Populations of egrets and other waterbirds in the Murray-Darling Basin of eastern Australia are declining. River regulation and increasing frequency of droughts are considered to be mainly responsible by reducing the frequency and extent of flooding to temporary wetlands. However, other changes have occurred to wetlands, such as the introduction of non-native fish, widespread severe declines in the abundance of native fish, increased salinity, sediment and nutrient concentrations, and removal of aquatic vegetation by livestock grazing. Current knowledge of the diets of egrets is inadequate for an assessment of any ecological implications arising from these changes. In this study the breeding season diets of Eastern Great Egret Ardea modesta and Intermediate Egret A. intermedia were quantified at a colony in the Murray Basin. Eastern Great Egrets depended mainly on introduced Common Carp Cyprinus carpio (94% of diet biomass), a species regarded as a severe pest of river systems and wetlands within the Basin. Attempts to eliminate carp, without adequate restoration of native fish populations, could result in further long-term declines in egret breeding success and survival. Intermediate egrets depended mainly on adult and larval frogs (80% of diet biomass), species that are under threat from habitat degradation and predicted increases in the salinity of freshwater wetlands. A holistic approach to the restoration of wetlands, involving the integration of improved flooding regimes, pest fish management and reinstatement of native fish and amphibians, salinity control and restoration of aquatic vegetation will be needed to ensure the future of egrets and other fish-eating birds in the Murray-Darling Basin.
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Vol. 31 • No. 4