HALIFAX -- The recent deaths of four endangered North Atlantic right whales in the Gulf of St. Lawrence is alarming, given that the massive mammals are just arriving in Canadian waters for the season, a leading marine biologist said Wednesday.

Boris Worm, a biology professor and well-known whale expert at Dalhousie University in Halifax, said the sudden spate of deaths this month stands in contrast to last season, when there were no recorded deaths linked to ship strikes or entanglements.

"It's heartbreaking," he said in an interview, noting the four recorded deaths mark the second-worst mortality rate in the past 10 years. "And it's only the end of June -- so it's probably not the end of the story."

There are about 400 North Atlantic right whales left, with deaths outpacing live births.

In 2017, the federal government introduced a series of measures to protect the whales after 12 of them died in Canadian waters -- mostly from collisions with boats or injuries caused by tangled fishing gear.

Those measures included increased aerial surveillance, restrictions on shipping lanes, slower speed limits and some real-time monitoring accomplished with the help of underwater listening devices.

"We were pulling all stops to track the right whales," Worm said, adding that some of the measures were quite costly. "Based on sightings, they closed areas dynamically .... It was innovative. It's rarely done anywhere else. And it's a very effective tool."

While it remains unclear what caused the latest deaths, Worm said it will be worth asking if any changes have been made to the mitigation measures introduced last year.

"Whatever was done (last year), if the cause of the deaths (this year) was entanglement, then clearly it wasn't enough," he said. "It means going back to the drawing board."

Fisheries and Oceans Canada did not immediately respond to a request for more information about the latest precautions.

Earlier this month, the department confirmed the cause of death for one of the first two whales found was not linked to a ship strike or entanglement, but there is still work to be done to determine what happened to the other whales.

The second carcass, found last week, was brought to shore on western Cape Breton for a necropsy involving a team of about 30 people.

On Tuesday, the department reported that another dead whale had been spotted near New Brunswick's Acadian Peninsula, and a fourth had been found west of Quebec's Iles-de-la-Madeleine -- a chain of islands northeast of P.E.I.

Sean Brillant, a senior conservation biologist with the Canadian Wildlife Service, said part of the problem is that the whales have changed their migration patterns since 2014-15.

Instead of heading to their traditional summer foraging grounds in the Bay of Fundy and the Roseway Basin off southwestern Nova Scotia, the population has shifted to a more northerly route.

"They're playing on a new highway," he said. "What's making it more difficult is that we don't actually have a good handle on where they are or how they're using the Gulf of St. Lawrence."

He said he was urging people not to panic when the first two deaths were reported, but that message changed with the latest grim news.

"I think we got really lucky last year ... and we can't expect to hit zero," he said. "Now that we're at four, and we're only in the early part of the season, people are really starting to wring their hands a bit more. There's a bit more of a panic."

Depending on the outcome of the latest tests, Brillant said it may be time to consider taking long-term steps to protect the whales.

"We didn't solve the problem last year and we're still in a crisis mode for dealing with this situation," he said.

He said more work needs to be done on keeping track of the whales, developing ropeless fishing gear and, perhaps, placing more restrictions on the shipping lanes.

The Sierra Club Canada Foundation says emergency actions are needed.

"We are going to have to act quickly if we are to see this species survive," national program director Gretchen Fitzgerald said in a statement.

"Over the last three weeks, we have seen the death of one per cent of the remaining right whale population and the summer has only begun."

She said the existing fisheries closures and shipping speed restrictions are only as good as the monitoring on the water.

"I fear that without greater protections, we will see the extinction of this species," she said.

The International Fund for Animal Welfare, based in Washington, D.C., suggested Ottawa has failed to adequately address human activities that continue to kill the whales.

"Unless ... additional protections are put in place immediately on both sides of the border, the extinction of the North Atlantic right whale will become a sad reality in our lifetimes," the animal welfare group said in a statement.

Meanwhile, Worm said he is working with a student to develop an automated satellite tracking system that will spot the whales from space and report their positions.

"They're big enough that you can see them on satellite images," he said.

A study released last week found more than half the 70 right whale deaths recorded over the last 16 years were caused by entanglements or vessel collisions.