Pulling the Plug-Draining an Alpine Lake Failed to Eradicate Alien Minnows and Impacted Lower Trophic Levels

Fish introduction into fishless high-altitude lakes has detrimental effects on biodiversity. Removal of alien fish through intensive fishing is cost-intensive and difficult to achieve in productive lakes. Lake Sulzkarsee is the only lake in the National Park Gesause, Austria, and was an important breeding site for amphibians until the lake was stocked with fish in the late 1970s. Salmonids were eradicated in 2005, but the lake remained degraded by the introduced minnows (Phoxinus sp.). In 2018, the lake was drained through a siphon pipe and then by pumping out water with dirt water pumps. The deepest part was treated with slaked lime, but several hundred adult minnows survived in sediment crevices and reproduced in the following season. After drainage, the phytoplankton biomass increased. Indicator species, such as Daphnia longispina and amphibians, showed signs of recovery, but they went back to an impacted state when minnows recovered after the failed eradication attempt. Purse seines proved to be the most efficient gear to catch minnows. These results indicate that deep mountain lakes are difficult to drain efficiently. Sediment treatment is required to eliminate all fish.

Automated summary

Introduction of fish into fishless high-altitude lakes negatively impacts biodiversity. Eradicating alien fish through intensive fishing is expensive and challenging in productive lakes. The draining of Lake Sulzkarsee in Austria, combined with sediment treatment, was partially successful in eliminating fish, but the introduced minnows survived and reproduced. The recovery of indicator species was short-lived when minnows returned after the failed eradication attempt. Purse seines were found to be the most efficient gear for catching minnows. These findings highlight the difficulty of draining deep mountain lakes and the need for sediment treatment to ensure complete fish removal.