They’re known as the rabbits of Queensland’s rivers. Tilapia were introduced into Australia in the 1970s through the aquarium trade, and these African exotics are now one of the country’s most destructive pest fish.

“They’re like little bulldozers in a river,’’ says aquatic ecosystems biologist Dr Dean Gilligan. “They dig around in the bottom of rivers, pull out vegetation, stir up mud and generally trash the habitat for native species. They’re also bullies. They’re extremely aggressive toward native fish – and, unfortunately, can breed up into a very large biomass, just like carp.”

Gilligan is a senior fisheries research scientist with the NSW Department of Primary Industries, and leads the CRC’s inland water pests research program, whose focus is to develop new technologies to detect and better control pest fish.

With researchers at the University of Notre Dame in Illinois, US, scientists from the Queensland Department of Agriculture, Fisheries and Forestry and James Cook University have been working to develop a DNA surveillance technique to detect the presence of tilapia in creeks and other waterways.

The spread of tilapia has so far been confined to Queensland, where their range includes one of the state’s biggest river systems – the Burdekin. Several outbreaks in West Australian rivers near Geraldton were controlled thanks to early detection. Preventing the spread of the fish, particularly to the Murray-Darling Basin, is a key concern of the CRC.

Tilapia can thrive in polluted and degraded waterways, and are fast, prolific breeders. Several were added to an ornamental pond at a hotel golf course in Port Douglas, near Cairns. Two years later, an eradication program removed 16 tonnes of tilapia from the pond.

Gilligan says the DNA surveillance technique being developed by the Invasive Animals CRC will enable fisheries officers to more efficiently detect pest fish, even in low numbers.

“Instead of sending a whole team of people out with a boat, nets and a pile of equipment for several days, we can send one person, with a bucket, to collect around nine to 10 litres of water from a river,’’ Gilligan says.

“They dig around in the bottom of rivers, pull out vegetation, stir up mud and generally trash the habitat for native species. They’re also bullies.”

The water is filtered, using fine filter paper, and when filtration is complete, the paper is analysed using a standard polymerase chain reaction laboratory test to detect DNA fragments.

“It’s not instantaneous. It takes a couple of days to filter the water and run the test, but it’s a much faster, more reliable [method] of measuring pest fish incursions in a river than using nets, lines and boats. Once the test result is back, we can run a risk assessment and move on to developing an eradication program.”

The DNA surveillance technique was originally developed in the US to detect carp, which are now among Australia’s most destructive environmental pests. The CRC is also evaluating a naturally occurring virus found overseas as a biological control agent to reduce carp impact. Dr Ken McColl, a veterinary virologist at the CSIRO Australian Animal Health Laboratory in Geelong, is leading the research.

McColl is conducting tests to confirm the findings that this carp herpes virus is effective and that it is safe for release into Australia’s waterways to control carp without affecting humans or native species. If successful, the strategic control program will open up new areas of research.

“We’d see unprecedented massive fish kills of carp in rivers, so we need to look at ways to manage collection and disposal of thousands of dead carp,” says Gilligan. “Do they go to council tips as landfill, or could they be ploughed into paddocks as fertiliser? That’s all part of the challenge of developing an eradication technique.”

– Rosslyn Beeby

www.invasiveanimals.com