CHARLOTTETOWN -- Analysis of six endangered North Atlantic right whales found dead since June in the Gulf of St. Lawrence suggests four were struck by ships and one died caught in fishing gear, says a report released Thursday.

The sixth was too decomposed to be sure. Preliminary findings of a seventh carcass assessed after the others and not included in the report indicate it too was caught in snow crab fishing lines, said co-author Pierre-Yves Daoust of the Atlantic Veterinary College.

There was no evidence that various toxins may have played a major role in the deaths, he told a press conference in Charlottetown. They were among 12 right whales discovered lifeless since June 7 in Canadian waters, plus another three in the U.S.

"They died because of human activity," Tonya Wimmer, director of the Marine Animal Response Society, said Thursday.

Fisheries scientists have scrambled to explain what they say is an unprecedented die-off in modern times. The curious, acrobatic animals have not perished at such a pace since they were hunted for their oily blubber in the 1800s.

The North Atlantic right whale population is now estimated at just 458.

Scar tissue, along with evidence of internal bleeding, helped veterinarians confirm the injuries did not happen to already deceased whales, Daoust said.

"Scar tissue doesn't form after death," he said.

Scientists say collisions with large vessels and getting tangled in fishing gear are the greatest threats to the docile surface feeders. But without actual footage of random strikes over a vast area, it's not yet clear what kind of ships are most often involved.

In August, the federal Fisheries Department announced new measures to improve the safety of whale migration in the Gulf, including rules around fishing gear and speed restrictions for large vessels.

Wimmer said the focus now has to be about preventing ship strikes and entanglements.

"Everyone really needs to sit down together -- all the players" from government, industry and animal protection in Canada and the U.S., she said.

"I think their fate is really in our hands and it's up to us to make sure they're still here."

Matthew Hardy, a Gulf region spokesman for the Fisheries Department's science branch, said there's no time to lose.

"That sense of urgency that we've had over the summer and the sense of urgency going forward to get ready for next season and have measures in place, I think we all feel that."

Federal Fisheries Minister Dominic LeBlanc said in a statement Thursday that efforts to slow ships and close some snow crab fishing areas were warranted.

"This has been a tragic summer for the North Atlantic right whale population and we are deeply concerned abut the future of these marine mammals," it said. "We will continue to monitor the migration of the whales and the efficiency of our measures in order to act accordingly, based on evidence."

For now, speed restrictions for vessels 20 metres or more to a maximum of 10 knots in the western Gulf of St. Lawrence will continue, said LeBlanc.

Right whales are expected to migrate out of the area later this fall but feeding patterns can be unpredictable. A big question is whether the massive animals, which can reach 18 metres in length and live at least 75 years, have shifted habitat.

Traditional feeding grounds include the Bay of Fundy and Roseway Basin, but the whales have increasingly appeared over the last five years in the Gulf. They prefer to eat small crustaceans called copepods.

Of the 12 killed in Canada this past summer, Wimmer said there were eight males and four females aged from two years to more than 37 years old. One was an 11-year-old female that might have added at least five to 10 calves to the dwindling population.