In the heart of the Alt Pirineu Natural Park, Tres Estanys del Mig and Tres Estanys de Baix have changed dramatically. Ten months after a targeted intervention, researchers from the Centro de Estudios Avanzados de Blanes (CEAB-CSIC) report that the invasive fish known as piscardo has been completely eliminated. The water is crystal clear again, and native amphibians and aquatic invertebrates not seen in over a decade have returned.
The ecological shift began years ago, when non-native fish, first trout and then piscardo, were introduced into these isolated glacial lakes. Piscardo is often used as live bait. Unlike rivers, these high-altitude waters historically lacked fish, supporting a delicate balance of microorganisms and invertebrates, alongside amphibians. Piscardo disrupted that balance, multiplying rapidly and feeding on zooplankton, insect larvae and amphibian eggs, while also outcompeting trout for resources.
As piscardo populations soared, the lakes’ ecosystems changed visibly. The fish ate the small crustaceans that kept microalgae in check, triggering blooms that turned the water green and turbid. They also stirred up sediment and released nutrients, costing the once-clear lakes their characteristic transparency.
Restoration Efforts
Conventional methods, including nets and traps as well as electric fishing, failed to eradicate the invasive fish completely. Even a handful of survivors could quickly repopulate the lakes. In September 2025, scientists turned to rotenone, a biodegradable piscicide that disrupts fish respiration and breaks down naturally within weeks. The operation was complex because the two lakes are interconnected, forming a larger system than those involved in previous projects in the region.
Sorelló Estudis al Medi Aquàtic and Associació La Sorellona coordinated the intervention, with scientific oversight from CEAB-CSIC and the Instituto de Diagnóstico Ambiental y Estudios del Agua (IDAEA-CSIC). The Alt Pirineu Natural Park and Forestal Catalana also supported the work. Post-treatment monitoring found no trace of piscardo in either lake. Chemical analysis confirmed that rotenone had not contaminated the downstream river, addressing one of the basic environmental concerns.
Signs of Recovery
With the invasive fish gone, the lakes are showing rapid signs of recovery. Water clarity has improved as microalgae levels drop, and surveys have detected amphibians and aquatic invertebrates for the first time since 2014. Recovery has been fastest in Tres Estanys del Mig, possibly because previous mechanical controls preserved dormant populations in the sediment, allowing them to recolonize sooner.
Marc Garriga, director of the Alt Pirineu Natural Park, says these early results bode well for conserving high-mountain aquatic habitats. The project is part of LIFE RESQUE ALPYR, a European initiative to restore mountain water ecosystems across the Pyrenees and Alps, co-financed by the EU’s LIFE programme. CEAB-CSIC describes it as a 2022-2026 conservation project spanning four Natura 2000 sites. Its work addresses pressures on mountain aquatic ecosystems, including climate change and hydrological alterations, as well as land-use change and invasive species. The project pairs introduced-fish removal with peatland and wet-meadow restoration, while also improving grazing management and working to recover sensitive wildlife.
Broader Context
Restoring these lakes shows the challenges of managing invasive species in fragile environments. Similar efforts are underway elsewhere in Europe, where invasive fish continue to threaten native biodiversity. Recent reports also point to the need for evidence-based conservation. An analysis by russpain.com debunked claims that tortoise reintroduction transformed the Sahara, showing that ecological recovery is rarely simple or immediate. See the full analysis here.
CEAB-CSIC says the project’s principal results are due to be presented at an international mountain-lake and peatland conservation congress scheduled for 2026. The event is expected to bring together about 90 experts. The programme places the lakes’ recovery within a wider effort to restore high-altitude aquatic ecosystems and the sensitive species they support. Full recovery of Tres Estanys will take years, while ongoing monitoring tracks native species’ return and water quality. For now, the clear waters offer a rare example of successful ecological intervention in the Pyrenees.
