Dead boars washed up on shores point to lethal hazard of poisonous gas and water pollution from algal slime

Fears are growing over potentially lethal green algae piling up on the north Brittany coast following corpses of wild boars washing up at the picturesque tourist destination of côtes d'Armôr.

Three dead boars were found floating in the water or slumped on muddy banks of the Gouessant estuary at Morieux on Wednesday after 18 were found dead the previous day. A total of 31 animals have been found dead this month.

Close to the estuary a beach at Saint-Maurice is now closed because of stinking piles of sludge from decomposing green algae. The local mayor said the beach had been cordoned off because the slime could harbour pockets of toxic gas fatal to humans if they slipped on it.

Local environmentalists have long campaigned against the dangers of what has become known as Brittany's "killer" green algae. It has been affecting the rugged north Breton coastline for decades but has increased in recent years, causing the death of a worker who was clearing it in 2009, as well as killing dogs and a horse walking on the beach.

Ecologists blame the spread of the algae on nitrates in fertilisers used in intensive pig, sheep and dairy farming in Brittany, saying the nitrates flow into the river system and enter the sea.

The government has launched a massive long-term plan to clear the beaches of algae, hauling away the noxious growth with bulldozers. But campaigners say nothing will change unless Brittany's powerful agriculture and food industry cuts its nitrates use. The algae is harmless until it dries and then decomposes, giving off a foul smell. Pockets of the toxic gas can become trapped under its crust.

Autopsies were being carried out on the boar corpses, which were also being tested for hydrogen sulphide, a poisonous gas.

But officials said it was too early to say how the animals died or whether the green algae was to blame. An official analysis of the water in the estuary found that green algae was "above the alert level but below the danger level".

A local police official, Philippe De Gestas, said of the dead boars: "They were not [otherwise] sick and they did not drown."

Yvette Doré, the Socialist mayor of the neighbouring area of Hillion, told Le Monde she was "under no illusion" and that the boars' death was "very certainly" linked to toxic gas from the green algae. In Hillion in 2009 a 27-year-old vet was dragged unconscious from a metre-deep patch of rotting algae after his horse collapsed and died within minutes from fumes given off by the sludge that was heaped on the beach.

The local environmental group Eau et Rivières de Bretagne suggested that the boars could have been killed by drinking water contaminated with the algae and noxious gas. Thousands of tonnes of green algae have been cleared from the Brittany coast this year. In Finistère the amount of algae has grown since last year.

In a front-page editorial entitled, Green Algae: The Unbearable Denial, Le Monde said the French state, in thrall to lobby groups, was downplaying the role of agricultural pollution in the proliferation of noxious algae.

Environmentalists have claimed that politicians are too scared of alienating the farming vote before next year's presidential election so had failed to take big measures on curbing nitrates.

Brittany is the biggest stock raising region in France, with a high concentration of pig, poultry and dairy farms.

On a visit to Brittany this month the French president, Nicolas Sarkozy, defended farmers, saying they were not to blame for the green algae. He dismissed as "environmental fundamentalists" those people who were loudly campaigning over the nitrate link to the algae.