Squilla took to motherhood. When she was first spotted with her new calf in January 2021 off the Georgia coast, mother and daughter stayed so close as they swam that they were touching. The baby rolled around in the water, as calves often do, and Squilla joined in, turning her belly to the sky.

The birth of Squilla’s calf was a momentous event for their species, the highly endangered North Atlantic right whale. As one of just 70 or so mothers, Squilla is part of a small group that represents the species’ last chance for survival. The fact that Squilla had a daughter made the birth more significant still, offering the possibility of a new generation of matriarchs.

For decades, North Atlantic right whales were slowly recovering after being devastated by centuries of whaling. But in 2011, their numbers suddenly started dropping. Now, they are one of the most endangered species in the United States.

In 2017, so many dead and injured right whales turned up that federal officials declared an “unusual mortality event” that’s still underway.

While the situation is considered unusual, the reasons are well understood. A document from NOAA Fisheries put it simply: “North Atlantic right whales are dying faster than they can reproduce, largely due to human causes.”

Whales are being killed and injured in vessel collisions. They are getting tangled in fishing gear. And females are giving birth to fewer calves. Biologists think that’s partly because the stress of nonlethal collisions and entanglements takes such a toll, and partly because it’s harder for the whales to find food as climate change alters the oceans.

Many females of reproductive age are not having calves at all, researchers say.

Some opponents of renewable energy say offshore wind projects along the East Coast are responsible for the increase in whale deaths, but so far there is no evidence to support that. Researchers say a better understanding of ocean noise is needed.

If the species is to recover, it will be because enough of the 70 or so mothers, Squilla among them, survive and bring more calves into the world.

“With the loss of a female, you’re losing her entire future of reproduction,” said Erin Meyer-Gutbrod, a marine ecologist at the University of South Carolina who studies right whales.

Squilla and her calf seemed to be off to a good start. Two months after they were first seen off Georgia, they were spotted some 700 miles north, in the waters off New York. They were still swimming side by side.

‘That’s a healthy calf’

When Squilla herself was a young whale, she spent summers feeding off the coast of New England and north into the Bay of Fundy, which stretches into Canada.

But in 2010, when she was about 3, right whales started abandoning those waters. They had little choice, scientists would come to understand. If the whales were humans, we might call them climate migrants.

Right whales feed largely on copepods, a fatty crustacean smaller than a grain of rice. In the early 2010s, researchers have found, climate change fueled a shift in water temperature that caused copepod populations to crash in the waters where whales had long found them.

The whales appear to have set off in search of a new supply. And they eventually found it farther north, in the Gulf of St. Lawrence. But if the move helped fill their bellies, it came at a high cost: They had ventured into a busy shipping and fishing zone without protections.

The first time Squilla was spotted in the Gulf of St. Lawrence, she was 10. It was 2017, a terrible year for her species. Seventeen North Atlantic right whales would be found dead, about 4 percent of the estimated population. Twelve of those fatalities were around the Gulf of St. Lawrence. In the cases where researchers were able to investigate the cause of death, most were linked to vessel strikes.

Eventually, the Canadian government would implement speed restrictions there for vessels. But up and down the whales’ migration routes from Florida to Canada, collisions remain a grave threat. National Oceanic and Atmospheric Administration Fisheries has said current speed limits in U.S. waters don’t offer sufficient protection. Two years ago the agency proposed stricter rules, but they faced fierce pushback from sport fishermen, recreational boaters and harbor pilots. So far, the rules have not been adopted.

At times, the everyday act of swimming in the ocean can be like crossing a highway. This year alone in U.S. waters, three right whale carcasses have exhibited signs of vessel strikes. An orphaned calf is also presumed dead, a fourth casualty.

Despite the dangers, when Squilla took her calf to the Gulf of St. Lawrence in June 2021, mother and daughter appeared to be doing well. The scientists who monitor right whales, identifying them by scars and distinctive markings on their heads, hadn’t given the younger whale a name. Instead, they used a number: 5120.

On a sunny day the next month, Gina Lonati, a doctoral student at the University of New Brunswick Saint John, came across 5120 while conducting research.

“That’s a healthy calf,” she recalled thinking as she looked at her drone videos. “She was chunky, which is a compliment to a whale.”

And soon, 5120 would make it safely to her first birthday. At around that age, she was spotted off New York alone, now apparently separated from her mother, Squilla. She’d spend the next months in the Northeast, moving to Massachusetts and then back into Canada.

Out on her own

It was sometime in those months, during the spring or summer of 2022, that the young one got into trouble.

In late August, the Canadian authorities spotted a whale off the coast of New Brunswick with fishing gear wrapped around her tail. It was 5120.

After reviewing photographs, NOAA biologists made a grim assessment. “As the yearling grows,” officials wrote, “the entanglement is likely to cause increasing harm and eventual death as it constricts the tail and other areas of the whale’s body.”

Experts compared it to a collar getting tighter and tighter around the neck of a growing puppy.

But hope was not lost. From Canada to Florida, there is a network of groups that makes dangerous excursions to try to free entangled whales. One, the Center for Coastal Studies, spotted 5120 from a plane in Cape Cod Bay in January 2023.

Disentangling a giant wild animal in the ocean requires bravery, grit and luck. Unlike with land mammals, you can’t just knock the whale out. Rescuers don’t get into the water; it’s too hazardous, and whales swim away too quickly, anyway.

In January, in a frigid wind, a team spent two days at sea trying to disentangle 5120. They got as close as they could from a small boat. They threw custom-made hooks with razor-sharp blades designed to latch onto and sever thick fishing line. They spent hours trying to stay with her as she tried to flee, invisible under the turbid water.

With right whales, such efforts succeed about half the time, the group says.

But not this time.

“Sunset came and we had to go home,” said Bob Lynch, who was on the boat. The team hoped for another chance to respond, but they never found her again.

“It’s a reminder of how much of a Band-Aid we are to the overall entanglement problem and how prevention is so clearly a better choice than relying on this kind of response,” said Mr. Lynch, operations manager for the center’s rescue team.

Most entanglements are thought to come from lobster and crab gear, because ropes connect traps on the ocean floor to buoys on the surface. In the mid-1990s, fishermen started switching to stronger ropes, which appears to have led to more severe entanglements for right whales. Separately, the population of lobsters started booming and people started catching them farther from shore.

“It’s just this perfect storm of all sorts of things ramping up: stronger ropes, more gear, more overlap with the whales,” said Amy Knowlton, a senior scientist at the New England Aquarium.

For years, the federal government has been working with fisheries to mitigate these effects. Lobstermen have reduced the amount of rope in the water by concentrating more traps per buoy and by connecting those traps along the bottom with line that doesn’t float. For the buoys, they have switched to ropes that are easier for whales to break. In Massachusetts, Cape Cod Bay and surrounding waters are closed to lobster traps from Feb. 1 to April 30, when right whales typically congregate there.

But in Maine, which produces about 90 percent of the country’s lobster, right whale sightings have been more diffuse. The gear changes largely allowed the state to avoid seasonal closures.

“Lobstermen care deeply about everything in the ocean and nobody wants to see right whales harmed,” said Patrice McCarron, policy director at the Maine Lobstermen’s Association, an industry group. “But they also very much feel like they’ve been overregulated and are implementing measures that are not necessarily benefiting the species, because we don’t have a significant amount of interaction with them.”

Scientists and environmentalists see a lot of promise in a type of new equipment, known as ropeless or on-demand gear, that releases a line or flotation bag only when the fisher is on hand to check the trap, sharply reducing the danger to whales.

But lobstermen have been skeptical, worried that this kind of gear will be inefficient and too expensive.

Just weeks before the failed effort to disentangle 5120, Maine’s congressional delegation added a provision to a huge federal spending bill. The move mandated a six-year pause on any new regulations for the lobster and Jonah crab fisheries related to right whales, and provided additional money for research.

“The fact is, there has never been a right whale death attributed to Maine lobster gear,” the Maine delegation and Gov. Janet Mills, a Democrat, said in a statement at the time.

Squilla’s calf would change that.

Half a lifetime tangled in ropes

Her body washed up in the surf on Martha’s Vineyard early this year.

Sarah Sharp, a veterinarian with the International Fund for Animal Welfare, was assigned to lead the necropsy. Arriving at the beach, she was first struck by how young and small the whale was, just 3, far from grown.

As she examined the carcass, she was astonished by the severity of the injury from the fishing lines encircling the base of 5120’s tail.

“They were so deeply embedded,” Dr. Sharp said. Inches of scar tissue had tried to heal over the wound. “The lines looked like they were coming out from close to her spinal column, and just coming out of the soft tissues.”

The wound could not heal, in part because the drag from the lines kept it open and bleeding. 5120 spent half her short life with that entanglement.

The Aquinnah Wampanoag Tribe received her body. In a ceremony, they said prayers and expressed gratitude for her life. Then they buried her.

“It hurt us very deeply,” said Cheryl Andrews-Maltais, chairwoman of the tribe. “It’s a child.”

This month, NOAA Fisheries announced the official cause of death: chronic entanglement.

In the past, it’s been hard to know the origin of fishing lines involved in entanglements. But in recent years, NOAA started requiring certain fisheries in New England states to mark their gear with specific colors.

The rope that was pulled out of 5120 was marked with purple cable ties, indicating that it was from Maine.

Among the state’s lobstermen, the news was met first with shock, then sadness for the whale and fear over what the consequences could be for their livelihoods, Ms. McCarron said.

Even entanglements that don’t kill right whales can contribute to killing off the species. The lines create drag in the water, making it harder for whales to swim and driving up the number of calories they need to survive, researchers say. “On average, an entanglement energy cost is the equivalent cost of producing a calf,” said Michael Moore, a scientist with the Woods Hole Oceanographic Institution. “And so if you have an entanglement, you’re not going to get pregnant.”

Scientists believe North Atlantic right whales used to give birth every three years or so. But recently, it’s been “six, seven to 12 to never,” Dr. Moore said.

More than 85 percent of right whales have been entangled in fishing gear at least once, according to research funded by NOAA Fisheries. Squilla has been seen with entanglement scars three times. Squilla’s mother, Mantis, has been seen with them twice.

Dr. Moore spotted Squilla this past spring, as he conducted research on right whales in Cape Cod Bay. Given her measurements, it is unlikely that she will give birth again this year.

But she wasn’t entangled. There were no signs of recent wounds. She was swimming strongly.