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SAVANNAH, Ga. — Endangered North American right whales gave birth last winter to the fewest calves seen off the U.S. coast in 17 years, troubling scientists who say the low births support other evidence that the imperiled species’ population may be declining.

Researchers estimate only about 500 of the rare whales still exist. Each winter they migrate to warm Atlantic waters off Georgia and Florida to give birth. Trained spotters who look for mother-and-calf pairs from planes during daily aerial surveys reported extremely few sightings this year— just three newborn whales swimming alongside their moms.

That’s the lowest number of births reported since 2000, when only one calf was sighted. The yearly average is 17. A single bad year doesn’t necessarily mean right-whale reproduction is in real trouble. Birth numbers can fluctuate greatly year-to-year. But researchers have seen below-average births every calving season since 2012, said Clay George, a wildlife biologist who oversees right whale surveys for the Georgia Department of Natural Resources.

“The preliminary data are starting to show the population may be flat or declining,” George said. “But considering how the population turned around in the 2000s, I think it’s too early to be too doom-and-gloom about it.”

After births bottomed out in 2000, the whales rebounded with a baby boom of 31 newborns the following year. One reason the numbers can seesaw so significantly is that female right whales typically give birth only once every three years or so.

It’s also possible this year’s calf total could increase. Researchers were trying Wednesday to confirm a report that a fourth mother-newborn pair had been sighted in Cape Cod, said Philip Hamilton, a right whale researcher at the New England Aquarium in Boston.

Still, the overall population is so fragile that even one bad birth year can affect right whales’ capacity to reproduce a decade from now, when this year’s newborns would reach sexual maturity, Hamilton said.

And there’s other evidence that right whales may be struggling. In the past five years, fewer right whales have been seen in waters where they’re known to gorge themselves on plankton in the northern Atlantic between New England and Nova Scotia. That points to a potential food shortage, Hamilton said.

For the first time since 2001, researchers saw no first-time mothers among the whales that had calves this past winter. They know this because mother whales are photographed and identified by unique markings on their heads. The three whales that did give birth had not had babies in seven to eight years — more than double the typical span between births.

Finally, only one other adult right whale — a male — was seen off Georgia and Florida during the calving season. In past years, as many as 200 total right whales have been spotted during winter.

“It’s really the combination of all the pieces of information that make it more alarming,” Hamilton said. “We will be really concerned if it’s another bad year” in 2018.

There are some signs that right whale births could improve next winter. More whales spent last the summer feeding in the Bay of Fundy off Nova Scotia than scientists had seen in several years. And Hamilton said right whales returning to Cape Cod appear more robust, and less thin, than in recent spring seasons.

“I’m somewhat hopeful next year will be better,” Hamilton said. “But it couldn’t be too much worse.”