The cold never bothered them anyway.

North Atlantic right whales have returned to the waters south of the Bay State, triggering NOAA to implement a slow speed zone for boaters.

At least 23 right whales were spotted south of Massachusetts last week.

Scientists from the New England Aquarium’s Anderson Cabot Center for Ocean Life were flying an aerial survey last Monday when they saw an aggregation of eight right whales about 37 miles south of Nantucket.

Then a second survey last Thursday found more than 20 right whales. Between both surveys, at least 23 unique whales have been identified so far.

Assistant Scientist Kate Laemmle, who was one of the observers aboard the plane, noted the high number of these social groups in such a concentrated area.

“All around us, we saw groups of whales rolling, splashing, and bursting out of the water,” Laemmle said. “Seeing these critically endangered right whales on our first surveys of the New Year is a great way to start the season and leaves us hopeful for more whales returning to the area.”

The group of whales included two female right whales that have calved in recent years, “Braces” and “Maple,” as well as “Atoll,” a young adult female that might be nearing calving age.

These three females were all seen in surface active groups, a social behavior that involves several right whales closely interacting at the surface. These surface active groups can also provide mating opportunities.

The majority of whales identified have been adult males, including “Batman” and “Dune.”

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The winter into spring season is the peak for right whale sightings in southern New England waters.

In addition to the Aquarium sightings south of Nantucket, the Center for Coastal Studies has also spotted right whales in Cape Cod Bay over the last month. Cape Cod Bay is subject to a mandatory slow zone for boats and ships known as a Seasonal Management Area until May 15.

Last week’s whale sightings by the Aquarium led the National Oceanic and Atmospheric Administration to implement a voluntary slow speed zone called a Dynamic Management Area (DMA) for mariners to reduce their speed to 10 knots to protect the whales.

The DMA is in effect until Jan. 20, one of several active voluntary slow zones from Massachusetts to Virginia due to right whale sightings.

Vessel strikes are one of the leading causes of injuries and deaths for this species, which numbers about 380 individuals. Scientists say it’s critical that vessels slow down to the recommended speed.

“These recent sightings are an important reminder that right whales can be found throughout New England waters during the winter and spring,” said Katherine McKenna, associate scientist in the Aquarium’s Anderson Cabot Center.

“Reducing vessel speed in mandatory and voluntary right whale management areas is one of the most effective actions mariners can take to reduce the risk of vessel strikes and help protect this critically endangered species,” McKenna added.

For the past 15 years, the New England Aquarium has conducted aerial surveys over the waters south of Martha’s Vineyard and Nantucket — collecting data on all marine animals with a focus on protected species of whales and sea turtles.

In recent years, the Aquarium has expanded survey coverage in New England, including coastal Maine, the Gulf of Maine, and the Canyons along the shelf break.

These flights help monitor changes in animal populations, identify various species, recognize trends using standardized data, and monitor aggregations. This provides crucial information that can be used to better protect endangered species.