The Cape Cod Canal closed for more than five hours Sunday to accommodate a right whale mother and her calf, who had entered the waterway, according to environmental officials.
The whales swam into the canal around 10:30 a.m., prompting the Army Corps of Engineers to close the waterway shortly after they appeared inside it, according to a spokesperson from the Executive Office of Energy and Environmental Affairs. Boat traffic was minimally impacted, she said, with one car carrier opting to navigate around the Cape instead of waiting for the canal to clear.
The whales entered from Cape Cod Bay, escorted by Massachusetts Environmental Police boats as they swam west toward a stretch of water between the Bourne and Sagamore bridges, wending their way around a tug and barge, the spokesperson said.
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Hours later, the pair turned around and left the canal, rejoining a group of at least 79 right whales congregating in the bay.
About half of those whales have been grouping by the entrance to the canal since Saturday. They were first spotted last week by the Center for Coastal Studies, the Globe previously reported.
The endangered whales now congregating around Cape Cod make up about a quarter of the world’s population of North Atlantic right whales. Right whales have faced unusually high mortality rates since 2017, often struck by vessels or tangled in fishing lines, outpacing birth rates, according to NOAA.
The Army Corps of Engineers, by protocol, closes the canal to traffic when a right whale is spotted inside the waterway. The corps allowed traffic to resume Sunday around 4 p.m., shortly after the whales left, according to the spokesperson.
Charles “Stormy” Mayo, director of research for the Provincetown-based Center for Coastal Studies’ right whale ecology program, praised the Corps of Engineers for moving quickly to close the canal to safeguard the mother and calf.
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“It’s not easy for shippers,” Mayo said, “but that little calf is a precious animal.”
Fewer than 350 of the right whales are left, with less than 70 breeding females, according to preliminary estimates by the National Oceanic and Atmospheric Administration.
Authorities are also trying to help an 8-year-old right whale that was first spotted south of Nantucket in February entangled in ropes, some caught in her mouth and wrapped around her body and flippers. On Wednesday, the center’s Marine Animal Entanglement Response team removed around 200 feet of rope from the whale and attached a tracking buoy, but the animal is not yet free.
Researchers with the response team were monitoring the whale Sunday, but could not attempt to remove the remaining ropes until conditions, including rough waves and high winds, calmed, according to Amy Jenness, director of marketing, communications, and outreach at the Center for Coastal Studies.
Mayo said whales have entered the canal in previous seasons, but this year has seen “more canal action” than before. Mayo told the Globe Saturday that he believes there could be closer to 90 to 100 whales in the bay.
Right whales have aggregated in the bay during the winter since at least 1984, Mayo said, when he began his research, drawn in by the region’s consistently high concentration of food.
“This is very much a nursery ground,” he said. “It’s probably a location where we see the highest concentrations of right whales of this species anywhere in the world.”
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He said right whales are “notorious” for swimming into shallow water — Mayo once worked with a whale that made it up the Delaware River — but researchers struggle to predict their individual behavior, since there are so few to study.
“When you get down to right whales, they live a lifestyle and live in areas that are really different than other whales,” Mayo said. “They socialize in different ways. They feed in radically different ways.”
Mayo said he typically sees whales that enter the canal turn around before making it all the way through.
Although, just a few weeks ago, he said, a year-old right whale crossed the entire length of the canal. Mayo called the trek “kind of extraordinary.”
Jenness said the Center for Coastal Studies had no planes in the air Sunday and so could not capture a bird’s eye view.
She also said that right whales do not typically swim all the way through the canal, which spans more than 17 miles, but there are exceptions.
“It depends on the food source, how deep into the canal the plankton has reached,” Jenness said. “I don’t think they like being in the canal, I don’t think it’s their favorite thing.”
The canal also closed at the beginning of March, causing a backup of traffic. One whale entered from its western entrance, near Buzzards Bay; an hour later, three more whales entered the east side of the canal to feed.
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Mayo said whales tend to exit the canal in a matter of hours, because the confined space is “not a good feeding area, that’s for darn sure.” He said more whales are likely to continue entering the canal as long as dozens remain near its entrance.
Daniel Kool can be reached at daniel.kool@globe.com. Follow him @dekool01.