• Ten months after the eradication programme, both lakes show a remarkable recovery in water transparency and biodiversity, including the return of amphibians and aquatic invertebrates, groups that had not been recorded since monitoring programmes first began in 2014

• The initiative, carried out within the framework of the European LIFE RESQUE ALPYR project, provides new evidence on the use of rotenone as a tool for ecological restoration

The first observations carried out following the intervention at the lakes of Tres Estanys del Mig and Tres Estanys de Baix, in Alt Pirineu Natural Park, confirm the complete eradication of minnows (Phoxinus spp.) and provide evidence of the first signs of ecological recovery. The presence of this fish in the Catalan Pyrenees is the result of its accidental introduction as live bait for recreational trout fishing, a practice that has caused significant alterations to these high-mountain ecosystems.

The intervention, carried out last September within the framework of the European LIFE RESQUE ALPYR project, involved the application of rotenone, a biodegradable piscicide, with the aim of restoring the ecological balance of these lake ecosystems and generating scientific evidence on the effectiveness and safety of this restoration methodology. This is the second pilot trial undertaken in lakes of the Catalan Pyrenees, following the intervention conducted in September 2024 in Lake Manhèra, in the Val d’Aran.

Data collected to date confirm that the intervention achieved its main objective, namely the eradication of minnows from both treated lakes. At the same time, the first monitoring results show clear signs of ecological recovery, including a marked improvement in water transparency and the return of amphibians and aquatic invertebrates, groups that are particularly sensitive to the impacts caused by this introduced species.

According to Barend L. van Drooge, researcher at the Institute of Environmental Assessment and Water Research (IDAEA-CSIC), “Monitoring of rotenone levels in the water has confirmed that it degrades at the rate originally predicted, with a half-life of between 10 and 20 days. This reduces its persistence in the aquatic environment and suggests a very limited impact on the ecosystem.”

Furthermore, no traces of rotenone have been detected in the river located downstream of the treated lakes. This result confirms that, as anticipated, the product remained confined to the treatment area and did not disperse beyond the lakes targeted by the intervention.

Restoring altered ecosystems

Fish introductions are one of the main causes of biodiversity loss in high-mountain lakes. Minnows are a species that has become established in numerous Pyrenean lakes as a result of their accidental introduction as live bait for recreational trout fishing. Their high reproductive capacity and adaptability favour their spread and cause significant disruptions to aquatic food webs.

The presence of this fish leads to a substantial decline in biodiversity, particularly among small aquatic organisms and amphibian populations, as it preys on amphibian eggs and alters the conditions required for successful reproduction. For this reason, its eradication is considered one of the most effective measures for promoting the recovery of high-mountain lake ecosystems.

Surveys conducted ten months after treatment revealed a highly positive response, with a notable recovery in both water transparency and biodiversity. The return of amphibians and aquatic invertebrates has also been recorded. These groups had not been detected since the first monitoring programmes began in 2014.

A benchmark experience for the conservation of Pyrenean lakes

The intervention at Tres Estanys represents the second pilot trial developed within the LIFE RESQUE ALPYR project, following the experience at Lake Manhèra in the Val d’Aran. In both cases, minnows were successfully eradicated and the first signs of recovery of the characteristic biodiversity of high-mountain lakes have been observed.

According to Quim Pou-Rovira, Director of Sorelló Estudis al Medi Aquàtic and Technical Lead of the intervention, “In the case of Tres Estanys, the challenge was particularly significant because the intervention was carried out in a larger and more complex system than Lake Manhèra, given that it involved interconnected lakes.” The results obtained demonstrate that this methodology can also be effective in larger and more complex lake systems.

An important difference between the two lakes treated in Alt Pirineu is that previous minnow control measures had already been carried out at Tres Estanys del Mig using mechanical methods such as nets, traps and electrofishing. Although these actions did not completely eradicate the species, they succeeded in keeping the population under control for several years.

According to Marc Ventura, researcher at the Centre for Advanced Studies of Blanes (CEAB-CSIC) and coordinator of the LIFE RESQUE ALPYR project, “The initial data indicate that ecological recovery is progressing more rapidly in Tres Estanys del Mig than in Tres Estanys de Baix and Lake Manhèra. One possible explanation is that previous efforts to control minnow populations helped preserve a latent reservoir of species within the sediments, which may now be facilitating a faster recolonisation of the ecosystem.”

Long-term scientific monitoring

Although the eradication of minnows has already been confirmed in both pilot trials, the project partners stress that the full recovery of these ecosystems is a gradual process requiring long-term monitoring. For this reason, monitoring activities will continue over the coming years in order to assess changes in biological communities and document the recovery of the lakes’ natural ecological processes.

According to Marc Garriga, Director of Alt Pirineu Natural Park, “These initial results are very encouraging for the conservation of high-mountain lakes. Beyond the success of the intervention itself, the pilot trial demonstrates that it is possible to restore ecosystems that had lost a significant part of their biodiversity and to generate valuable knowledge for addressing similar challenges in other Pyrenean lakes.”

The intervention was coordinated and implemented by Sorelló Estudis al Medi Aquàtic, S.L. and the La Sorellona Association, under the scientific supervision of the Centre for Advanced Studies of Blanes (CEAB-CSIC) and the Institute of Environmental Assessment and Water Research (IDAEA-CSIC), with the direct support of Alt Pirineu Natural Park and Forestal Catalana.

The conference will feature over 60 speakers, including a significant representation of scientists from the LIFE RESQUE ALPYR project, coordinated by the Center for Advanced Studies of Blanes (CEAB-CSIC).

This event brings together scientists, managers, and local entities with the goal of disseminating the scientific knowledge gained from the research conducted in the area, while also providing insights into how this knowledge can be applied to the management of natural spaces.

It will be a space to foster collaboration among different sectors to conserve the Pyrenean ecosystems.

The project’s researchers will give three presentations related to the project’s results (and the results of preceding projects).

The three presentations will take place on September 4th, starting at 3:00 PM in the multipurpose room of MónNatura. These will be part of the session dedicated to ponds, moderated by Núria Catalán, another researcher from CEAB-CSIC. The first presentation, by Danilo Buñay, will discuss primary production as the main determinant of transparency in ponds. Next, Marc Ventura, coordinator of the LIFE RESQUE ALPYR project, will talk about littoral crustaceans in ponds and their relationship with fish presence. Finally, Víctor Osorio will present on the recovery of macroinvertebrates after the eradication of invasive fish species.

The other two presentations will take place on September 5th from 5:00 PM in the session dedicated to rivers and wetlands, moderated by the researcher of the project and of the University of Barcelona, ​​Josep M. Ninot. The first presentation will be that of Eulàlia Pladevall, on the vegetation of the wetlands after five years of livestock exclusion. Then, Josep M. Ninot, will talk about the distribution of organic carbon in the soil in four Pyrenean wetlands.

The conference program includes a total of 76 participations (66 presentations, 10 in poster format) spread across 10 thematic sessions: landscape evolution, archaeology, geology, cultural heritage, ponds, monitoring networks/snow science, socio-ecological systems, animal ecology and biodiversity, forest ecology and biodiversity, and rivers and peatlands. There will also be a poster session covering various topics.

More information about the III Research Conference of the Alt Pirineu Natural Park can be found at this link.

The scientific meeting was held in Tortosa and Vic on June 18 and 21, 2024. This time, the congress focused on promoting the study and conservation of native fish in the continental and marine aquatic ecosystems of the Iberian Peninsula.

The members of the LIFE RESQUE ALPYR project contributed knowledge obtained from their research in the field of conservation and restoration of high mountain lakes. They did so through presentations and the display of scientific posters.

For example, among other things, they presented work related to the use of genetic tools for the early detection of invasive fish species in lakes. Metabarcoding is an accurate tool for identifying and conducting phylogenetic analysis of species, allowing the detection of bioinvasions in their early stages.

The history of fish introductions in high mountain lakes was also narrated and explained in detail. This history was studied by researcher Marc Ventura along with Didier Galop and Olivier Baudier.

SIBIC congresses are held every two years to address scientific and management challenges related to various aspects of ichthyology. This edition, among other new topics, will include issues such as freshwater and marine fish, and conservation aquaculture.