André Ollivro stepped carefully down the grassy banks of an estuary in the bay of Saint-Brieuc, Brittany, not far from his beachfront cabin. The pungent smell of rotting eggs wafting from decomposing seaweed made him stop and put on his gas mask. It was a strange sight in what is usually a tourist hotspot.

“You can’t be too careful,” said the 74-year-old former gas technician, who is leading the fight against what has come to be known as France’s coastal “killer slime”.

“When I was 16, I used to bring a boat here with my uncle,” Ollivro said. “In those days, it was all about natural beauty and you didn’t see seaweed piled up. It’s a shame this place has come to be associated with death.”