Why There’s Been a Sudden Population Crash for Rare Whales

North Atlantic right whales had been making a slow recovery, but ocean changes linked to global warming are pushing down their numbers again, scientists say.,

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Climate change is the quiet force behind a sudden decline in the population of North Atlantic right whales, according to a new study that bolsters a growing body of research into why the critically endangered animals have veered from slow recovery to alarming decline.

An analysis of data on plankton, oceanic conditions and whale sightings, published Wednesday in the journal Oceanography, showed that the whales abandoned their traditional feeding grounds in the Gulf of Maine in 2010, the same year that warming water caused the fatty crustaceans they eat to plummet in the area.

Many of the whales eventually followed their food north to the Gulf of St. Lawrence, but the protections from fishing gear and ships that had safeguarded them in their previous habitat did not exist in their new one. Entanglement in gear is the leading cause of death for North Atlantic right whales, followed by collisions with vessels.

“They moved so fast that our policies didn’t move with them,” said Erin Meyer-Gutbrod, a quantitative marine ecologist at the University of South Carolina and one of the study’s authors. “The environment is just not as predictable as it used to be, so I think that we all need to think on our feet more.”

To make matters worse, researchers found that the decline in crustaceans had led to a drop in reproductive rates. “We’re slowing their births and we’re increasing their deaths,” Dr. Meyer-Gutbrod said. “You don’t have to be a super mathematician to guess what that change is going to cause.”

Scientists have counted only 356 individuals remaining.

The research helps explain why deaths spiked in 2017, leading marine mammal officials to declare an “unusual mortality event” that is still in effect. The whales tend to get tangled in ropes used for crab and lobster fishing. The ropes can drown them, sever parts of their bodies or lead to slow deaths from impaired feeding or swimming.

Canadian officials have responded by adding protections in the Gulf of St. Lawrence, including speed restrictions for ships and temporary closures of certain fisheries when a whale is detected. On Tuesday, NOAA Fisheries announced new rules in American waters for catching lobster and Jonah crab that are aimed at reducing the number of lines that run from marker buoys to traps. They also weaken the lines so entangled whales can break free.

Conservationists reacted with disappointment to the long-awaited rules, criticizing them for falling far short of what the whales need.

The most effective solution, according to the study’s authors, would be a transition to ropeless fishing gear. “That would have a tremendous impact,” said Charles Greene, a senior fellow with Ocean Visions, a research and advocacy group, and one of the study’s authors.

Fishermen worry that such gear is prohibitively expensive and less efficient. The key, activists and researchers say, is making the gear attractive to them.

“We have to figure out how we can finance it properly so that they can make this technological shift,” said Dr. Greene, a former professor in the department of earth and atmospheric sciences at Cornell University. “That’s what got people to put rooftop solar on their houses.”

Various speed restrictions exist in United States waters to protect whales from collisions, but operators often disregard the rules, according to research by the nonprofit organization Oceana.

Perhaps 10,000 right whales swam the North Atlantic before the advent of widespread whaling. They were considered good hunting targets because they moved slowly, stuck close to shore and floated when dead, all while yielding abundant oil and baleen, according to the International Whaling Commission. By the 1890s, the species was hunted close to extinction. But the whales’ numbers slowly recovered after commercial whaling was banned, reaching about 500 before the current decline took hold.

Earlier research suggested that plankton loss and warming water had prompted the whales to move, but this study presents the strongest case yet that human-driven climate change caused that reduction in prey, drove the whales into new areas and led to a decline in reproduction rates.

“Though it is upsetting because of the ultimate danger the whales faced, it is not surprising that they appear to move north in response to these changes,” said Jaime Palter, a professor of oceanography at the University of Rhode Island who was not involved with the study.

Distinctive calluses on whales’ heads have allowed researchers to document every single individual.

Although their natural life span is about 70 years, dead ones, when they are found, are inevitably much younger. The latest victim was an 11-year-old male known as Cottontail, found dead off the coast of South Carolina in February. Researchers had been tracking him since October, when he was spotted with his head and mouth entangled in a line that trailed behind him for three or four body lengths.

There is hope for the whales, scientists say. Nineteen calves were born this year, more than any year since 2013. Dr. Meyer-Gutbrod said she thought their mothers somehow found more food.

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