The dwindling river: Problems with introduced species

The Murray River, Australia's longest navigable waterway, is home to a diverse range of ecosystems, native animals and plants. Over time, introduced species of animals and plants have altered and damaged the biodiversity of the Murray. Rabbits, foxes, goats, pigs and cats are amongst the introduced species that have established along the River.

There are eleven introduced fish species in the Murray-Darling system. Of these, six are regularly found in South Australian waters including brown trout, rainbow trout, redfin, carp, goldfish and mosquito fish. These introduced fish have become dominant and compete aggressively with native fish for food and habitat, as well as introducing new diseases, parasites and pathogens which have the potential to destroy native species.

The European Carp (Cprinus carpio) is particularly problematic. It is an omnivore (an animal which feeds on many types of food including both plants and flesh) that forages on the riverbed, stirring up sediment, dislodging water plants, and creating high levels of turbidity. Their activity also damages riverbanks and irrigation drains. Carp behaviour is detrimental to the Murray environment as it destroys the habitat for native fish, invertebrates and waterfowl.

The European Carp is only one form of environmental degradation affecting native fish. Many introduced plants cause significant damage to the river system. These include the water hyacinth, lypia and willow.

The water hyacinth is a South American plant which home gardeners choose for use in fish or lily ponds, but has been recognised as the world's worst aquatic weed. It was introduced into Australia as an ornamental plant in the late 19th century. However, water hyacinth spreads quickly and chokes rivers and their tributaries, blocking off the oxygen supply needed by native plants and fish. This occurs particularly when the plant dies and the decaying process uses up the oxygen in the water. If not controlled, the water hyacinth weed threatens to block parts of the Murray-Darling system as it has in parts of the Mississippi and Amazon.

Lypia is another weed seriously impacting the environment along Murray-Darling, particularly in Queensland and New South Wales. It is a major threat to stream banks and waterways as it forms dense carpets, preventing the growth of other riparian vegetation. This results in decreased bank stability and increased soil erosion, affecting the overall health and quality of the waterway.

The willow, despite being attractive and familiar along the river's banks, is a plant which threatens the river system. Willows were planted by European settlers for several reasons which include aiding paddle-steamer navigation by marking the main channels in the 1860s, and the stabilisation and prevention of erosion of the banks which occurred due to the clearing of riverside and catchment vegetation. Willows were also introduced to the banks of the River Murray for their aesthetic appeal. At least three willow species have naturalised along the Lower Murray in South Australia: the Weeping Willow (S. babylonica), Basket Willow (S. rubens) and Crack Willow (S. fragilis). These willows do provide food and habitat for some riverine life, but they compete with native gums for water, and some plants, fish, and animals such as platypus and tortoises are discouraged by their dense shade. Many native fish, such as the Murray Cod, rely on snags provided by River Red Gum branches for spawning sites and shelter. Willows may also increase the river's salinity problems as they only take water from the upper fresh layers, allowing salt to rise into the river, whereas red gums take 40-50 per cent of their water from these layers, helping to keep the saline water table low. As the willow is deciduous (losing its leaves in autumn), it deposits large amounts of organic matter into the river quite quickly. These extra nutrients encourage the growth of algae (such as Blue Green Algae) to an already slow moving river, and can poison native animals.

Further Reading

Lazarides, M. CSIRO handbook of Australian weeds, Collingwood, Vic.,CSIRO Publishing, 1997

Sainty, G.R. Waterplants in Australia: Australian water weeds, Darlinghurst, N.S.W.: Sainty & Associates, 1988

Water Research Foundation of Australia. The menace of water hyacinth and other aquatic weeds: proceedings of a symposium held at Australian Mineral Foundation, Adelaide, on 1st April, 1977, Adelaide, 1977.

Links

Atlas of South Australia 1986 See: Environment resources: Environmental change

Feral.org.au See: Species: Carp

Native fish Australia

New South Wales Department of Primary Industries See: Fishing and aquaculture: Pests and diseases: Freshwater pests: Carp

NOVA [Australian Academy of science] See: Environment: Sustainability: Toxic algal blooms - a sign of rivers under stress

River Murray Urban Users Committee See: Resources: RMUUC Fact Sheets: A river in decline

Save the Murray See: Environment: Flora and fauna