Controlling carp in the River Torrens in key to improving water quality. Find out what’s being done to manage these pest fish in our much-loved Torrens.

Photo credit: City of Adelaide

Photo credit: City of Adelaide

Common carp are conspicuous invaders of many South Australian waterways. They reduce water quality and cause issues for native fish, irrigation and fishers.

Since their spread through the Murray-Darling system in the 1960s and 70s, carp now completely dominate much of South Australia’s River Murray. This invasion is not limited to the Murray system though, but also the rivers that are topped up with River Murray water such as Adelaide’s Torrens and Onkaparinga rivers.

What is the impact of carp in the Torrens?

The dirty feeding habits of carp change waterways.

They suck up mud in search of food, stirring up fine silts and making water turbid and murky. This means that sunlight cannot reach life-giving aquatic plants which would normally absorb excess nutrients and clean the water.

Carp dominate and engineer water conditions at the cost of plankton, causing problems in the food chain for our invertebrates, water birds and native fish. These factors can contribute to blue-green algal blooms, impact on recreational use of rivers which is a common problem in summer for Adelaide’s Torrens Lake.

Photo credit: City of Adelaide

Photo credit: City of Adelaide

How are carp being controlled in the Torrens?

A new project, supported by Green Adelaide and the City of Adelaide, is working towards controlling carp to improve water quality in the Torrens Lake.

The project involves scientific research to determine how many carp are in the lake and how many need to be removed to achieve real water quality benefits.

South Australian Research and Development Institute (SARDI) has estimated the Torrens Lake carp population is more than three times what is needed to achieve acceptable water quality.

Carp harvesting techniques have been trialled to reduce the carp numbers. This includes electrofishing, gill netting and fyke netting. Electrofishing was most effective as it reduced the lake’s estimated carp population by 13% (1,286 carp) over a three-day period. Whereas the netting techniques only captured 0.08% of the population during the same timeframe.

Electro-fishing involves sending an electrical surge through the water from the safety of an insulated boat. The surge temporarily stuns fish causing them to float to the surface where they can be scooped up with a dab net. The carp can then be taken from the water and any native species can then to swim away. The electrical surge itself has a very limited range, therefore knowledge of preferred carp habitats and skill of the operator are essential for this technique to be effective.

Photo credit: City of Adelaide

Photo credit: City of Adelaide

What’s next for carp control?

Carp control programs elsewhere have demonstrated that if carp densities are less than 50 kg per hectare, water quality is greatly improved and aquatic plants can grow – enhancing the habitat and reducing competition for native fish species.

Using the science, this past summer a carp harvesting program of 10 electro-fishing events was delivered. If repeated annually, it is anticipated that significant water quality benefits can be achieved by 2031.

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