Consequences of rotenone treatment for benthic fauna in Rastadammarna

Abstract

In 2024, goldfish (Carassius auratus) were discovered in the artificial ponds Rastadammarna, south of Gislaved, Sweden. These ponds function as natural treatment plants and absorb nitrogen from wastewater. The water ultimately exits the ponds into the natural waterway Nissan. The invasive goldfish pose a threat to local biodiversity due to their rapid reproduction, resilience and ability to outcompete native species. As the river Nissan is considered ecologically valuable and is of national interest for nature conservation, preventing the goldfish from spreading was of high priority. In collaboration with the Norwegian Veterinary Institute, the ponds were treated with rotenone to eradicate the goldfish. Prior to the treatment, a pre treatment survey was conducted on the abundance and composition of zooplankton and benthic fauna. This thesis is a follow-up survey conducted nine months post-treatment to monitor changes in abundance and community composition of benthic fauna through sampling at two stations using the standardised kick-net sample method (SS-EN ISO 10870). The sampling took place in June 2026 and consisted of five quantitative subsamples per station and one qualitative sample. Both stations had high individual densities (3,396 and 2,507 ind./m2, respectively), considerably higher than the pre-treatment levels of 1,537 ind./m2 and 652 ind./m2. With regard to species composition, rotenone-sensitive taxa like caddisflies and mayflies had decreased substantially, resulting in rotenone-tolerant species dominating. Species richness remained low at both stations, with 13 taxa each. The results suggest that functional diversity and community composition were altered by the rotenone treatment and/or the removal of the goldfish, whereas the total abundance of benthic fauna appears to have recovered relatively quickly.