In response to the deaths, ships in Canadian waters have slowed and shifted their routes, but fishing gear remains a problem, Mr. Hamilton said. In 2018, three right whales died in the United States and none in Canada, through mid-October, according to federal data.

Changes in feeding patterns because of warming waters may also be affecting their fertility, Mr. Hamilton said. In the last eight years, the whales have been feeding much more in Cape Cod Bay, and less in some other habitats.

“They’re having to search new places to feed,” Mr. Hamilton said. “The increased calving interval suggests many are not getting as much food as they need in order to support a calf,” which can cost a mother one-third of her body weight.

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To track North Atlantic right whale births, two teams fly over the Atlantic coastline from Savannah, Ga., to Jacksonville, Fla., every clear-weather day from December through March.

A few weeks ago, looking out of the home they share on the coast, a Florida Fish and Wildlife Conservation Commission worker noticed the first mother of the season swimming with her calf, said Jennifer Jakush, a biological scientist with the commission’s North Atlantic Right Whale Project. That was whale 3317 and she had last given birth three years ago, suggesting that at least some female whales are getting the nutrition they need, Ms. Jakush said. “That in and of itself is a good sign,” she added.

The calf born to 1204 was spotted by aircraft during its regular survey, near Amelia Island, north of Jacksonville, Ms. Jakush said. The mother hadn’t been seen earlier this season, so it’s not clear how old her calf is yet.