Fearing an existential threat to one of the largest mammals in the sea, the United States government has launched an investigation into a string of deaths of endangered North Atlantic right whales.

At least 13 of the whales have been found dead this year off Atlantic Canada and New England, an unprecedented number experts say threatens the survival of the species.

"The North Atlantic right whale is one of the most critically endangered populations of large whales in the world," David Gouveia with the U.S. National Oceanic and Atmospheric Administration's fisheries branch said Friday.

"The population numbers are very low and recovery is very slow. It's a significant die off."

In response to the troubling raft of deaths, NOAA Fisheries has declared "an unusual mortality event."

The designation triggers a sweeping investigation into the cause of the deaths, including environmental and habitat conditions, threats from commercial fishing and shipping and other risk factors.

NOAA Fisheries officials will work with counterparts at Fisheries and Oceans Canada on sampling and data collection, analysis and recommendations for future responses.

Matthew Hardy, aquatic resources division manager with Fisheries and Oceans, said Canadian officials expect to have necropsy results finalized by the end of September.

"This is a tremendous amount of work for the six necropsies that we completed in the Gulf of St. Lawrence," he said. "We have specialists in zooplankton, food distribution, oceanography and toxicology to help feed into our understanding of what is actually happening."

Ten North Atlantic right whales have been found dead in the Gulf of St. Lawrence and two have been found in U.S. waters. There have also been four whales found entangled in fishing gear and two whales successfully released.

In April, a North Atlantic right whale was found dead in Cape Cod Bay.

In all, 13 of the marine mammals have been found dead this year, more than triple the annual average of 3.8 in Canada and the U.S.

Some of the whales have apparently died due to ship strikes or fishing gear entanglement.

No more than 500 still exist in the wild, prompting conservation groups and marine scientists to warn that the survival of the North Atlantic right whale is at imminent risk of extinction.

The whales, which summer off New England and Atlantic Canada, are among the most imperilled marine mammals on Earth. Populations have only slightly rebounded from the whaling era, when the blubber-rich baleen whale became nearly extinct.

Sean Hayes, the protected-species branch chief at NOAA's Northeast Fisheries Science Center, said compounding the whale deaths is the declining number of calves.

"The two parts of population growth are numbers of deaths and numbers of births," he said. "When the number of deaths is high and births is low it's obviously from a simple algebraic standpoint hard to recover a population."

Governments in Canada and the U.S. are already taking steps to protect the endangered whales.

In an effort to reduce the frequency and severity of ship strikes, Ottawa ordered large vessels to slow down in the Gulf of St. Lawrence earlier this month.

Vessels of 20 metres or more are required to slow to 10 knots – or about 19 kilometres per hour – while travelling in the western Gulf of St. Lawrence, from the Quebec north shore to just north of Prince Edward Island.

Ships that don’t comply with the speed limit can face a fine of up to $25,000.

Meanwhile, in addition to seasonal speed limits up and down the eastern seaboard, the U.S. also has what it calls "dynamic restrictions" on vessel speeds based on whale sightings.

If a pod of whales is spotted in an unexpected area by an aerial survey team, for example, speed limits are imposed and vessel operators asked to slow down.

The U.S. has also ushered in gear modification rules to reduce fishing gear entanglement, especially with lobster trap pot gear, gill nets and floating rope.

"We've taken approximately 30,000 miles of line out of the water column over the years and we've also closed seasonally about 32,000 square nautical miles of fishing territory during the course of the years to protect whales," said Gouveia, branch chief of NOAA's protected species monitoring program.

The issue is a new one in the Gulf of St. Lawrence, which until the last few years has seen far fewer right whales.

"It wasn't that long ago that sightings of right whales in the gulf were considered a rare occurrence," Hardy said. "We've obviously seen a much greater number in recent years, particularly this year."

Last month, lobster fisherman Joe Howlett was killed in waters off eastern New Brunswick after he freed a right whale caught in fishing gear.

The tragic death prompted U.S. officials to ban whale entanglement rescues, a restriction since lifted for all whales — except right whales.

Sarah Wilkin, national stranding and emergency response co-ordinator for NOAA Fisheries, said the U.S. has developed heightened safety training for responders.

She said right whale entanglements could be authorized on a case-by-case basis by the most experienced and qualified responders, but she said there has not been an entanglement since Howlett's death.

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