Biologists target 30 tonnes of fish to clear three lakes

Biologists will remove 30 tonnes of roach and bream from three lakes in a project intended to clear the water within three years, DR reports.

Rørbæk Sø, Kulsø and Nedersø, located between Ikast-Brande Municipality and Vejle Municipality, are dark green with algae at this time of year.

The project uses a method known as biomanipulation, which involves removing fish species that contribute to algae growth. Roach and bream eat zooplankton that would otherwise feed on the algae, allowing the algae to multiply and turn the water cloudy.

The fish will be sorted by hand. Small roach will be used as animal feed, while larger pike and perch will be released.

Anette Bonde, the project manager for Vejle Municipality, described the situation as an imbalance in the lakes.

“The large number of small roach changes the composition of algae and plankton, making the water cloudy,” she said.

In addition to removing 80 per cent of the fish, a layer of clay will be placed on the bottom of Rørbæk Sø to bind phosphorus, which promotes algae growth. The clay product contains added lanthanum, a chemical element that can capture the phosphorus, Bonde said.

Biologists and assistants have spent several weeks on Rørbæk Sø locating shoals of fish and analysing the species found in the lakes. They will now begin catching the fish.

If the water becomes clear again, predatory fish such as pike and perch will have an easier time seeing and eating roach and bream. More water plants are also expected to grow when sunlight can reach the bottom.

The restoration is expected to take three years and has received 22.7 million kroner in funding from the Agency for Green Land Transformation and Water Environment.