Fisheries officers are attempting to eradicate introduced fish from some Perth waterways following research that suggests less than five per cent contain only native species.
Initial results from a survey of lakes in the metropolitan area suggest that more than 80 per cent have only introduced fish, while between five and 10 per cent contain a mix of both native and feral species.
Department of Fisheries acting principal research scientist (freshwater) Rodney Duffy says the survey has so far uncovered more species of introduced fish than native fish.
He says at least two introduced fish discovered – the convict cichlid (Amatitlania nigrofasciata) and a species of platy – had not been recorded previously in WA.
At Lake Marmion in Myaree, an attempt is underway to eradicate the eel-tailed catfish (Tandanus tandanus) after laboratory tests revealed the fish is carrying a bacterial disease never seen before in Australia.
Department of Fisheries aquatic biosecurity officer Bruce Mackay says the catfish is an aggressive predator and was carrying Pseudomonas anguilliseptica, also known as red spot disease.
He says the population was well established in the lake and was "breeding like crazy".
Mr Mackay says in the 12 months leading up to the eradication attempt fisheries officers conducted a "fish down", in which thousands of catfish were removed using nets.
Then in February the fish-specific toxin rotenone was applied to the lake.
"Prior to the eradication attempt in just one night's netting we pulled out 500 fish," Mr Mackay says.
"Directly after the rotenone application with an even greater number of nets… we only pulled out five fish and they were tiny, less than a 5 cent coin in length."
The next step is to remove the catfish completely and Mr Mackay hopes to one day restock the lake with native species.
Prevention key to management
He says it is important to focus on prevention to stop introduced species becoming established in the first place.
"If a pest species becomes established and starts spreading into a full river system the chances of eradication are just about zero and the costs would be astronomical," Mr Mackay says.
Dr Duffy says the survey project has assessed more than 400 water bodies since the start of last year and he hopes the total number, 600-700 in all, will be completed by the end of this year.
He says a lot of the feral species seem to be aquarium releases but with others it was less clear how they happened.
"For instance carp in a lot of the lakes, often they might be put in by a community member or a developer to enhance the amenity," Dr Duffy says.
Explore further: Climate change costing soybean farmers