Milky green, even neon instead of crystal clear waters. Occasional hikers who wore out their boots on the heights of the Pyrenees this summer must have sometimes looked gray when they discovered that the vaunted pure water of the small altitude lakes was no longer transparent. At the beginning of July, the Pyrenees National Park published a photo of a lake in the Cirque d’Estom, near Cauterets, in the Hautes-Pyrénées. It was all green, just like the lake of Bethmale in Couserans in the heart of summer, or that of Areau, still in Ariège, just last week.
“It’s not always so obvious, but we can see that all our lakes are changing color, that there is more turbidity”, ask Adeline Loyau and Dirk Schmeller, researchers at the functional ecology and environment laboratory of Toulouse* and which have been monitoring the state of the mountain lakes in the massif for almost a decade. They met a lot of hikers worried about this sudden touch of green during their summer surveys.
Importing live bait
These mountain specialists, who have already revealed the presence of a “toxic cocktail” in the high oases, never go around the bush. Admittedly, for them global warming, and consequently that of the water, is one of the factors of the proliferation of algae to which we owe this green color. But they essentially see the hand of man, and especially the tail of a small fish, ten centimeters at most: the minnow. “If these algae are there, it means that the organisms – zooplanktons or small crustaceans – which normally eat them are disappearing. And if they disappear, it is because they are eaten by minnows. These fish make the plankton disappear,” explains Adeline Loyau.
However, minnows, like trout for that matter, are not found in high altitude lakes in their natural environment. They were transported in these clear waters, and very poor in nutrients, by sportsmanship, for the happiness of the fishermen. This does not date from yesterday and is not confined to France. “There is a whole lot of literature on the subject”, recall the Toulouse researchers. Minnows are used as live bait to fish for larger catches. But sometimes they accidentally come off the hook. “And probably also that fishermen put the minnows they have left back into the lake so they don’t have to bring them back down,” says Adeline Loyau.
The Pyrenees National Park is on the same wavelength concerning the responsibility of minnows, and “forage fish” in general. At the beginning of the summer, after the spectacular greening of the Cirque d’Estom lake, he recalled that it was forbidden “to transport live fish, whatever the species, including minnows, and also to introduce into the lakes and watercourses” in the heart of its perimeter. Prohibition also valid for all lakes in the Hautes-Pyrénées perched at an altitude of more than 1,000 meters.
A phenomenon “ reversible »
Questioned by AFP, Sébastien Delmas, president of a fishing association bringing together those of the Pyrenees, recognizes that the minnow poses a problem and recommends “harmonizing the regulations” from one department to another to limit live fishing. in the mountains.
Adeline Loyau and Dirk Schmeller plead on their side to limit the authorizations of nursery, “paradoxical” now that the damage of the minnows is known. They are also counting on the awareness of the younger generations of fishermen, according to them “much more ready to make concessions”. They also recommend “sharing the mountain”, reserving only a few lakes for fishing and preserving the transparency of the others for hikers. Finally, they insist on the fact that “the phenomenon is reversible”. On the Spanish side of the Pyrenees, on the initiative of scientists, lakes are “patiently and gradually” emptied of their fish. And they gradually regain their clarity.
*CNRS/UT3/INPT