Three dead boars were found at the Saint Brieuc bay at the mouth of the Gouessant estuary in Brittany on Wednesday, bringing the death toll to 31 this month.
“They were not ill and they did not drown,” according to local police official Philippe de Gestas, who said that autopsies were being carried out on some of the boars.
“There is no rational explanation,” regional water official Gilles Huet, told Ouest France newspaper. “But it has to be said that the Saint Brieuac bay is the only place in France where we can see this exceptional death rate.
“It is also the site where there are the most blue-green algae per year, between 25,000 and 45,000 tons. That’s a troubling coincidence.”
A local authority statement said tests on the water revealed a level of blue-green algae, which let off toxic hydrogen sulphide when they rot, "above the alert level but below the danger level".
Some of the dead animals found on Sunday showed signs of pulmonary embolism which would be consistent with hydrogen sulphide posioning, according to Ouest France.
Officials and environmentalists say the spread of algae is the result of nitrates used in fertiliser, sped up by unusually hot weather early this summer.
A person died after working to clear the algae in 2009. A horse also died that year.
The number of wild boars in France has risen sharply over recent years. A herd of about 30 was observed in the area before the deaths came to light but officials say more deaths are possible.