CHATHAM, Mass. -- Cape Cod officials are reminding swimmers and boaters to beware as more great white sharks have been spotted this week, with as many as six seen off the coast of Chatham.

A professional fish spotter managed to spot one of the fish, a 1500-pound great white, which came within 100 yards of the shore and within 20 feet of a swimmer.

"It's terrifying in the airplane. I've seen this on a couple of occasions where the shark will swim directly at a swimmer and then he'll veer off, you know. He probably senses there's something in the water, maybe it's dinner, it's a seal," said spotter George Breen.

In fact, the burgeoning population of gray seals off the Chatham coast is what's thought to be attracting the sharks. The once-endangered population has grown, making for a big attraction for the predators.

"We've seen two or three dead seals this year, with a shark implicated in its demise," said Greg Skomal, a state biologist with the Division of Marine Fisheries.

The danger comes when humans swim near the seals. The crew of Breen's boat, the Easy Does It, managed to tag the shark that came in close in order to monitor and study it.

"Just have to exercise caution. That's why we're doing the scientific research. Putting the tags in. Finding out their habits. When they come in shore, what tides, what water temperatures," said Breen.

Chatham officials have not closed any beaches, but said people should stay near shore and not venture close to seals, a favorite food of sharks.