaped by waves and currents.
- Westerport is essentially a marine, rather than estuarine inlet (as is Port Phillip). Today vast quantities of muddy sediment in and around the bay form shoals and marshlands. The mud (consisting of silt, clay and organic matter) is partly from river sediment washed into the bay, and partly the result of reworking by waves and tidal currents of fine-grained material derived from outcrops around and beneath the bay.
- Western Port is subject to the maximum tidal range on the Victorian coast, of up to three metres. The tides are the key to understanding Western Port’s marine life. As the oceans rise and fall with the moon, seawater is flushed in and out of bays like Western Port. These tidal currents take time to wind around the islands and there is quite a time lag between high tides at the mouth and head of the bay. Consequently the difference in height between low and high tides is be much greater than along the open coast, and can reach over 3 m towards the north.
- Because of the relatively sheltered waters of the open coast, mud has formed extensive layers across much of Western Port. These muds are generally black and have a strong sulphur smell, because little oxygen is able to penetrate them. More than 270 square kilometres of mud flats are exposed at low tide and other areas of the bay are very shallow.
- As water streams off these flats, it forms a network of channels in the soft mud. These channels gradually join together to form large channels that eventually drain into the deeper areas in the south.
- Around the fringe of the bay are large areas of mangrove, saltmarsh and tee-tree swamp. The North Arm of Western Port Bay contains significant and unique intertidal mudflats and channel habitats, as well as extensive seagrass beds, mangrove and saltmarsh. The area’s well developed tidal channel system of varying depth, profiles and orientations, contributes to the high diversity of habitats.
- The mudflats of this area are nationally significant as feeding habitat for wader birds. They are also of State geomorphological significance.
- The northern shore of French Island has one of the most extensive areas of saltmarsh and mangrove communities in Victoria. These plants are unusual in that they are able to cope with the highly salty conditions of this environment. Mangroves are also able to cope with living in thick, airless mud. Mangroves are a vital part of the bay ecosystem and provide important habitat for numerous invertebrates including crustaceans (crabs, shrimps and sand hoppers), marine snails and bivalves, adult and juvenile fish. Unfortunately, these important habitats are not adequately protected by marine national parks or sanctuaries.
- Quail and Chinaman Islands are considered to be of State botanical and zoological significance. The relatively undisturbed mangrove and saltmarsh habitats of Watson Inlet and Quail Island are also of State significance as some of the most intact communities in Victoria.
- Another important feature in northern Westernport is Crawfish Rock, which has deep reefs, pinnacles and canyons and underwater channels with extremely high conservation value.
- Extensive seagrass meadows are found in the area, including some areas where little loss of seagrass has been recorded. Seagrasses have disappeared from more than 80 per cent of Western Port during the 1970 and 1980’s. No single cause has been identified for this loss, but a reduction in the amount of light reaching the seagrass is often the culprit. This can be caused by an increase in the amount of sediment in the water, or from a bloom in the algae growing on the leaves, caused in turn by increased levels of sewerage or fertilisers entering the sea.
- Seagrass loss remains a major concern for Western Port. Seagrasses are extremely important habitats and play a major role as nursery areas for many fish species, including commercially and recreationally important species such as King George Whiting, Bream and Mullet. The boundaries of the existing French Island and Yaringa Marine National Parks must be extended to protect these critically important habitats.
- The most developed and extensive Victorian mangrove populations occur in Westernport Bay. White mangroves are the only mangroves that grow in Victoria and are actually flowering plants that have evolved strategies for surviving in a challenging environment. They can survive in very salty soils and mud by actually taking up salt through their roots and getting rid of it through specialised salt glands on the back of their leaves. They can grow in the thick airless mud because of a series of breathing roots that let them get oxygen directly from the air.
- The mangroves are vital habitat for the life cycles of crabs, shrimps, sand hoppers, marine snails and bivalves, and well as important feeding areas for adult and juvenile fish. Scavenging and filter-feeding animals do not eat the mangroves directly, however, mangrove leaves that fall into the water decompose to form detritus, which is a major food these animals in the Bay.
- Many small animals also live in the mud and seagrass beds. The mudflats support a wide diversity of deposit feeding animals including worms, and bivalve molluscs like pippies. These animals convert the debris that accumulates in the bay into animal tissue which is then available as food for animals like birds and fish.
- Fish gorge themselves on these animals and the birds feed on them all. Over 65 per cent of Victoria’s birds have been sighted around the bay. Western Port as a whole is very important as a bird habitat, and over 295 species have been recorded in the area. These include large populations of Black Swans, Pied Oystercatchers and Royal Spoonbills.
- Many of the migratory wader birds that spend summer in Victoria depend on Western Port because their main habitat requirements are mudflats for foraging and high tide roosts where birds wait for the next feeding period. Up to 15,000 wading birds and 10,000 black swans feed on the flats and seagrass meadows.
- Westernport is recognised as one of the world’s most precious areas for wading birds, some of which migrate all the way to Siberia. These include the Eastern Curlew, whose non-breeding birds migrate from north-eastern Asia, the Bar-tailed Godwit, which breeds in Arctic regions of Eurasia and Alaska, and the Curlew Sandpiper, which breeds in the Arctic regions of eastern Siberia.
- The French Island Marine National Park includes the waters around Barralier Island, which is a significant feeding habitat for the 32 species of migratory waders and one of the bay’s 13 high tide roosts. This habitat is particularly sensitive to disturbance. This island is an important high tide roost for birds, but due to its location on the very edge of the park it is not adequately protected.
- The marine community at Crawfish Rock is unique, with very high invertebrate diversity. A number of threatened species make their homes here. The area qualifies to be listed as a threatened community because of the unique and vulnerable nature of the marine community that it supports, which includes threatened Hydroids.
- Other rare species found in the area include three species of threatened Ghost shrimp (Michelea microphylla, Eucalliax tooradin and Paraglypturus tooradin), a threatened Sea-cucumber (Apsolidium handrecki), and a threatened Snapping shrimp (Alpheus australosulcatus).
Marine Protected Areas:
Yaringa Marine National Park
French Island Marine National Park
Western Port Ramsar Site