Along the Odet River in Brittany, fisher Mathieu Le Bouter and a dozen volunteers are catching hundreds of trout and young salmon - not for sport but to save them. The water is barely up to their knees, which is problematic when you're a fish that needs more than a puddle to thrive.

“The situation is catastrophic, far worse than we thought,” said Le Bouter, of the Finistère fishing federation. The team transfers the fish to tanks of oxygenated water and releases them 10 or 12 miles downstream where tributaries offer more water and hope. “The fish are under a great deal of stress. When you combine very high temperatures with low levels of dissolved oxygen, problems arise,” he added, as if describing a bad spa day.

The Loire, France’s longest river, is at a historical low, with some branches reduced to a trickle between sandbanks. Two weeks ago, 16% of French rivers were reported to have dried up, and a quarter of small streams vanished or stagnated, according to the national low water observatory. The Seine is now “on life support,” declared Le Monde, with half its flow drawn from reservoir lakes upstream - a measure not normally needed for another month. In Vernon, Normandy, the flow dropped from 200 cubic metres per second last July to 110.

Ninety-nine out of 101 French departments are under water restrictions, and a state of emergency has been declared in 58. The ministry for ecological transition noted a rise “in the death of fish linked to the rise in water temperature.” An emergency agriculture bill passed on Tuesday facilitates water storage tanks for irrigation, sparking fears that farmers are being prioritized over fish and fair sharing.

Claude Roustan, president of the national fishing federation, blames not just drought and climate change but “excessive extraction” of water for crops. Le Bouter says water levels in the Odet and nearby Isole River always drop in summer, but never this early. “As the days go by, the environment becomes increasingly suffocating for them. Without doing what we are doing, those that remain are doomed,” he said.

Le Bouter and his volunteers will keep wading through the Odet and Isole for as long as necessary. “We seem to be the only ones who are concerned about the fish. Unlike birds, deer and foxes, which are more visible, they tend to get forgotten.” He added: “It’s the holidays, and we’d rather be doing something else, but at the moment we’re the only ones out there carrying out these rescues and systematically raising the alarm.”