Pyrenees: High-altitude lakes turn green, an “imported” fish in the crosshairs of researchers

green algae To the great dismay of summer hikers, high altitude lakes in the Pyrenees, usually crystalline, are turning green. Toulouse researchers point to the minnow, a fish used as bait by fishermen

The Ariège pond of Areau, perched at 1,800 meters above sea level, this summer. — Dirk Schmeller

• In the Pyrenees, several high-altitude lakes have darkened or turned completely green due to the proliferation of algae.
• For Toulouse researchers Adeline Loyau and Dirk Schmeller, specialists in bodies of water of the massif, there is no mystery: the minnow, a fish introduced as bait by fishermen, is at the origin of the phenomenon.
• An example of “counterproductive” biodiversity that, on the other side of the Pyrenees, the Spanish are trying to subdue.

Milky green, even fluorescent, instead of crystal clear waters. Occasional hikers who have worn out their boots on the heights of the Pyrenees this summer, must have sometimes I felt gloomy when discovering that the much-vaunted pure water of the small high-altitude lakes was no longer transparent. At the beginning of July, the Pyrenees National Park published a photo of a lake in the Estom cirque, near Cauterets, in the Hautes-Pyrénées. It was all green, just like Lake Bethmale in Couserans in the heart of summer, or Lake Areau, still in Ariège, just last week.

“It’s not always so obvious but we see that all our lakes are changing color, that there is more turbidity”, say Adeline Loyau and Dirk Schmeller, researchers at functional ecology and environment laboratory of Toulouse* and who have been monitoring the state of the mountain's high-altitude lakes for almost a decade. They crossed quite a few 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 high altitude oases, we never beat around the bush. Certainly, for them, global warming, and consequently that of water, is one of the factors in the proliferation of algae at sea. who we owe this green color. But they essentially see the hand of man, and above all the tail of a small fish, about ten centimeters at most: the minnow. “If these algae are there, that means that the organisms – zooplanktons or small crustaceans – who normally eat them are disappearing. And if they disappear, it’s because they are eaten by minnows. These fish make the plankton disappear,” explains Adeline Loyau.

However, minnows, like trout, are not found in high altitude lakes in their natural environment. They were transported in these clear waters, and very poor in nutrients, out of a sporting spirit, for the happiness of fishermen. This is not new and is not confined to the past. à France. “There is a whole pile of literature on the subject”, recall the Toulouse researchers. Minnows are used as live bait to catch larger catches. But they sometimes come off the hook accidentally. “And no doubt also that fishermen pour the minnows they have left into the lake so as not to have to worry about fishing. take them back down,” says Adeline Loyau.

The Pyrenees National Park is on the same wavelength regarding the liability of the Pyrenees. minnows, and “forage fish” in general. At the beginning of summer, after the spectacular greening of the lake in the Estom cirque, he recalled that it was forbidden “to transport live fish, whatever the location”. ;species, including minnow, and also to introduce them into lakes and streams” from the heart of its perimeter. Prohibition also valid for all lakes in the Hautes-Pyrén&es; more than 1,000 meters above sea level.

A phenomenon “ reversible »

Interviewed by AFP, Sébastien Delmas, president of a fishing association bringing together those in the Pyrenees, recognizes that minnows pose a problem and advocates “  to’harmonize regulations » from one department to the other to limit live fishing in the mountains.

Adeline Loyau and Dirk Schmeller plead on their side. to limit nursery authorizations, “paradoxical” now that the damage from 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”, by reserving a few lakes only for use. fishing and preserving the transparency of others for hikers. Finally, they insist on the fact that “the phenomenon is reversible”. On the Spanish side of the Pyrenees, at At the initiative of scientists, lakes are “patiently and gradually” emptied of their fish. And they find themselves little by little; small their clarity.

*CNRS/UT3/INPT