Disturbing new research on the food of waterbirds suggests we may need to reconsider our policy on the management of carp.
According to a senior aquatic ecologist any plans to eradicate carp from the Murray Darling Basin need to consider the effects this could have on Australia’s fish eating waterbirds as many have come to depend on carp as a food source, as a replacement for dwindling native fish stocks.
“Birds are good environmental monitors and can often tell us when things start to go wrong,” says Dr Iain Taylor, a principal researcher with the Institute for Land, Water and Society – Charles Sturt University’s institute for environmental, social and economic research.
Dr Taylor is concerned that if we are successful in removing carp from our waterways before native fish have been properly re-established, up to 10 species of Australian water birds such as the Great Egret could suffer very severe population reductions.
“We need to control carp and other introduced fish but the best way to do this is to tackle the management of the entire system, to reinstate native fish populations and be aware that dealing only with one problem may cause another,” says Dr Taylor.
Dr Taylor and Mike Schultz, from the Fivebough and Tuckerbil Wetlands Trust, recently conducted the first detailed study in the Murray Darling Basin on the diet of egrets – two species of waterbirds, the Great Egret and the Intermediate Egret–at a breeding site in the Murrumbidgee Irrigation Area near Leeton in NSW.
“At present the two species are not threatened but their numbers are declining,” says Dr Taylor.
While the sample study was conducted at only one site, Dr Taylor says given the fact that carp now dominate the waterways in the Murray Darling Basin and that native fish numbers are very low, the results “almost certainly” represent what you would expect elsewhere.
They found that 96% of the weight of the food taken by the Great Egrets was fish, almost all of which was carp, with smaller amounts of goldfish and mosquito fish, also introduced species. There were no native fish.
The study found that the Intermediate Egret depends almost entirely on frogs and tadpoles to feed its chicks.
“This indicates to us that the Intermediate Egret is also vulnerable,” says Dr Taylor. “In Australia, the increasing salinity of our wetlands is a big threat as frog eggs and tadpoles have limited ability to cope with high levels of salt.
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