As of the year 2000, an estimated 27% of high-mountain lakes in the Pyrenees were infested with minnows, a problem researchers believe has increased in the years since. Aerial imagery captured in July 2026 now shows a stark transformation at two of those sites, Tres Estanys del Mig and Tres Estanys de Baix within the Parc Natural de l’Alt Pirineu, where the water has returned to a clear, natural state after researchers applied rotenone to eradicate invasive fish populations.
Introduced Fish Triggered Food Chain Collapse in Pyrenees
The high-mountain lakes of the Pyrenees were historically fish-free due to natural geographical barriers. Everything changed with the introduction of trout for recreational fishing and the subsequent release of leftover minnows (Phoxinus phoxinus)—often used as live bait by anglers.
Marc Ventura, a coordinator for the project at the Center for Advanced Studies of Blanes (CEAB), noted that these small fish, which can reach 12 cm, successfully colonized the waters. Their arrival triggered a collapse in the local food chain. The fish consumed significant amounts of zooplankton and amphibian tadpoles, leaving no tadpoles to graze on algae and no zooplankton to regulate microalgae levels. The result was rapid algal blooms and the turbid, green water documented in July 2025.
Targeted Chemical Intervention With Rotenone
Traditional methods like netting and trapping proved ineffective against the invasive minnow. To break the cycle, researchers turned to rotenone, a plant-derived piscicide.
The chemical functions by blocking oxygen intake in gill-breathing organisms. Because of the potential impact on non-target species, the application required precise, controlled conditions as part of the European LIFE RESQUE ALPYR project, supervised by the Spanish National Research Council (CSIC) and the Generalitat de Catalunya.
Native Species Return After Fish Populations Are Eliminated
Ten months later, post-treatment monitoring confirmed the target fish populations were successfully eliminated.
Researchers detected the return of native species, including amphibians and invertebrates, that had previously disappeared. Environmental assessments indicate that the rotenone degraded as expected, with no remaining traces detected in the downstream water.
Scaling Up Pyrenean Conservation Efforts
While the initial results are successful, the research team is still investigating why the rate of ecological recovery varies between the two treated lakes. Scientists from the LIFE RESQUE ALPYR project are currently analyzing the data to develop a standardized framework for restoring other affected high-mountain ecosystems.
The findings from these two lakes represent a pilot effort to determine the feasibility of scaling such interventions across the broader Pyrenees region.
Why Rotenone Was Used to Clear Invasive Minnows
Why was rotenone necessary for these specific lakes?
Conventional removal techniques, such as manual netting or trapping, were unable to control the invasive minnow population effectively. Researchers turned to rotenone as a targeted tool to clear the lakes of fish so that native amphibian and invertebrate populations could re-establish their roles in the ecosystem.
Are there long-term environmental risks from using this chemical?
The project reports that the rotenone degraded at the anticipated rate, leaving no detectable traces in the water downstream. The restoration of water clarity and the return of native species suggest that the short-term intervention allowed the ecosystem to reset, though researchers continue to monitor the long-term recovery process.