BOSTON -- An abundance of seals off the coast of Cape Cod near Chatham has attracted a bounty of great white sharks, prompting officials to close beaches and causing at least one close call for a diver.

The diver was in the water recovering an electronic tags when a great white started swimming in his, or her, direction. The pilot of the spotter plane said the shark was moving fast and headed straight for the diver.

Flying high above, the spotter pilot George Breen radioed the boat, sending a warning of approaching sharks. Seasoned harpoonist William Chaprales of Marstons Mills knew he had only seconds to nail a satellite-tracking tag into the shark.

"It's one of the greatest things I've done in my life to tag a Great White Shark," said Chaprales. A great white was tagged off Naushon Island in 2004 but the tag quickly became loose and was lost within days.

Since Saturday, Chaprales, his son and a state marine biologist have tagged five great whites with data loggers that they hope can provide information about the species, tracking how deep they're swimming and where they go.

"It is hard to get a real number because we don’t know if we are double-counting, but it wouldn't surprise me if there were eight or nine or 10 or more fish out there," marine biologist Greg Skomal said.

Breen said there were two close calls in the water Tuesday, another one involving a swimmer who managed to get out quickly.

"There were swimmers near sharks and ... were the sharks stalking the swimmers? Yes, they were. Would they eat 'em? Would they bite 'em? I don't know, but they knew the swimmers were there and they were coming up to them," Breen said.

The other incident involved a diver who was recovering a tag that had come loose.

"She sensed that swimmer in the water from a quarter of a mile away. And we all know that sharks have these extra-sensory ways of seeking their prey and this was visual proof to me," Breen said.

The sharks in the area have ranged from 6 to 15 feet and state biologists said they're likely to stay in the area until the water turns colder, which could be several more weeks. The sharks routinely visit the area, drawn by a sizeable gray sea population.

Researchers said the last time a great white attacked a human was in 1936 in Buzzards Bay.

Swimming is prohibited at North Beach, Lighthouse Beach, South Beach and Andrew Harding’s Lane in Chatham.