We have been entrusted with the care of a pond in the centre of České Budějovice for breeding common bream. The pond is fed by a stream with the possibility of flow regulation. The water in the stream is extremely fertile and this water flowed through the pond, resulting in ponds overgrown with perch. There is a big problem with the perch because there is cooler water underneath with no light, little oxygen and little natural food for the fish, so the fish grow slowly. Redfish usually don't even spawn under the sedges. We have tried several times to flush the perch out through the outlet device at increased flow, this has worked, but within 3 weeks the perch had overgrown the pond back to its original state. The next spring we stopped the inflow, no water was coming in or going out. During the spring, the perch reestablished itself (Figure 1), so we prepared several logs, tied them together, and pulled the perch down to one bank (Figure 2). We used the jacket to remove the perch (Figure 3). This left 1% of perch in the water and was enough to re-grow the pond entirely with perch within 3 weeks. So we had to repeat the whole procedure.
Next, we deployed grass carp of 10-12 cm in size to eliminate the perch growth, and then removed the perch remnants with a jacket. Since then, the perch have not grown any more. Soon after the removal of the perch, algae multiplication occurred, followed by a mass multiplication of coarse zooplankton and an increase in water clarity, and the fish recruitment was large. There was a multiplication of the introduced common bream and common minnows. The recruitment of common bream was around 12 cm. Bream stocked in April 2024 measured 8 cm, and by September 2024 they measured 20 cm (Figure 4). For grass carp, growth increased from an initial 10 cm (June 2024) to 30 cm (September 2024).
The process of pond overgrowth has been reversed and biodiversity has also increased. After the removal of the perch, water clarity was more than one meter by eye and clumps of coarse zooplankton were present. In addition, tadpoles and many larvae and adults of aquatic insects were present in the water. This intervention was thus beneficial not only for the common bream and the common minnow, but also for other biota of the backwaters.
Author of article and photos: Vojtěch Kresl