Two high-altitude lakes in Spain's Alt Pirineu Natural Park are showing early signs of ecological recovery after scientists removed invasive minnows from the water bodies in September 2025. Researchers treated Tres Estanys del Mig and Tres Estanys de Baix with a biodegradable fish-killing chemical as part of the European LIFE RESQUE ALPYR project. Ten months after the intervention, monitoring found no minnows in either lake, while amphibians and aquatic invertebrates had returned. The researchers said neither group had been recorded in the lakes since monitoring began in 2014, making the early recovery a significant result for the restoration effort.

High mountain lakes in the Pyrenees are naturally often free of fish because natural barriers prevent fish from reaching them from lower streams. Researchers have linked the arrival of minnows in the region to human activity, particularly their use as live bait for trout fishing. Once introduced, the small fish can reproduce rapidly and adapt to new environments, spreading through mountain lakes and altering their ecosystems. Minnows feed on amphibian eggs and other small aquatic organisms, which can reduce native populations and change the biological balance of lakes that evolved without fish. A 2020 study published in Biological Conservation also examined the impact of removing non-native fish from Pyrenean lakes and found evidence of amphibian recovery after eradication efforts.

For the 2025 intervention, scientists used rotenone, a chemical that kills fish and naturally breaks down, with the aim of removing the invasive minnows and restoring the ecological balance of the two lakes. The operation was the second pilot carried out under LIFE RESQUE ALPYR, following an earlier treatment at Lake Manhèra in the Val d'Aran in September 2024. Project coordinator Marc Ventura said the initial results showed that recovery was progressing more quickly at Tres Estanys del Mig, while recovery at Tres Estanys de Baix and Manhèra was slower. The different timelines are being considered as part of the project's continuing assessment of how mountain lake ecosystems respond after invasive fish are removed.

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Researchers also monitored the environmental persistence of the treatment. According to IDAEA-CSIC researcher Barend van Drooge, the concentration of rotenone in the water declined as expected, with a reported half-life of between 10 and 20 days. The team also found no traces of the chemical in the river downstream from the treated lakes. The researchers said the relatively short persistence of the substance, together with its breakdown after treatment, indicated that its wider impact on the ecosystem was limited. Water clarity also improved after the minnows were removed, while the return of amphibians and aquatic invertebrates provided an early indication that ecological conditions were changing.

Previous research provides further evidence that native species can return relatively quickly once invasive fish are removed. A 2020 study by Alexandre Miró and colleagues examined eight naturally fishless Pyrenean lakes where researchers had eradicated between 95% and 100% of non-native trout or minnows between 2015 and 2019, comparing them with 56 nearby lakes. Within a year of removal efforts, amphibian communities in treated lakes were consistent with typical levels in the surrounding area, while numbers generally reached expected levels within three years, with the common frog being an exception. Researchers noted that all newly recorded species came from the same valley, highlighting the importance of healthy nearby populations in helping restored ecosystems recover.

The scientists involved in the latest project stressed that the early results do not represent the end of the restoration process and that long-term monitoring will continue in the coming years. Recovery can vary between lakes depending on factors including the previous abundance of invasive fish and the presence of nearby native populations. At Manhèra, for example, earlier efforts using nets, traps and electric fishing had kept minnow numbers low for several years before the chemical treatment, potentially leaving a hidden reserve of species in the lake sediment that could contribute to recovery. Alt Pirineu Natural Park director Marc Garriga described the initial findings as encouraging, saying they demonstrated the possibility of restoring mountain lakes that had lost biodiversity after invasive species were introduced.

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