Researchers have confirmed the complete eradication of introduced minnows from two high-mountain lakes in Catalonia’s Alt Pirineu Natural Park, with early signs of ecological recovery recorded 10 months after the intervention. Surveys at Tres Estanys del Mig and Tres Estanys de Baix found the return of amphibians and aquatic invertebrates. These groups had not been recorded since monitoring programmes first began in 2014. The intervention was carried out in September 2025 under the European LIFE RESQUE ALPYR project and involved applying rotenone, a biodegradable piscicide, to eliminate the introduced fish. The minnows, identified as Phoxinus spp., had been accidentally introduced as live bait for recreational trout fishing and had caused significant changes to the biodiversity of these high-mountain ecosystems. Researchers say the results provide further evidence that the method can eradicate minnows from a more complex, interconnected lake system, although they stress that longer-term monitoring is still needed to determine the extent and durability of the ecological recovery.
Minnows have altered the lakes’ aquatic ecosystems
Minnows (Phoxinus spp.) have become established in numerous Pyrenean lakes following their accidental introduction as live bait for recreational trout fishing. The fish have a high reproductive capacity and are adaptable, allowing them to spread and form populations in high-mountain lakes. Their presence can substantially affect aquatic ecosystems, particularly by preying on amphibian eggs and altering conditions needed for successful reproduction. Researchers also identify disruptions to aquatic food webs and declines among small aquatic organisms and amphibians as consequences of the introduced fish.At Tres Estanys del Mig, researchers had previously attempted to control the minnow population using mechanical methods including nets, traps and electrofishing. These measures did not completely eradicate the species, although they kept its population under control for several years. The previous control work also distinguishes Tres Estanys del Mig from Lake Manhèra, the site of the project’s first rotenone intervention in September 2024. The September 2025 intervention at Tres Estanys was designed both to remove the introduced fish and to assess whether rotenone could be used effectively in a larger and more complicated high-mountain lake system. The two Tres Estanys lakes are interconnected, making the operation more technically demanding than the earlier treatment at Manhèra.
Rotenone degraded within weeks and was not detected downstream
Researchers applied rotenone, a biodegradable piscicide, to the lakes as part of the restoration intervention. The substance is used to eliminate introduced fish because it interferes with respiration through the gills. This means its effects are not limited exclusively to the target fish, which is why the project included monitoring of the chemical and its environmental effects as part of the intervention.Following the treatment, researchers monitored rotenone concentrations in the water. Barend L. van Drooge, a researcher at the Institute of Environmental Assessment and Water Research (IDAEA-CSIC), said monitoring confirmed that the chemical degraded at the rate predicted before the intervention, with a half-life of between 10 and 20 days. The relatively short persistence reduced the time during which rotenone remained in the aquatic environment. Researchers also reported that no traces of rotenone were detected in the river downstream of the treated lakes. The project says this indicates that the product remained confined to the treatment area and did not disperse beyond the lakes targeted by the intervention.
Amphibians and aquatic invertebrates returned after treatment
The first observations after the intervention confirmed that minnows had been completely eradicated from both Tres Estanys del Mig and Tres Estanys de Baix. Surveys conducted 10 months after treatment also recorded a marked improvement in water clarity and biodiversity, including the return of amphibians and aquatic invertebrates. According to CEAB-CSIC, neither group had been recorded in the lakes since monitoring programmes began in 2014. The researchers describe these findings as the first signs of ecological recovery rather than evidence that the lakes have already returned completely to their previous ecological condition. The two lakes are also recovering at different rates. Marc Ventura, a researcher at the Centre for Advanced Studies of Blanes (CEAB-CSIC) and coordinator of LIFE RESQUE ALPYR, said initial data indicated that recovery was progressing more rapidly at Tres Estanys del Mig than at Tres Estanys de Baix and Lake Manhèra.Ventura suggested that the earlier mechanical control efforts at Tres Estanys del Mig could be one reason for the faster response. Because nets, traps and electrofishing had previously reduced the minnow population without completely eliminating it, researchers suggest that a latent reservoir of native species in the sediments may have remained and could now be helping recolonisation. This is presented as a possible explanation for the difference in recovery rates, rather than a confirmed mechanism.
Researchers will continue monitoring the lakes
The Tres Estanys intervention was the second pilot trial using rotenone under LIFE RESQUE ALPYR. The first took place at Lake Manhèra in the Val d’Aran in September 2024. In both cases, the project reports successful minnow eradication followed by early signs of the return of characteristic high-mountain biodiversity. Quim Pou-Rovira, director of Sorelló Estudis al Medi Aquà tic and technical lead for the Tres Estanys intervention, said the operation was particularly challenging because it involved a larger and more complex system than Manhèra, with interconnected lakes. The results indicate that the methodology can also be effective in such systems.However, the project does not regard the initial recovery as the end of the restoration process. Researchers say long-term ecological monitoring is necessary to assess how the biological communities develop after the fish have been removed and to determine whether the recovery continues. Earlier project researchers similarly stressed that longer-term monitoring is needed to confirm the results of the interventions. LIFE RESQUE ALPYR is a European conservation project focused on restoring mountain aquatic habitats and species. It covers protected sites in the Pyrenees of northeastern Spain and the Alps of northwestern Italy. CEAB-CSIC coordinates the project, with partners working across the two mountain regions. The Tres Estanys results add evidence from the Catalan Pyrenees to the project’s broader work on reducing the ecological effects of introduced species in high-mountain lakes.
