The shark advisory, issued by the U.S. Coast Guard district that covers waters from Maine to New Jersey, came several days after the crew of a tuna boat caught and later released a 7-foot juvenile great white shark in the Stellwagen Bank fishing area, about 20 miles off Massachusetts.
Several great white sharks were spotted off Cape Cod last summer, and experts believe more will return this summer, attracted by the exploding local population of seals, a favorite shark food.
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With a hot, sunny July Fourth weekend forecast, residents and tourists are expected to flock to beaches.
Massachusetts officials held a news conference this week to stress that no great whites - the kind of shark that taunted swimmers in the book and movie "Jaws" - had been spotted near beaches, and that beachgoers and swimmers should not be worried so long as they exercise "common sense" and avoid swimming near seals.
In the Coast Guard advisory, Al Johnson, a recreational boating specialist, said boaters and paddlers in the Northeast generally don't have to concern themselves with predators, and noted that shark attacks are extremely rare in the region.
"But I have no doubt that a great white shark that swims into your comfort zone would surely find a splashing paddle or dangling hand inviting," Johnson wrote. "I also expect that same passing shark would spend little time differentiating between boater, paddler and prey."
Sharks spotted off New England have been as long as 15 feet, Johnson said.
Recreational boaters and kayakers should steer clear of seal colonies or passing pods of seals, Johnson said. And anyone who spots a shark should resist any temptation to go near it.
"I can also imagine the excitement most boaters and paddlers would have if they visually encountered a shark on our waters," he said. "However, things can and do go wrong on the water, and since a close encounter could easily have tragic consequence, I recommend an extreme degree of caution."
Secretary of Environmental Affairs Ian Bowles said he anticipated more great white sightings this summer but did not foresee any beach closings.
The last fatal shark attack in Massachusetts was in 1936.