Great white sharks are making their way north. Two were detected near Marshfield beaches

Great white sharks are making their way north of Cape Cod as the water warms, and two have been detected off of Marshfield.

The Chatham-based Atlantic White Shark Conservancy offers a real-time shark activity map through its Sharktivity app. The map shows that two tagged sharks, named Chewbs and Bruce, were detected by receivers in Marshfield this week.

Chewbs was swimming near Ocean Bluff on Sunday, while Bruce was near Green Harbor on Tuesday. The names of the sharks are chosen by donors to the Conservancy, which has tagged more than 700 white sharks in the Northwest Atlantic since 2014.

Chewbs and Bruce are the first two tagged great white sharks to be picked up on receivers on the South Shore this year, according to the app, but those are just sharks that have been tagged and happen to swim by a detection buoy. A shark must be tagged for the buoy to gather information.

Atlantic White Shark Conservancy last month shared a video of a great white shark in Scituate coming snout to lens with an underwater camera.

How many great white sharks as being detected off Massachusetts in the fall

Marshfield has five detection buoys, two of which provide real-time data. The other three have acoustic sensors that receive sound emissions from tagged sharks within a 250-yard radius, said Marshfield Harbormaster Mike DiMeo. Those buoys have to be hauled out of the water before researchers can upload their data.

DiMeo said the sharks typically don't make their way to the South Shore until late summer, when the water temperatures are highest.

"They come in and then move along. They aren't really staying here," he said. "They cover 25 to 30 miles a day, easy."

He said there were several reports of shark sightings over the summer, but those ended up being basking sharks, which are not dangerous to humans.

Why are there more sharks in the waters off the South Shore and the Cape?

The sharks are attracted to the area by a dense population of seals, whose numbers continue to grow in the northwest Atlantic because of federal protections put in place in the 1970s. The gray seals were protected after intense hunting over the previous couple of centuries had nearly wiped them out.

The number of white sharks dropped in the 1980s and 1990s with the expansion of commercial and recreational fishing. Since aquatic mammals were listed in 1997 as a species that people could not hunt, capture, or kill in the northwest Atlantic, their numbers have rebounded.

This article originally appeared on The Patriot Ledger: Great white sharks are making their way north. Two were detected near Marshfield beaches