Location of Mud Islands
Port Phillip Bay lies in a sunkland formed by faulting and movement of the earth's surface during past geological eras. This low-lying area is a natural discharge point for the rivers draining southern central Victoria. It was a swampy lake even when sea levels were lower and the Yarra entered Bass Strait near Queenscliff. As the sea rose, it filled the sunkland, and wetlands formed further north at the deltas of a number of rivers and creeks.
The area contains five sites of State geomorphological significance, one of which is Mud Islands - Ridges and Lagoons.
Significance: This is the only known locality in Port Phillip Bay where consolidated dune rock is exposed above high water mark. The outcrop of cemented beach rock is the only known occurrence in Port Phillip Bay and is unusual on Victorian coasts. Mud Islands is the most obvious surface expression of the Port Phillip Sands, the shoal area overlying the Nepean Bay Bar. The islands are an unusual feature in Victoria and superficially resemble an atoll.
Swan Bay, Mud Islands, Werribee-Avalon, Lake Connewarre and Hospital Swamps are all recognised as wetlands of international importance to migratory waders (Watkins 1993).
One per cent of the known Australian population of four migratory wader species: Pacific Golden Plover, Grey
Plover, Mongolian Plover and Ruddy Turnstone have been recorded at Mud Islands and the islands are used as
a high tide roosting area by 5 % of the Victorian populations of Red Knots, Great Knots, Eastern Curlews and
Bar-tailed Godwits. Nearly one quarter of the White-faced Storm Petrels in Victoria breed on Mud Islands
Pt. Cook Metropolitan Park is a site of State botanical significance. This is the only Reserve in the western region of Melbourne that contains examples of four vegetation types in proximity (dune vegetation, salt marsh, swamp and grassland). The salt marsh is an important habitat for the rare Orange-bellied Parrot. Several sites around the bay (including Mud Islands) contain Grey Glasswort (Halosarcia halocnemoides) which more commonly occurs in northwestern Victoria.
Port Phillip Bay is home to a vast number of birds dependent on its coastal wetlands and sheltered waters. The area is of international significance due to the presence of large numbers of migratory wading birds, seabirds and because of its importance to waterfowl the endangered Orange-bellied Parrot. It is the sixth most important area in Australia for migratory waders and the most important in Victoria. It is also of national significance due to the large number of different bird species (many of them relatively rare) and the large concentration of cormorants, Pied Oystercatchers, Banded Stilts and Red-necked Avocets. The presence of large numbers of terns, crakes, rails, coots, Great Crested Grebes, Straw-necked Ibis and Royal Spoonbills also give it State significance.
Seabirds are a prominent feature of the birdlife, particularly in southern waters where shearwaters, skuas, albatrosses, prions and petrels feed. About 5500 White-faced Storm-petrel, one-quarter of the Victorian population, breed on Mud Islands and nearby South Channel Fort Island.
Eleven species of tern have been recorded in the bay, including unusually large numbers of migratory Common Terns and lesser numbers of the rarer Arctic Tern. One of the largest breeding colonies (nearly a thousand) of Crested Terns in Victoria is situated on Mud Islands and a few Caspian Terns also nest there. Fairy Terns also breed at several locations including Mud Island, the Spit and Swan Bay.
Three species of gull occur in Port Phillip Bay, the Silver, Pacific and Kelp. The Silver Gull is the most conspicuous and breeds in the area, at Mud Islands. Its numbers have increased dramatically in the last thirty years, possible due to an increased availability of food associated with nearby urban areas and rubbish tips.
Waders are the most numerous of the birds of Port Phillip. Most spend the spring, summer and early autumn in the bay, and banding has shown that the same individuals return to the same part of the bay every spring. Between 48000 and 65000 waders feed on the shores of the bay during summer, making it the sixth most important site for these birds in Australia.
The bay holds more than 1% of the known Australian population of fourteen species: Pied Oystercatcher; Grey, Lesser Golden, Mongolian and Double-banded Plovers; Banded Stilt; Red-necked Avocet; Ruddy Turnstone; Eastern Curlew; Greenshank; Marsh, Sharp-tailed and Curlew Sandpipers and Red-necked Stint. It holds more than 5% of the Victorian population of another dozen species: Sooty Oystercatcher; Large Sand and Redcapped Plovers; Whimbrel; Wood and Common Sandpipers; Grey-tailed Tattler; Latham's Snipe; Bar-tailed and Black-tailed Godwits; and Great and Red Knots.
Most waders occur on the sites along the western side of the bay, where five of the wetlands - Altona, Werribee-Avalon, Point Henry, Swan Bay Mud Islands - can be considered of international importance on the basis of their wader populations alone.