A red algae infestation in the St. Lawrence Seaway caused by heavy rain this summer may have caused the death of several marine animals, including birds, fish and a handful of beluga whales, say scientists in the area.

Residents in the Lower St. Lawrence region and greater Rimouski area have reported finding hundreds of dead sturgeon and birds on the shore.

And seven beluga whales have been found dead in the past week, about half of the average annual death toll recorded for the species, according to Fisheries and Oceans Canada.

The pattern of death is unusual for the region, said Nadia Ménard, a marine biologist based in Tadoussac.

"It's mostly in the numbers in the last week," she said. "If we're talking about 15 [dead whales] a year, now in one week we're about at half of what we normally have in a year, so it is quite particular."

The algae, known as the "red tide" but actually pink in colour, produces a toxin that in high concentrations can affect whales' brains. The algae normally spikes during the spring melt, but has re-emerged this summer because of unusually high precipitation.

Recent tests found 50,000 algae organisms per litre of water. The toxic level is considered to be 2,000 organisms per litre.

The toxin could cause mental confusion and slow the marine mammals' reflexes, making them more vulnerable to collisions with boats, Ménard said.

Some of the whale carcasses had propeller marks on their bodies, leading scientists to speculate the mammals had been affected by the toxin.

Biologists at Fisheries and Oceans are running toxicology tests on the whale remains.

The other indicator that suggests a link between the algae and carcasses is the range of species found dead.

It's unusual to see mammals, birds and fish dying at the same time, Ménard said.

"What sparked our concern is that we're talking about different components in the food web that seem to be affected by something — the seabird, the fish, the seals and the whales," she said.

"It could be pure coincidence of independent events happening simultaneously," or it could be linked to the red algae, she said.

The department is asking people in the region to be sensitive to the whales. Their reflexes may be slow because of algae poisoning, so boaters and sailors should try to steer clear of the large mammals.

Any unusual whale behaviour should be reported to the Quebec Marine Mammal Emergency Response Network, Ménard said.

With files from the Canadian Press