'Giant' feral goldfish invade WA's river systems

Scientists remove a feral carp from the Swan River.

The number of invasive fish species that have been dumped in Perth's waterways has more than doubled in the past decade, according to Murdoch University fish expert.

Dr David Morgan says scientists have now discovered that the pests, which kill or maim native species and compete for limited space and food, are introducing disease and parasites to the river system.

"Recently we have found a few diseases which have not been found in Western Australia before," Dr Morgan said.

"There is the ectoparasite Lernea cyprinacea, a little crustacean that anchors onto most of the native species, has spread across the Southern River and Canning and into the Serpentine and Murray Rivers.

"We are a bit concerned about that one and that's just one we know of, there are bound to be viruses and all sorts of things that we really don't know anything about that come with introduced species."

The feral fish find their way into the river system after ignorant homeowners dump unwanted aquarium species into waterways near their homes.  Those waterways then flow into the Swan and Canning Rivers.

Dr Morgan said that his team had discovered 40 centimetre-long feral goldfish that weighed up to two kilograms.

"In WA, we have the fastest growing goldfish in the world," Dr Morgan said.

"They just eat and eat and grow and grow."

The giant goldfish stir up soil sediment and nutrients on the river floor, sparking and acerbating deadly algal blooms.

"There has been some work done elsewhere that has shown that some dead blue-green algae passing through a goldfish's intestine can get reinvigorated and begin to grow again," Dr Morgan said.