Rotenone, a plant-derived ichthyocide, was used to eliminate invasive fish from two high-altitude lakes in the Pyrenees, and within 10 months, the lakes were teeming with life again. This successful restoration project, part of the European LIFE RESQUE ALPYR initiative, offers valuable insights for managing similar ecosystems across Europe.
Background: The Problem with Invasive Fish
High mountain lakes in the Pyrenees naturally contained no fish due to natural barriers. However, over time, humans introduced species like trout for fishing purposes. These non-native fish preyed on native amphibians and invertebrates, disrupting the delicate ecosystem balance. The introduction of fish, particularly minnows and trout, led to the decline of species that had evolved in fishless environments. The longer history behind the story is traced in Siluros Found in Lebrija Irrigation Pond 12 km from Guadalquivir.
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The Rotenone Treatment: A Drastic but Necessary Measure
In September 2025, under the supervision of CSIC and the Generalitat, rotenone was applied to two lakes: Tres Estanys del Mig and Tres Estanys de Baix in the Alt Pirineu Natural Park. Rotenone is a natural compound derived from plant roots that is toxic to fish but degrades quickly in the environment. The goal was to eradicate all fish, allowing the ecosystem to recover. The background to this story is set out in Lake Whitefish at Risk: Great Lakes Icon May Vanish. A related thread runs through Discover Europe's Best Lakes and Mountains for Summer Travel. Earlier coverage in Lake Whitefish Decline: Great Lakes Icon Faces Extinction shows how the argument developed.
Results: Rapid Recovery of Native Species
Ten months after the treatment, monitoring reports confirmed that no fish remained, the water clarity had improved significantly, and native amphibians and invertebrates had returned. The rotenone degraded as expected, with no traces found downstream. The lakes, once turbid and green, were now blue and clear, indicating a healthy ecosystem. A closer look at the same subject appears in Lake Whitefish Could Vanish from Great Lakes in 5 Years. Anyone following the wider picture can start with 7 lakes in Trentino that are as beautiful as Lake Garda. The same theme surfaces in Europe's Lakes and Mountains: Top Summer Travel Tips, which takes a different angle.
- Rotenone effectively eliminated invasive fish within weeks.
- Water transparency improved dramatically within 10 months.
- Native amphibians and invertebrates recolonized the lakes.
- No residual rotenone was detected downstream.
- The project demonstrates a viable method for lake restoration.
- Lessons can be applied to other European mountain lakes.
Comparison of Lake Conditions Before and After Treatment
| Parameter | Before Treatment (July 2025) | After Treatment (July 2026) |
|---|---|---|
| Fish Presence | Invasive fish abundant | No fish detected |
| Water Clarity | Turbid, greenish | Clear, blue |
| Native Amphibians | Absent or rare | Detected and increasing |
| Invertebrate Diversity | Low | Recovering |
| Rotenone Residue | Not applicable | None detected |
Why This Matters for European Lake Management
This case study shows that removing invasive fish can lead to rapid ecosystem recovery, even in sensitive high-altitude environments. It provides a model for other restoration projects aiming to protect native biodiversity. The success underscores the importance of addressing human-induced changes to natural habitats. Context on how this played out before sits in Invasive Species Cost Australia Billions: How to Respond. A companion piece, Invasive Species Cost Australia $24.5B: How to Respond, approaches the same subject from another angle.
FAQ
What is rotenone and is it safe for the environment?
Rotenone is a natural substance derived from the roots of certain plants. It is toxic to fish but breaks down quickly in water and soil, leaving no harmful residues. It has been used in conservation projects worldwide.
How long did it take for the lakes to recover?
Within 10 months, the lakes showed significant recovery: water cleared, and native species returned. Full ecosystem recovery may take longer but the initial results are promising.
Were there any negative impacts on other species?
Rotenone is not harmful to amphibians or invertebrates at the concentrations used. In fact, these species benefited from the removal of predatory fish.
Can this method be used in other lakes?
Yes, with careful planning and monitoring. Each lake requires an assessment to ensure that rotenone use is appropriate and that native species will not be harmed.
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