MONTREAL—More than half of the North Atlantic right whales examined after being found dead since this summer were killed by blunt-force trauma, according to a new report.

Twelve of the animals, which are classified as an endangered species with less than 500 in the world, have been discovered dead since June in the Gulf of St. Lawrence, sending shock waves through the scientific and animal conservation communities.

Another three of the right whales were found dead in American waters.

“This makes it pretty much the deadliest year we’ve seen for North Atlantic right whales since the days of whaling,” said Tonya Wimmer, director of the Marine Animal Response Society.

There are only believed to be 458 of the whales in existence.

The long-awaited report includes the results of six autopsies. Four died from blunt-force trauma, one died after becoming tangled in fishing lines and another was in an advanced state of decomposition and the cause of death could not be determined, said Pierre-Yves Daoust, a pathologist and professor at the University of Prince Edward Island’s Atlantic Veterinary College.

Autopsy results on a seventh whale show that it died after getting tangled in fishing line, though the findings from the autopsy came too late for inclusion in the report. The carcasses of the remaining whales could not be recovered from the waters.

The trauma, which showed up as a fractured skull in one of the animals and internal bleeding in three others, could only be the result of a collision with large ships given the whale’s size and its protective cushion of blubber and muscle that lies between the skin and the vital organs, said Émilie Couture, a veterinarian with the Université de Montréal.

“For a whale of that size, to have a collision that causes sufficient trauma that it results in death, it’s the only known cause in this species,” she said.

The autopsy results rule out the possibility that the whale deaths studied so far were due to infection, starvation or other illness, and will put additional pressure on the federal government to put in place preventative solutions to protect the endangered species.

Since the deaths came to light in June, the Canadian government put in place mandatory speed limits on large shipping vessels, limiting them to a maximum of 10 knots in an area of the Gulf of St. Lawrence stretching from Quebec to Prince Edward Island.

Fisheries Minister Dominic Leblanc and Transport Minister Marc Garneau said in a joint statement that, as of Oct. 2, four vessels have been fined for not respecting the speed restrictions.

“We are committed to doing what is necessary to help keep our right whales from harm, and we are considering all options in order to protect this iconic species,” the statement said.

Jane Weldon, Transport Canada’s director-general of marine safety and security, said officials are examining the findings of the autopsies and working with organizations that track the whales’ movements.

While the whales do not yet appear to have moved out of the Gulf of St. Lawrence for the season, she said Transport Canada was prepared to move the mandatory slow zone to other areas of Canada’s territorial waters to protect the mammals.

The shipping industry is also working closely with the government on potential solutions, while coping with the delays, costs and interruptions that have resulted from the speed restrictions, said Sonia Simard, director of legislative and environmental affairs with the Shipping Federation of Canada.

“We are fully aware of the need to ensure that whales and ships can safely coexist,” she said from Vancouver, adding that solutions could include shifted shipping lanes and improved surveillance of whale movements within the Gulf of St. Lawrence that allows shipping traffic to quickly adapt.

North Atlantic right whales are black in colour, often with white patches. They can grow up to 18 metres long and a normal lifespan is at least 75 years. They are generally present in Canadian waters through the spring, summer and fall months, but swim to the southern United States for the winter.

Wimmer said the species is considered to be an “urban whale.”

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“Where they live their lives is actually some of the busiest in terms of human activities, along the eastern seaboard of Canada and the U.S.,” she said. “These human activities are among the primary reasons that these animals die.”

Daoust said there is an “obvious” urgency to introduce protective measures.

“A single death each year in this at-risk population is already something that is very serious. Now that we have at least 12 deaths,” he said. “It’s obvious to me that we have to start as soon as possible having discussions about the probably causes and how we can implement solutions that will diminish the impact.”