MONTREAL—A researcher monitoring belugas in the St. Lawrence estuary is warning of a looming "catastrophe" after another difficult calving season for the endangered whale.

The belugas have been in a slow population decline for the past decade, according to Robert Michaud, the scientific director of Quebec's Marine Mammals Research and Education Group.

His team has found the carcasses of at least five baby belugas so far during the calving period, which officially ends on Oct. 15. The number of dead beluga calves turning up on the shore has been unusually high since 2008, Michaud said.

"It's a catastrophic trajectory we're observing, and we don't yet know exactly what are the causes for that," he said.

"The only way this population can reverse its trajectory would be to increase the survival rate and the birth rate, and what we've been observing for the last years is totally the opposite.

If the population is to recover, Michaud said there must be a concerted effort to reduce the sources of stress on animals, particularly in areas frequented by beluga mothers and their calves.

The latest figures on beluga calves come amid a debate over whether to allow exploratory drilling off shore of Cacouna, Que., near the breeding ground at the mouth of the St. Lawrence.

A judge suspended drilling for the TransCanada oil terminal last Tuesday following objections from environmental groups.

The marine terminal would allow the company to export oil transported from the Alberta oilsands as part of its Energy East pipeline project, which has been submitted to the National Energy Board.

TransCanada has argued it had all the proper permits and was sensitive to the belugas in the area.

Michaud is among those opposed to the oil terminal.

He said its construction, along with "all the phases that are preliminary to the construction, and then afterward the exploitation of such an activity," could be detrimental to the whales.

Researchers don't have an exact count on the number of belugas in the St. Lawrence estuary, but the population was estimated at 889 in 2012, according to a recent Fisheries and Oceans Canada report.

The population was stable or increasing at a slow rate after unregulated hunting closed in 1979 until the early 2000s, when it reached approximately 1,000, but it has declined in the years since, according to the report.

The worst year for dead calves was 2012, when 16 were found dead, said Stephane Lair, a veterinary professor at the University of Montreal who has conducted autopsies on the carcasses.

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Whales were found to be dying of cancer at an unusually high rate in recent years as well, according to a study led by Lair.

Lair said it's difficult to know the exact cause of the calves' deaths, but the findings are troubling.

"What we think is that either the calf is too weak to follow the mother, or there is a bonding problem between the calf and mother," said Lair.