Scientists have established firm links between the warming of deep waters in the Gulf of Maine and the reduction of food for the North Atlantic right whale, the world’s second-most endangered marine mammal.

The researchers, led by Nick Record of the Bigelow Laboratory for Ocean Sciences in Boothbay, showed the warmer water in the eastern gulf has sharply reduced the numbers of the whales’ favorite prey, the copepod Calanus finmarchicus, a tiny flealike creature they scoop up by the millions with their sieve-like baleen. The copepods that supply the eastern gulf typically sink into deepwater basins and hibernate over the winter, but the rapid warming of these basins since 2010 has triggered winter population declines of as much as 90 percent.

“People had guessed that the right whales were leaving the eastern Gulf of Maine because of food, so this nailed that down and found some really strong links with climate-driven changes to ocean circulation,” Record said. “We know their migration patterns have changed, but we don’t know if they’ll lock into a new, predictable pattern now.”

Over the past decade, the right whales, whose worldwide population is fewer than 450, have largely abandoned summer feeding grounds off Lubec, Maine, and on the Roseway Basin, near the southwest tip of Nova Scotia, and in late 2017 began showing up unexpectedly in the approaches to the Gulf of St. Lawrence, where several were killed by passing ships.

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Record was among many ocean scientists who suspected a climate-change link, as copepods, like many of the other coldwater species in the Gulf of Maine, are at the southern end of their range here. To test this hunch, he pooled data with scientists from more than a half dozen other institutions to look at temperature and population changes across the gulf.

In Cape Cod Bay and the western Gulf of Maine – where right whales still come in large numbers in the spring – copepod numbers have remained strong. But in the eastern Gulf, where copepods are more dependent on seasonal hibernation, the data showed steep declines and deepwater warming much more severe than Record had expected.

The surface of the Gulf of Maine has been warming faster than any other place on Earth, save a stretch of current off Japan, but he found the deep, cold-water currents that feed into the eastern Gulf of Maine have been warming even faster, particularly in winter months, sometimes by nearly 1 degree Fahrenheit a year, or more than twice the rates seen at the surface.

Calanus is one of the key forage species in the Gulf, providing food for herring, seabirds and many other creatures. It grows fat in spring, making little food packets that other creatures eat later in the year, an energy-storing function that Record likens to serving as the “batteries” for the gulf ecosystem.

“It’s not really news that the Gulf of Maine is changing – that’s now part of the conversation,” says Andrew Pershing, chief scientist at the Gulf of Maine Research Institute in Portland and one of the co-authors of the study. “But it’s surprising to me how we are still figuring out what that means, and this is one of the clearest signals yet that something strange is going on in the food web.”

Pershing, the scientist whose work first revealed how fast the Gulf of Maine was warming, said that just a few years ago most researchers assumed the atmosphere was the primary driver of the phenomenon, but studies like this one showed the role ocean currents – themselves disrupted by the meltdown in polar regions – were playing.

“I think it’s much clearer that a big portion of the warming, especially in the long-term trend, is driven by warmer water pushing into the Gulf of Maine at depth,” he said.

Erin Meyer-Gutbrod, a right whale researcher at the University of California, Santa Barbara’s Marine Science Institute, agreed the study strongly supports the theory that right whales have shifted north in pursuit of food. She said it will help policy makers target likely feeding grounds and better understand links between food supply and successful calving.

“If limited prey availability is causing a decline in reproduction, then reducing human-caused mortalities becomes even more critical to prevent the population from going into decline,” she said via email.

The findings, which appear in the June issue of the journal Oceanography, raise questions about how best to protect the endangered whales, which feed on the surface where they sometimes get caught up in lobster fishing gear or are struck by ships. The whales might stop visiting the warmer, copepod-depleted waters of eastern Maine entirely, which would reduce the chance they will wind up in eastern Maine lobstermen’s gear, Record said. But with the ecosystem changing, the copepod may be on the move as well.

Scientists have had early success at forecasting where copepod blooms might occur by monitoring oceanographic data, Record said, but with federal funding for ocean research on the wane, scientists haven’t been able to perfect the system. The research relied on deepwater temperature data recorded by the offshore buoys of the Northeastern Regional Association of Coastal Ocean Observing Systems, a network that’s suffered from budget cuts in recent years, with some buoys having to be pulled out of the water. Surveys of the copepods themselves have been cut back. (Disclosure: This reporter is a Bigelow trustee.)

“Unfortunately,” Record said, “our monitoring of the Gulf of Maine has been getting worse and worse.”