A Roseate Tern adult and chicks. Chris Linder

Seas & Shores

An Overheating Gulf of Maine Is Forcing Wildlife to Adapt—or Move

Rapid warming in the Gulf of Maine is shifting the marine food web, putting already endangered Roseate Terns and their broods at even greater risk. Figuring out how to help these seabirds could point the way for safeguarding other species.

It's lunchtime on Seavey Island, and the thousands of terns on this rugged five-acre outcropping off the New Hampshire coast are none too happy to have company for their meal. Their shrill, kipping call creates an otherworldly din intended to scare away the human intruders. No one but me seems to notice. 

A crew led by Jennifer Seavey, executive director of Shoals Marine Laboratory, blithely inches across precarious, seaweed-covered rocks, weighing and counting tern chicks. I’m sandwiched between them, a procession deliberately ordered to minimize the chance that my newbie feet will accidentally make contact with a precious egg. Exacerbating that likelihood is my comically oversized straw hat. Just shy of a film-set sombrero, its crown is crammed with reams of towels, bandannas, and socks—anything to blunt a sharp beak. 

The birds, however, are undeterred by our defense system. They alternate between relentless assaults on our hats and flights off the island for food. Of the nearly 6,000 adult terns here this summer, most are common, along with 70 pairs of endangered roseates and a single breeding arctic pair. When they return from sea with tiny silver fish in their mouths, plum-size chicks emerge from low-lying vegetation and crevices. They swallow their meal in a single gulp, retreat to their safe havens, and their parents head out  again.

Last summer, at the height of hotter-than-normal temperatures, Seavey and her staff noticed a disturbing trend: Adults were returning to the colony not with the usual small herring or sand lance or hake. Instead, they carried butterfish, a species found in greater abundance much farther south. Seen from above, a juvenile butterfish looks a whole lot like a herring or hake; from the side, though, it’s much wider. For generalists that regurgitate food for their young, like Herring Gulls, that difference doesn’t pose a problem. For tern chicks, it can be deadly. 

“We’d sit there and watch a chick try to swallow the butterfish for several minutes and then eventually just give up,” says Elizabeth Craig, the laboratory’s tern conservation program manager. “The whole island was covered in dead fish and hungry birds.” 

Craig and Seavey were especially concerned about young Roseate Terns. The population that breeds in the Northeast, once prolific from Nova Scotia to Virginia, experienced two catastrophic collapses: the first at the hand of the millinery trade at the turn of the century, and again beginning in the late 1930s, when human development and unrestricted predation wiped out many remaining colonies. (Roseate Terns that breed in the Caribbean, Europe, and the Indian Ocean remain healthy.) Today the overwhelming majority of Roseate Terns in the northeast nest at just three sites located in the Long Island Sound and south of Cape Cod—a dangerous predicament for an already beleaguered species. With such a concentrated nesting area, a single predator or disease outbreak could decimate the population. So, too, could a tropical storm, the likes of which will intensify as climate change amplifies and reroutes weather systems. 

That’s why the recovery plan calls for restoring at least six Northeast colonies with more than 200 breeding pairs at each. Seavey is one of five such sites in the Gulf of Maine. 

Restoring roseates to a healthy population—5,000 pairs, up from around 3,200 today—depends upon a complex system of relationships. Roseate Terns rely upon an abundance of the more bellicose Common Terns for protection. Both species have evolved to return each summer just in time to capitalize on a rich supply of a few particular types of fish. Once chicks fledge, terns travel to Cape Cod to fatten up for the roughly 5,000-mile flight to their South American wintering grounds. Remove one step in this cycle, and a colony could collapse. 

Butterfish (left) and sand lance (right) collected in the colony. Chris Linder

“Everything that goes on at this island depends upon synchronicity,” says Seavey. (She should know: The island is named for her eighth great grandfather, who shed these waters for cod four centuries ago.) Hence her concern at the appearance of butterfish in terns’ beaks last year. Researchers reported the phenomenon at colonies of terns and puffins, which also feed their chicks whole fish, throughout the Gulf of Maine. “With shifts in climate resulting in changes in temperature, we expect to see changes in the food web and the distribution of fish,” says Craig. “But we were all alarmed it happened that fast.” 

The ocean absorbs 90 percent of Earth’s excess heat, which is why sea temperatures are rising everywhere. But few places are warming as fast as the Gulf of Maine, a robust cold-water ecosystem that spans from Cape Cod north to Nova Scotia’s Bay of Fundy. Here, changes expected to take decades are materializing in a few years. It’s a little like watching a movie on fast-forward. And in many ways, the star is the Roseate Tern. 

As an apex predator, the roseate offers a view into the intricate happenings beneath the waves. Its success, or failure, reveals a lot about the ecosystem as a whole. And so if scientists like Seavey and Craig can succeed at shepherding terns through the tumult, they may well create an effective model for safeguarding other seabirds across the world’s  oceans.

A Roseate Tern chick pipping (top); Common Tern chicks (bottom). Photos: Chris Linder

Last year wasn’t the first time strange species started showing up in the Gulf of Maine. The office of Andrew Pershing, chief scientific officer at the nonprofit Gulf of Maine Research Institute, looks out on one of Portland’s commercial fish piers, where boats bring in halibut, haddock, and cod. In 2012, then one of the hottest years on record there, fishermen returned with reports of unusual creatures in their nets: warm-water dwellers like black sea bass, longfin squid, and blue crabs—mid-Atlantic species, just like butterfish. Meanwhile, beachgoers observed other warmer-water species such as loggerhead turtles and seahorses. 

Puzzled, Pershing and his peers pulled up temperature records and satellite data for the past 40 years. What they discovered surprised even them: The Gulf of Maine wasn’t just warming; it was doing so more than four times faster than the global average, faster than 99 percent of other regions. 

Picture two spinning gears: one in the north Atlantic, one to the south. The northern one spins counterclockwise, pushing water from the Labrador Current south. The southern gear spins clockwise, with the powerful Gulf Stream on the western side bringing warm water into the north Atlantic. The gears overlap near the Gulf of Maine. 

Tern leg bands. Chris Linder

Manmade warming is changing that historical pattern in several ways. Rising ambient temperatures are melting polar ice caps, sending massive amounts of fresh water into the North Atlantic. Fresh water doesn’t sink as quickly, and so stalls the engine that powers the gears. As a result, the point where the gears meet shifts and more warm water flows into the Gulf of Maine. Records bear that out: The average sea temperature there has risen about 3 degrees Fahrenheit over the past 30 years. 

That incremental increase has allowed some species to gain a foothold. Lobsters, for example, prefer temperatures between 54 and 68 degrees Fahrenheit, and the gulf is currently in that sweet spot. While shell disease decimated populations in warmer waters off southern New England, the Gulf of Maine is experiencing an unprecedented boom. Last year Maine harvesters brought in nearly 120 million pounds, about 80 percent of the nation’s total, worth about $485 million. “Just a small amount of warming crossed an important biological threshold for lobster that really helped fuel the explosion,” says Richard Wahle, director of the University of Maine’s Lobster Institute. 

But the boom may be short-lived. To understand why, you have to begin at the base of the food chain and work back up to predators like lobsters and terns. Historically, minuscule organisms called phytoplankton have been in rich supply here, but climate-driven changes are causing their decline. More rainfall, for instance, dilutes ocean salinity. It also increases terrestrial runoff, which darkens the water and limits phytoplankton’s ability to absorb sunlight and thus photosynthesize and grow. 

That, in turn, affects keystone species that consume phytoplankton, most notably the zooplankton Calanus finmarchicus. This copepod lives its life like a tiny oceanic bear: It gorges on phytoplankton much of the year, then hibernates and conserves energy in winter. That strategy makes copepods calorie-dense—Pershing likens them to floating drops of butter—and everything from juvenile lobsters to right whales to herring makes a living off them. 

Without healthy phytoplankton, copepods can’t store up the energy they need. And when water gets too warm, copepods skip their slumber and use up their energy stores, making those that survive less nutritious. Meanwhile, changes in oceanic currents are slowly shifting healthy populations north. 

The well-being of this one species has myriad impacts. Wahle has observed a strong correlation between copepod availability and the health of larval lobsters. Marine mammal experts, meanwhile, believe that copepods’ changing distribution has altered the range of the endangered North Atlantic right whale, forcing more away from traditional feeding grounds off New England into heavily trafficked areas like the Gulf of St. Lawrence (at least three have been struck and killed by ships there this year alone). 

Adult herring also depend upon Calanus, which could at least partly explain why the fish are trending to cooler waters in the northeastern gulf. That, in turn, may be drawing unusual seabirds south, including the Common Murre, which reared chicks in the Gulf of Maine in 2018 after a 27-year hiatus. “It all seems to link back to this one copepod,” says Wahle. 

Video: Chris Linder

Scientists are tracking animals like copepods and how they respond to incrementally warming waters in order to make educated guesses about what’s to come. Complicating that research are marine heat waves that have hit the gulf in recent years. 

If global warming is the constant pressure on an ecosystem, marine heat waves—five-plus-day stretches when temperatures exceed the 90th percentile of historical averages—are best understood as a sudden shock, explains Alistair Hobday, research director for oceans and atmosphere at CSIRO, Australia’s national science agency. “Marine heat waves impact right through the food chain,” he says. “Some species die; other species arrive; diseases occur. So management gets challenged by new things that have not been previously considered or encountered.” 

In 2018, the Gulf of Maine experienced 250 days that qualified as marine heat waves, making it one of the hottest years on record. But even it was eclipsed by both 2012 and 2016, which witnessed 360 and 302 marine heat wave days, respectively. 

That statistic floored Pershing. But what really interested him was how the marine ecosystem responded to these changes. “There are a few species that react really fast to these events,” he says. Squid, for instance, can arrive after just a few days or weeks of warm water. Herring, meanwhile, make a hasty departure. “And if you see an impact on herring,” Pershing warns, “that’s going to have an impact throughout the ecosystem.” That’s particularly true for seabirds, as we saw last year. 

Seabird species respond to these changes in ocean temperature differently. Species whose young take to the ocean right away, such as Razorbills, abandoned the waters around their breeding colonies faster than other species, presumably in search of suitable food. Atlantic Puffins, meanwhile, surprised scientists with their resiliency to the temporary food shortage. As sea-surface temperatures rose in early summer, puffin parents slowed down their feedings. When cooler waters, and the fish that occupy them, returned a few weeks later, the adults redoubled meals, feeding their chicks 10 times or more a day. They also extended the time they spent rearing their single chick—in some instances nearly doubling it, from 40 days to 83. Nevertheless, last year’s fledging rate was one of the lowest on record. 

It’s far more challenging for terns to adapt, explains Don Lyons, director of conservation science for Audubon’s Seabird Restoration Program. Audubon manages four islands in the gulf that host Roseate Tern colonies. “They have a pretty hard-wired growth-and-development pattern,” says Lyons. “Terns start out with multiple chicks. If they run into a food shortage, it gets really hard to feed the whole family, and so they really struggle.” To give terns and other seabirds a better chance of finding food, says Lyons, we have to reconsider how we manage our fisheries. 

Herring, for instance, were once so prolific that they formed shoals 30 miles long and five miles deep. Overfishing caused the collapse of the industry in the 1960s and 1970s, which led to stiff quotas. The total fishery catch limit today is about 15,000 metric tons per year, down from over 180,000 metric tons just 15 years ago. That number will drop further next year. Meanwhile, stock assessments conducted by the New England Fisheries Management Council show that the number of baby herring is declining, perhaps in response to copepod declines. 

Purse seiner boats, used to catch herring, dock in Portland, Maine. Chris Linder

At the same time, terns’ other preferred staple, the sand lance, faces its own struggles. On Stellwagen Bank off Cape Cod, the survival of eggs and embryos decreases as much as twentyfold when carbon dioxide levels increase in ocean water, suggests a forthcoming study. “Sand lance might be the most CO2-sensitive fish species tested to date,” says Hannes Baumann, senior author and a fisheries biologist at the University of Connecticut. “This means that long-term marine climate conditions are likely to worsen for sand lance.” Baumann says industries related to fracking and construction have expressed interest in mining sand in the lances’ habitat, posing additional threats to the species, which are commercially exploited for fish meal. 

Given the dissipating numbers of these key fish, it’s more important than ever that management of these species takes a truly ecological approach, Baumann, Lyons, and others say. In the past, fish quotas have been set strictly on analysis of historical data. Allowing for seasonal forecasts that include marine heat waves would do a lot to improve management as temperatures rise, argues CSIRO’s Hobday. “It would lead to more-rapid decision making.” 

If ocean warming here has taught us anything, it’s that more-nimble management is necessary, says Mary Beth Tooley. She coordinates government and regulatory affairs for O’Hara Corporation, a herring fishing company, and served on the New England Fishery Management Council until 2017. “Our job is to leave this company in good shape for the next generation,” she says. “We’re not interested in taking the last fish. If you rely long-term on fishing, you’r