Cleaner water for town drinking from mass carp death

The National Carp Control Plan is in its last six months of finalisation, with a comprehensive report leading to the possible release of the virus within two years, if approved by all levels of government.

A reliable estimate of the tonnage of carp in the river systems is expected in the report. It is estimated that carp already make up 80 per cent of the biomass in many rivers.

Worryingly, the carp problem is increasing with the fish entering cleaner water systems for the first time, including the pristine Bega river system, and a well-known wetland. The Northern Territory is the only jurisdiction in Australia that doesn’t have a carp problem.

Carp Plan co-ordinator Matt Barwick said there would be large scale benefits if carp were removed including improving the quality of water for councils to access from rivers. “Some councils are already finding ways to control carp to improve drinking water quality but we expect that to increase if carp are removed, and there will be savings,” he said.

The largest area of savings or benefits will come through increased tourism and fishing if carp are removed, he said.

The biggest issue confronting the release of the virus is what will happen if there is a mass kill of carp and how that could damage rivers and native fish populations.

Mr Barwick said. “We know this virus can kill carp very efficiently and if there is a lot of caracasses that is one of our biggest challenges. Significant carp kill events, how will we manage that ?”

 In-depth research was going into carp organic matter and how to remove it to protect the existing river systems. The agency had conducted experiments with this by putting 6 tonnes of dead carp in a 2.5ha wetland to see how they could deal with it. “We know this issue is one of the most important parts of the project,” he said.

Local communities were working with the agency to understand how to manage any large kill off.

Mr Barwick said after the plan was finalised in about six months it would go to all levels of government. The release of the virus would have to satisfy many different pieces of legislation at both a state and federal level. There were four pieces of federal legislation alone that needed to be satisfied that the virus was suitable for  release. He said all levels of government had shown their support for the plan if it met stringent environmental conditions.

Mr Barwick said another consideration was also the right time for release of the carp virus, that rainfall and river flow had to be considered and climate conditions.

Carp have a massive impact on river systems, killing off native fish populations, muddying waters, killing off water plants and thereby killing invertebrates and slugs and snails. It is classified as one of the top 10 most invasive fish species in the world. It is estimated carp causes about $500 million of damage a year in Australia through various losses such as tourism, fishing and water quality.

The National Carp Plan agency has linked with a researcher at Water NSW to see what impact dead carp could have on water quality.

“The NCCP is tackling this issue with research starting in ‘mesocosms’ (small pools)  before scaling up to larger experiments,” the agency said. “Joe Pera – Water NSW – is looking into what’s going to happen to water quality with the potential release of the carp virus.

“There could also be impacts on the ecosystem if an area becomes anoxic (low oxygen) – which could impact on other fish species. Joe’s team has undertaken an experiment looking at biomass levels ranging from no carp to 6000kg carp per hectare to simulate a complete range of densities of carp.

“A mesocosm – experimental pond – is great way to simulate a water body. Small 2m round pond which can be replicated – so 20 mesocosms were set up in a small area. Different volumes of dead carp were put into each mesocosm and water quality was checked over a month - looking at algae, bacteria and other water quality parameters like dissolved oxygen levels - to see what the changes were as the fish decomposed.”

It found “dead carp led to increases in algae and bacteria in line with higher biomass. If the fish were removed after they died, there was not much impact on water quality at all. What this means for the clean-up is that we now know at what levels water quality could be impacted and can allocate resources into doing thorough clean-ups in those areas.”