It’s summertime on Stellwagen Bank, 50 miles north of Chatham, Massachusetts, in the Atlantic Ocean’s Gulf of Maine. As the sun rises on this late June day, the bank hosts seabirds, many of them Great Shearwaters, and humpback, finback, and minke whales. For all these species, the season depends on a feast of tiny fish called sand lance, abundant in the region at this time of year.
Shearwaters wheel above the 50-foot research vessel Auk. Alongside, humpback fins and tails break the ocean’s surface. Aboard ship, researchers have a front-row seat to a simultaneous aerial and underwater ballet. While they marvel at the show, they also work to keep tabs on all the players — from huge whales to small fish — to track the health of their ecosystem at a time of unprecedented ocean warming.
Great Shearwaters spend most of their lives at sea, sailing over white-capped waves on long, stiff wings or feeding in groups on small fish, often with other seabirds and whales such as humpbacks.
“Shearwaters are named for their gliding flight, which scales these slim-winged seabirds just above the wave tops,” writes Nathalie Ward in Stellwagen Bank: A Guide to the Whales, Sea Birds, and Marine Life of the Stellwagen Bank National Marine Sanctuary. “Tipping from side to side to trim sail, the birds effortlessly ride updrafts between the waves, occasionally skimming low into the wave’s trough, only to reappear beyond the next crest.”
As shearwaters coast from swell to swell, a humpback calf twirls up and out of the sea, pirouettes in a full breach, and sprays sparkling water droplets in all compass directions before slipping beneath the waves.
In 1992, Stellwagen Bank became New England’s first, and to date only, national marine sanctuary, a wild ocean place not far from the urban world of Boston. It was historically important as a fishing ground. Today, it’s an oasis for marine life from the microscopic to the behemoth.
A decade later, in 2002, Stellwagen was designated an Important Bird Area (IBA). The IBA program, administered in Massachusetts by Mass Audubon, is part of BirdLife International’s work to conserve bird habitat. An IBA provides essential habitat to breeding, wintering, and/or migrating birds and supports high-priority species, major concentrations of birds, and exceptional bird habitat or has high research or educational value. Stellwagen Bank meets all these criteria, says marine biologist David Wiley, research coordinator for Stellwagen Bank National Marine Sanctuary.
The Great Shearwater is one of the few bird species that breeds in the Southern Hemisphere and migrates north to its nonbreeding range in the Northern Hemisphere. From November until April, the seabird is found along the Patagonian shelf off Argentina and in large nesting colonies on a few rocky islands in the South Atlantic, including Tristan da Cunha. Then, in May, the shearwaters head north up the coasts of South American and North America. From June through November, they can be found around the Caribbean and along the eastern coasts of the U.S., Canada, and Greenland before crossing the ocean and flying south along the western coasts of Europe and Africa toward their breeding islands.
The shearwaters use Stellwagen and the Gulf of Maine as feeding grounds mostly from July through November, although the first birds arrive in June, and some linger into December. The species also is reported in low numbers from January through April along the coasts of Alabama, Florida, the Carolinas, Virginia, and the Bahamas, suggesting that at least some individuals don’t fly all the way to the South Atlantic each year.
More than 40 species of seabird summer on Stellwagen Bank; in addition to Great Shearwaters, the bank attracts Sooty and Manx Shearwaters, and Wilson’s and Leach’s Storm-Petrels.
“Seabirds are excellent indicators of the health of the marine ecosystem,” says Wiley. “They allow us to track changes in the environment over time, from their summer feeding range on Stellwagen to their migratory routes to their breeding ranges.”
Humpback whales also migrate across long distances, up to 10,000 miles each year. The whales feed in summer in northern waters such as Stellwagen, then migrate to tropical or subtropical waters to breed and give birth in winter, when they live on fat reserves.
Catching seabirds
What’s sustaining the bank’s summer visitors — avian and cetacean? To find out, scientists are tracking both shearwaters and whales. The answer, it turns out, lies in countless small, silvery sand lance.
Since 2013, Stellwagen scientists and their colleagues have placed tags on Great Shearwaters. To catch their quarries, the researchers enlist crew members aboard the Auk, which serves as the mothership of two smaller watercraft, the rigid-hulled inflatable boats (RHIBs) Balena and Luna.
The biologists use Luna to motor to where shearwaters are foraging (Balena is for whale research). Then they entice the birds in close with cut fish and squid. Once a shearwater comes near the boat, it’s caught with a long-handled net and ultimately fitted with a solar- or battery-powered satellite (platform terminal transmitter, or PTT) tag and released. The birds are normally tagged during research cruises in June, July, and/or September.
Why study Great Shearwaters instead of another of the many seabird species that frequent Stellwagen in summer? As Wiley, Kevin Powers, a biologist at the marine sanctuary, and colleagues wrote in 2017 in the journal Marine Ecology Progress Series: “In the western North Atlantic, Great Shearwaters are among the most abundant seabirds during summer months, yet little is known about their movement ecology and habitat requirements in this ecosystem.”
Data from PTT tags allow the researchers to monitor the shearwaters’ movements. The scientists can compare the birds’ flight paths to water temperatures, chlorophyll concentrations, ocean fronts, and other factors that might affect the birds’ prey. While the analyses primarily focus on understanding the shearwaters’ food source, the tags have also allowed the biologists to track the birds on their southbound migration.
In celebration of Black Birders Week 2021, for example, that year’s class of tagged Great Shearwaters included a bird named Shuri after a character in the Black Panther films. DNA from blood sampled when the bird was tagged showed that she was a female.
From the day she was tagged, July 14, 2021, through January 1, 2022, when her tag failed, Shuri traveled 26,465 miles from Massachusetts coastal waters to the south-central South Atlantic. She was the distance leader in the 2021 shearwater class.
Tagging whales
Shearwaters are but one part of the Stellwagen Bank picture. Then there are the whales.
If placing a tracking tag on a shearwater is no easy feat, doing the same on a humpback whale is orders of magnitude more challenging. On a bright morning after summer solstice, the Balena ferries biologists to the center of a whale pod. There, in an attempt to fix a tag on a humpback, the researchers hope to nose up to a whale whose flank, where the tag will be placed, is exposed.
The scientists are using a suction-cup tag called CATS, for Customized Animal Tracking Solutions. CATS tags carry two video cameras, along with accelerometers, gyroscopes, and sensors to monitor temperature, light, and other variables. The instruments observe the mechanics of whale feeding, as well as “see” other humpbacks in the same area as the tagged whale.
Wiley takes the helm of the Balena; the Luna is the chase boat that follows along, ready to see where a whale goes immediately after it’s tagged. Suddenly, a humpback surfaces near Balena’s starboard side. The crew is all-systems go. Wiley guides the boat alongside the whale, while team member Mike Thompson, a researcher at Stellwagen, extends a 40-foot-long pole toward the whale. A CATS tag is mounted on its end. With luck, Thompson will gently place the tag on the humpback. “Tag on!” he yells as the whale disappears beneath the waves.
“Look at this place,” says Wiley as he half-turns from the wheel of the Balena to view a pod of humpbacks with shearwaters soaring overhead. “Seabirds and whales, just everywhere.”
‘It all depends on sand lance’
Stellwagen Bank, with its sandy bottom and relatively shallow waters, is prime habitat for the sand lance that sustain the birds and whales. The bank’s sand lance, which travel in huge schools reaching the tens of thousands, offer seabirds and whales high-calorie meals.
At night, sand lance tunnel into sandy sediments or form schools close to the seafloor. During the day, the fish often swim in dense mats along the seabed. Where sand lance are found in the Gulf of Maine, so, too, are shearwaters and humpbacks. “Shearwaters and other seabirds, whales, pretty much anything looking for a meal here is feeding on these fish,” says Wiley. “On Stellwagen Bank, it all depends on sand lance.”
Great Shearwaters and humpback whales head toward areas where sand lance are plentiful, according to data Wiley and others published in October 2020 in the journal Conservation Science and Practice. “The strong co-occurrence between these predators and their sand lance prey suggests that it’s important to identify the locations where sand lance live,” says Wiley. “Discovering more information about sand lance habitats may indicate other areas where shearwaters and whales aggregate.”
More than 70 species rely on the tiny fish, reports Michelle Staudinger of the Northeast Climate Adaptation Science Center at the University of Massachusetts Amherst. She and colleagues, including Wiley, published a report in 2020 in the journal Fish and Fisheries that further documents the dependence of myriad species — from Great Shearwaters to Roseate Terns to Atlantic Puffins, humpback whales to bluefin tuna to Atlantic cod — on sand lance.
To see where the fish are hiding out, Wiley and crew use a seabed observation system known as the SEABOSS, or SEABed Observation and Sampling System. This boxy eye-in-the-sea allows the researchers to collect sediment samples and obtain videos of sand lance seafloor hangouts, according to Thompson. Sand lance surveys are conducted at 44 stations on or near Stellwagen Bank at various times of the year, including about a month before the research cruise each June.
Waters warming
If trends continue, shearwaters and humpbacks may soon need fish finders. Fish communities on Stellwagen Bank have changed dramatically, according to marine scientists Peter Auster of the Mystic Aquarium and Christian Conroy of the University of New Haven. A 2019 report they co-authored on Stellwagen Bank fish details species shifts that began in the early 1980s.
Those years ushered in a two-decade period of steadily rising sea-surface temperatures. Waters across the northeastern United States continental shelf warmed at a rate three times the global average. In response, many species in the Gulf of Maine moved to greater, cooler depths. For some species, however, such as sand lance, deeper realms may not offer the right habitat. Sand lance rely on sand or fine gravel, sediment that makes up most of the bottom in Stellwagen Bank’s shallows but is not as common in its depths.
“The relatively minimal number of sand lance collected in trawl surveys conducted since 2010 may reflect, in part, the results of this squeeze between warming waters and preferred habitat,” state Auster and Conroy.
Indeed, sand lance seem to change location each year, taking the seabirds and whales with them.
Although the June research cruise was scheduled to depart from Provincetown, the sand lance — and seabirds and whales — took up residence off a different Cape Cod port, Chatham. To keep up with the shifting scene, the scientists moved the Auk from its berth in Provincetown to a dock in Chatham. “We may be all about birds and whales,” says Wiley, “but they’re all about sand lance. So, we look at where sand lance are concentrated.”
Feeding frenzy
To catch sand lance on Stellwagen, humpback whales engage in bubble-netting, a means of feeding on small fish. One or more whales sound, or dive, then exhale together underwater. When their bubbles reach the surface, they form a large ring with seafoam in the center. The bubble ring becomes a net, trapping countless sand lance. Seconds later, one whale — then several — surface in the ring’s center, huge baleen-lined mouths open, straining the water, or dragging, as marine biologists call it, for sand lance.
Humpbacks release the bubbles while swimming in upward spirals, often during a behavior called double loops. “Double loops” start with an ascending spiral to corral fish, then the smack of a fluke on the ocean surface and a second lunge to capture the corralled prey. The whales work in teams of two to 10 or more, emerging at the ocean surface in a boiling cauldron of open mouths and flipping fish.
Waiting just above are Great Shearwaters and other seabirds, which take the liberty of walking on the whales as they surface, the better to cheekily snatch sand lance right out of the leviathans’ mouths.
Will the buffet of sand lance be enough to keep the seabirds and whales going until they return the following summer? On Stellwagen Bank, the fates of shearwaters, humpbacks, and countless other seabirds and cetaceans, are intertwined with that of one very small fish.
Stellwagen Bank National Marine Sanctuary eBird list
Seabird populations declined 70 percent throughout the modern industrial era
This article appears in the May/June 2023 issue of BirdWatching magazine.
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