Five brigades coordinated by the biologist Marc Ventura, from the Center for Advanced Studies in Blanes (CEAB-CSIC), continue this summer with the work to eliminate trout and minnow from ten high mountain lakes, in the Parc Nacional d'Aigüestortes i Estany de Sant Maurici and in the Parc Natural del Alt Pirineu. “All the fish in the ponds are invaders because they have not arrived naturally, most of them were introduced by fishermen in the middle of the 20th century indiscriminately, creating conservation problems; The objective is to restore the original state of these lake spaces and promote the protection of endangered amphibian populations, such as the Pyrenean newt, the grass frog and the midwife toad”, stresses Ventura.
The initiative is part of the European Life Rescue Alpyr project, which takes over from the LimnoPirineus, and which is also carried out in the Italian Alps. In fact, in the previous program, which was carried out in the period 2014-2019, it was possible to completely eradicate, and for the first time in the world with traditional techniques (shrimp, electric fishing and nets), the minnow from the Closell and Rovinets ponds. Trout was also wiped out in those of Cap de Port, Cabana and Subenuix, and both species in the two small ones in Dellui, Ventura said.
Now, the efforts are focused on the lakes of Naorte, Montanyó and two of the Tres Estanys enclave, in the Alto Pirineu park, and those of Llastra, la Coveta, Gargoles Inferior, Cabidornats, Dellui and Manhèra, in Aigüestortes and Estany de Sant Maurici or in its peripheral area.
Ventura details that the action focuses on three species of trout (common, char and rainbow) and three minnow, a fish that measures no more than ten or twelve centimeters but leaves a harmful impact. This cyprinid devours the larvae of insects and crustaceans that feed on algae. Without crustaceans, aquatic plants proliferate, the water turns green and visibility is reduced.
Ventura highlights the challenge of promoting a series of sectors in the parks free of trout and minnow so that the Pyrenean newt can go from one lake to another without the risk of being eaten. "This newt is endemic to the Pyrenees and in Aigüestortes there is a unique genetic lineage that is threatened by the exaggerated presence of fish," adds the researcher.
If in Catalonia there is awareness of the need to completely neutralize the presence of trout and minnows, in France it is more difficult to convince of the urgency to implement programs aimed at restoring the health of lakes by suppressing salmonids. "In France there is no pond without fish, while, for example, in Aigüestortes, which is where there are more of Catalonia, in 30% there are none," says Ventura. “Now we are collaborating with the French to explain the situation to them, they said we were crazy, they did not understand it, now they are starting to act with a pilot test in La Cerdanya”, he adds.
It should be noted that within the national park fishing is prohibited, although it is allowed in the peripheral zone of this protected space.
The Generalitat has become involved in this project by financing an action to also eradicate trout from Lake Collada and soon plans to do the same in those of Travessani, all in the national park.
The systems used are nets, shrimp farms and electric shocks that numb them and make it easier to remove them one by one. Likewise, the CEAB-CSIC team is finalizing a pilot test to use a piscicide, a biodegradable chemical substance already tested in other countries that inhibits the gill respiration of animals, according to Ventura.
Calculations carried out on the ground indicate that in a medium-sized pond there can be between 500 and 1,500 common and rainbow trout and up to 5,000 char, while the number of minnows rises to between 100,000 and 200,000. In France, they use the latter species as bait and throw the excess into lakes.
The works will culminate in 2026 to meet the objective of restoring the species composition and ecological function of high mountain lakes in the Pyrenees, and also in the Italian Alps, with the eradication of fish.
