Water for the trout, but not too much!

Les crues, sous certaines conditions, redonnent une dynamique salutaire au cours d’eau. Midi Libre – ALAIN PERNIA

Alors que la saison débute ce samedi, Julie Marais, ingénieure à la fédération de pêche du Gard, rend compte d’études sur la recolonisation des truites après les crues.

September 2020. On the night of the 21st, precipitation in Gard defied weather forecasts and reached 718 mm in Valleraugue. The Gardon and Hérault watersheds then suffered a hundred-year flood, the violence of which is still remembered.
However, three years later, an inventory conducted by the fishing federation, upstream of the town of Saint-André-de-Valborgne, revealed a population of brown trout of all size classes and a good proportion of juveniles".
The study, published on the federation website in October 2023, makes it possible to understand the behavior of fish in the event of heavy flooding. And therefore to better protect them as these events see, under the pressure of climate change, their frequency and intensity evolve and thus shake up a certain number of benchmarks.

Several kilometer slides

Julie Marais, deputy director of the fishing federation and engineer in water science, revealed that "in the event of small floods, the populations will be looking for areas “refuge” in tributaries. The problem then lies at the level of the thresholds, which are sometimes impassable, making it impossible for the fish to move. In the event of a strong 100-year flood, impacting a watershed, the fish flood before or during the flood (from a few hundred meters to several tens of kilometers, Editor's note) &nbsp ;before reconquering the environment. They know where they were before and will try to find the original location. Weirs or lack of fish passage are then always a problem." 
What is less common is the phenomenon of flooding, all things considered. "It’is a fantastic ecosystem booster"  according to the expression of Yann Abdallah in an article published in 2020 in the magazine Truites et Cie.

What is troublesome is when the rearing is done with a genetic strain that is not used to living in these extreme situations< /p>

"Residents often analyze the phenomenon as a destructive evil of the environment but in fact, a river needs this dynamic, support Julie Marais. A flood moves the sediments and mobilizes an organic load between the stream, the river and the sea. Which is important in terms of biodiversity. And then, before the trout spawn between November and January, when the environment is filled with algae following very hot summers, the floods between September and November allow these environments to be unclogged."
The question of the timing of precipitation is, however, crucial since a flood is logically more impactful when the spawning is recent or the fry are too weak to resist the current. A situation taken into account regarding federal fish management.
"What is troublesome is when the breeding is done with a genetic strain that is not used to living in these extreme situations; very hot, dry summers and winters with intense flows. The trout then have no tendency to return to their initial location and do not recolonize the environment." 
Floods whose impact also depends on the level of anthropization of the environment. With the aggravating factor: the waterproofing of soils, the reduction of wetlands or the recalibration of the river bed accentuating the violence of the waves.
"It is necessary to favor meanders on watercourses rather than dikes and concreting, provide for flood expansion zones and significant riparian forests because the more we retain the water upstream the less violent it will be downstream " insists Julie Marais. And what is good for fish is just as good for man…hellip;

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