Water-quality crucial

Researchers have found risks to many Australian inland waterways associated with the potential release of a carp virus would be minimal in many areas and manageable in others.

Principal investigator Justin Brooks from University of Adelaide said ecosystems where the virus might be released would need to cope with a surge in the number of dead carp, an increased demand for dissolved oxygen and the presence of more nutrients.

“This work has shown the impact on water quality would be minimal in many waterways where there is good flow and mixing and manageable where the flow is slower,” he said.

Wimmera catchment leaders have suggested the Wimmera River as a potential release site for the virus, pending outcomes of in-depth investigations.

The study’s models show that areas with good flow, depth and cooler waters would likely cope best, whereas shallower waters with low or no flow and warmer temperatures were more likely to experience water-quality issues, especially if the density of carp was high.

The Wimmera River system has minimal-flow and shallow areas and Mr Brooks said waterbodies might require intervention to clean up dead carp to avoid significant impact on water quality.

He said the main threats to water quality included a reduction in the concentration of oxygen in the water and the possibility of associated algal blooms.

The first was due to the oxygen consumption of organisms working to decompose dead carp, whereas the second would be a result of an increase of nutrients in the water as dead carp broke down.

The research showed that at existing carp biomass levels, it would be unlikely oxygen concentrations would drop below levels that were dangerous for native fish in most waterways.

In many locations, the research also showed the existing biomass of carp would have to be increased by more than four times before water-quality impact would become noticeable.

The research means management of carp carcasses could focus on higher risk sites prone to still water and relatively high temperatures.

This research project is part of the $10.2-million National Carp Control Plan led by Fisheries Research and Development Corporation on behalf of the Federal Government.

The plan aims to address two questions: Is it feasible to release the carp herpes virus to control carp? If so, what is the most effective way to release and manage the virus?

Crucial estimate

Wimmera Catchment Management Authority officials, meanwhile, believe the region’s waterways are in a good position as part of the overall carp-control program, based on national carp biomass figures.

Authority water planning and policy officer Greg Fletcher said biomass estimates would be useful in terms of understanding a release of the virus.

He said Wimmera CMA had provided information for the Carp Biomass Project and the NCCP through carp surveys of the catchment’s waterways.

“We’re in a good position because we have fed that local information through,” he said.

“It will help give us a more specific understanding of the impact of virus release.

“The CMA has been preparing in case the virus goes ahead, and we’re really trying to be proactive. We need to do a lot of background research to understand the impact of the virus on the environmental waterways.”

The Carp Biomass Project has produced the first estimates of carp weight across the continent. This information will also help guide the best approaches to control the destructive pest.

Project leader Jarod Lyon from Victoria’s Arthur Rylah Institute for Environmental Research said estimates came from fish-survey data from more than 150 studies and 4381 sites across 24 years.

He said biomass estimates varied significantly between years depending on how wet conditions were at the time.

“For example, during a single wet year such as May 2011, the carp biomass estimate was 368,357 tonnes, with a lower and upper limit of 184,234 and 705,630 tonnes respectively,” he said.

Dr Lyon said additional modelling suggested carp biomass could reach a maximum of 1,200,000 tonnes after a series of three flood years.

He said estimates had helped create a national map of aquatic environments, which would become an ‘invaluable’ planning tool for water managers.

“This work has provided us with a key understanding of how this highly adaptable pest can respond to different environmental conditions,” he said.

“A major project outcome is that sites where carp biomass is high enough to cause significant environmental damage can now be identified. This means carp-management resources can be allocated where they are most needed.”

The Federal Government is scheduled to receive the NCCP this month.