Correction: An earlier version of this story misstated the number of calves of the endangered right whales spotted by aerial survey teams during the past winter. The teams in Florida and Georgia counted six new calves, including one that probably died, not six in addition to the one that probably died.

A right whale sounds in Cape Cod Bay. Aerial surveys have counted only six new calves this year, down from the average of 20. (BILL GREENE/BOSTON GLOBE VIA GETTY IMAGES)

Normally for a few days in spring, beachgoers on this hook of land stretching into Cape Cod Bay witness one of the rarest scenes in the animal kingdom: dozens of surface-skimming North Atlantic right whales, lumbering just a few hundred yards from shore.

But that rite of spring was upended this year. The critically endangered animals, which usually arrive in late March or early April to graze on shrimplike plankton, began arriving before Christmas, as water temperatures hovered several degrees above normal, dispersing only recently.

“It’s a confluence of remarkable things. We’ve got extraordinarily rare animals, nearly extinct, acting very unusually,” said Charles “Stormy” Mayo, director of the right whale habitat studies program at the Provincetown Center for Coastal Studies.

Before arriving early in Cape Cod waters, one of their main feeding grounds, right whales had a difficult winter off Florida and Georgia, where they gave birth to fewer calves.

Scientists are reluctant to draw a straight line between warmer water and changes in whale behavior, but some feel that they’re seeing more than coinciden-
ces. Water temperatures in and around Cape Cod Bay were more than 3.5 degrees above average this winter, although scientists say this is probably a short-term anomaly that can’t be directly attributed to climate change.

“To me or you 3.5 degrees isn’t a big difference, but in an ocean system it means different oceanography, different currents and different biological processes,” Mayo said. He suspects this could be driving changes in the distribution and timing of plankton blooms, in turn influencing the whales’ odd behavior.

With only about 400 of the animals in existence, North Atlantic right whales inevitably attract attention when they venture close to shore. The vast majority of them bear the scars of ship strikes and entanglement with fishing gear, two leading causes of right whale mortality and a threat to the species’ survival.

Humans have long posed a threat to the docile, huge-headed animals, which were hunted to near extinction in the 19th and 20th centuries because of their slowness and buoyancy after being killed (thus making them the “right” whale to hunt for their oil and baleen). But there are indications that in the coming decades the whales will be affected by a changing planet.

“It was a terrible year for right whale calves,” says Tony LaCasse, a spokesman for the New England Aquarium, a leader in right whale research.

Female right whales venture as far south as Florida to give birth, and the past decade has witnessed an encouraging uptick in calving numbers, with a yearly average of 20 and a high of 39 born in 2009. But this winter, aerial survey teams in Florida and Georgia have counted only six new calves, including one that likely died, apparently from malnutrition.

According to scientists, the disappointing numbers could be linked to changes in the animals’ northern feeding grounds brought on by water that is warmer but also less salty because of melting Arctic sea ice.

Female right whales have a gestation period that can last more than a year; for that reason, researchers are looking at a change in the food supply in Canada’s Bay of Fundy in the summer of 2010 as a possible culprit for this year’s low number of calves.

Right whales, which can weigh as much as 70 tons, consume 2,200 to 5,500 pounds of tiny crustaceans, or copepods, a day. Females do not get pregnant if they are underfed, and with good reason: They can lose up to 30,000 pounds on the journey from Canada to their southern calving grounds, and once a mother gives birth, she must feed nursing calves, which can put on several hundred pounds a day. Time spent feeding can be crucial in ensuring an animal’s reproductive success.

Along with Cape Cod Bay and a channel east of Nantucket Island, the Bay of Fundy — a part of the Gulf of Maine between New Brunswick and Nova Scotia — is one of the right whale’s major feeding areas. It is also the preferred feeding ground for nursing and reproductively active females.

Amy Knowlton, a research scientist at the New England Aquarium, was part of a team that studied the animals in the Bay of Fundy in the summer of 2010. “The few right whales that were there were [moving quickly and] diving and showing up a half-mile away, suggesting that they were looking [for] but weren’t finding food,” she said. The whales normally make long dives and come up in generally the same area.

Sperm whales were also observed swimming in the bay that summer, according to Knowlton, which was unusual. In more than 30 years of research in the bay, she said, sperm whales had been spotted only once, and for no longer than a day. In 2010, a large group was there for close to two months.

The strange summer in the Bay of Fundy in 2010 was accompanied by warmer water; researchers think that the right whale’s favorite plankton, a type of cope-
pod called Calanus, was not as plentiful, while the preferred prey of sperm whales, squid, flourished.

These observations track with data from the late 1990s, when current shifts in the Bay of Fundy resulted in a huge drop-off in copepods and, subsequently, some of the worst calving years for right whales on record.

“If we look over the last 30 years, if the water is warmer or fresher, then we see lower abundance of Calanus,” said Andrew Pershing, a scientist at the Gulf of Maine Research Institute and the University of Maine. “We can expect in the future that the Gulf of Maine is going to get warmer and fresher.”

The right whales that Knowlton has seen this spring in Cape Cod Bay show signs of malnourishment.

“We saw a female looking extremely thin,” she said. “She’s a reproductive female who had a calf in 2010, and now she’s just not looking well. The animals aren’t looking as good as we might hope. Their condition and their nutritional fitness has declined.”

Strangely, while the whales were drawn to Cape Cod months earlier than usual this year, Calanus has been less abundant than in years past. But other plankton have thrived in the warmer conditions.

“We began to see an organism this winter called a pteropod that we had not seen before,” Mayo said. “We’ve seen them, but it’s one here, one there, out of a thousand [organisms per sample]. In February we were seeing them dominant in a couple of samples. Hundreds of them.”

He compared the right whales’ behavior this winter to the tourists who flock to the Cape’s seafood joints every summer. Imagine, he says, that one year all of them showed up in February, then failed to arrive as usual in July.

“The whales are just simply not doing what we expect them to do,” Mayo said. For a species as intensely monitored as right whales, much of their lives — such as where they go in the winter — remains poorly understood.

Recently Mayo led a group of scientists by boat to an area off Provincetown where fin whales and dolphins gathered to feast and where the right whales had been surface skimming in large numbers for weeks.

During the trip, Christy Hudak, one of Mayo’s colleagues at the Center for Coastal Studies, spotted a normally sedate whale hurling itself fully out of the water over and over again.

“They’ve been doing that a lot this year,” she said. “We don’t know why.”

Brannen, a writer living on Martha’s Vineyard, is a recent Ocean Science Journalism Fellow at the Woods Hole Oceanographic Institution.