Summer solstice. Since prehistory, the date has been significant to cultures around the world. It’s no less so to the humpback whales of Stellwagen Bank, 50 miles north of Chatham, Massachusetts, in the Atlantic Ocean’s Gulf of Maine. As the sun rises over a still June sea, the underwater bank hosts a promenade of humpback, finback and minke whales. A whale solstice is celebrated with a feast of tiny fish called sand lance, abundant in the region at this time of year.

Humpback fins and tails break the ocean’s surface on all sides of the 15-metre research vessel Auk. Aboard the ship, an audience with front row seats watches more than 30 humpback whales perform a ballet. A calf born this year peacefully swims alongside its mother. Suddenly, it 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.

Humpbacks are baleen whales that filter-feed with ‘strainers’ made of keratin inside their huge mouths. Adults range from 12 to 16 metres long, and weigh about 36,000 kilogrammes.  They have distinctive body shapes, with long pectoral fins and knobby heads. Males ‘sing’ complex songs lasting 10 to 20 minutes, which they repeat, sometimes for hours. The whale music may have a role in mating.

Found in oceans around the world, humpback whales migrate up to 25,000 kilometres each year. Humpbacks feed in summer when they’re in northern waters like the Gulf of Maine, then migrate to tropical or subtropical waters to breed and give birth in winter when they live on fat reserves. 

What are humpbacks eating – and how do they get enough of it – to tide them over until the following summer? On Stellwagen Bank, the answer lies in a cloud of bubbles and countless tiny fish.

Auk serves as the mother ship of two smaller watercraft, the rigid-hulled inflatable boats (RHIBs) Balena and Luna. Balena ferries biologists to the centre of a whale pod. There, in an attempt to place a tracking tag on a humpback, the researchers nose up to a whale whose flank, where the tag will be placed, is exposed. 

Called a digital tag, or DTAG, it’s attached with a suction cup. This acoustic recording tag provides data on the whale’s orientation (pitch, roll and heading) and depth – 50 times per second. “The DTAG also records all sounds made and heard by the tagged whale,” says Dave Wiley, a cetacean biologist and research director at the Stellwagen Bank National Marine Sanctuary.  Visualisation software called TrackPlot provides fine-scale information on the behaviour of the whale, including how, when and where it feeds.  

DTAGs remain on whales anywhere from minutes to hours. “After they fall off, they’re retrieved from the ocean, the data they’ve collected are downloaded, and the tags are ready for redeployment on other whales,” explains Wiley. 

The scientists are also using suction-cup-attached tags called CATS, for customised animal tracking solutions. CATS tags carry two video cameras, along with accelerometers, gyroscopes, and sensors to monitor temperature, light and other variables.  They observe the mechanics of whale-feeding, as well as ‘see’ other humpbacks in the same area as a tagged whale.  It’s important information, the researchers say, about marine mammals that until recently were on the U.S. endangered species list, a result of overharvesting in whaling days.