Three dead boars were found on Wednesday at the mouth of the Gouessant estuary in Brittany, an AFP journalist witnessed, bringing to 31 the number found this month, floating in the water or washed up in the area.
The nearby beach has been closed for safety, its cove stinking with algae which give off a poisonous gas when they rot.
"One of the theories we have is that the animals could have drunk water that could contain algae," said Gilles Buet, a Brittany water official.
"They were not sick and they did not drown," said local police official Philippe De Gestas.
"We found two carcasses in the morning, then five more, then it went up to 17" and later 18, he said on Tuesday.
Local authorities said in a statement that tests on the water revealed a level of blue-green algae "above the alert level but below the danger level".
Officials and environmentalists say the spread of algae is driven by nitrates used in fertiliser. The proliferation of the minute organisms was speeded by unusually hot weather early this summer.
In 2009 a person in France died after working to clear algae, as well as a horse.
Officials were testing for hydrogen sulphide, a poisonous gas given off by the blue-green algae, also known as cyanobacteria, when they decompose.
Gestas said they were also carrying out autopsies on some of the boars.
Further test results were due Wednesday, the statement said.