Scientists and naturalists working on and near the Vineyard worry that recent reported sightings of great white sharks near the Island will feed fears that get ahead of the facts.

Naturalist Gus Ben David, shark expert Greg Skomal and oceanographer Anthony Wood downplayed the threat to humans from sharks.

“In the cases of humans being hit, it is mistaken identity,” Mr. Ben David said. “They do not selectively seek out humans for food. In most cases they spit them out after they have had a taste, we do not have what is necessary for their need.”

On July 10, the sighting of a large shark off South Beach led to the closure of three Vineyard beachs and attracted media interest from around the world. No large sharks have been reported in Vineyard waters since, although a dead 6 1/2-foot great white shark was found washed up on Nantucket this past Monday.

The great white shark, its notoriety enhanced by the movie Jaws, is the largest of predator sharks. This shark can measure as much as 20 feet in length and weigh 20 tons.

Of all the animals in the sea, great white sharks are the most feared by swimmers and surfers and are the most sensationalized in the media. They are called great white because of the appearance of their underbelly.

Mr. Ben David, who runs World of Reptiles and Bird Park in Edgartown, spent his childhood out on the waters of the Vineyard, hunting swordfish south of the Vineyard with his family and friends.

For 36 years, he was the director of the Felix Neck Wildlife Sanctuary in Edgartown. He is an authority on birds, reptiles, and creatures of the Island ashore and offshore.

Mr. Ben David doubts the sightings of great white sharks that have been reported so far this summer.

“There is a correlation between the bird watching community and that of wildlife,” he said.

“Birders are always enthusiastic about sighting rare species. When it comes to birds here in Massachusetts, every Tom, Dick, and Henry wants to call in and report that they’ve seen a rare bird. We now have a panel in Massachusetts, in order to accept a rare sighting of a bird, we have a strict list of criteria. Unless you were at the highest level of experience your report won’t be accepted,” unless there are photographs and collaborated information by a professional.

“I’ve looked at all these different articles about a great white shark being seen by different captains in boats and the average Joe on the beach,” he said. “I can tell you now that unless [Vineyard shark expert] Greg Skomal said he saw a great white shark, I wouldn’t accept it.”

Further, he said, “The difference between a fin on a great white shark and a big mako, and even a basking shark is not easy to tell.”

Basking sharks are the most common large shark that swims in Vineyard waters during the summer. The animals are huge and they come close to shore. Basking sharks are harmless to humans; instead, they feed on phytoplankton and little creatures.

“Just because you’ve done years of bluefish tours doesn’t make you a shark expert,” Mr. Ben David said.

Earlier this summer, there were two reports of great white sharks being seen in the waters off Chilmark and Aquinnah. Mr. Ben David questions both of them.

“Great white sharks don’t breach,” Mr. Ben David said. “Let me tell you something: from all my years of studying sharks — one of my favorites, near and dear to me as birds of prey — great white sharks don’t breach.”

Mr. Ben David said that the only time great white sharks breach is when they are feeding.

“Their killing technique is to come up from deep in the water. They will come straight up like a nuclear Atlas missile, coming out of a submarine.”

“In most cases, in almost all cases it is mistaken identity,” Mr. Ben David said. “In the case I read about, they were not making a kill.”

Basking sharks do breach, Mr. Ben David said. “But once you tell that story of a great white shark beaching, you set the tone,” he said. “Everybody that sees a fish breach is now going to think it is a great white shark. It is mistaken identity.

“An Atlantic sturgeon breaches and they sometimes breach off Chappaquiddick and East Beach. These big fish can be nine feet in length. I can guarantee to you that if one Atlantic sturgeon were to breach off East Beach tomorrow, you would have 100 fishermen saying they saw a great white shark breach. You got a psychological factor going on. Everyone’s mind is set.”

Mr. Ben David said there are a number of different sharks in Vineyard waters, as there is across the ocean. But he cautions people from projecting too much of the Jaws movie into real life. To catch a great white shark fishermen have to chum deep waters using bits of bait and that takes deliberate effort.

“The great white shark has been persecuted all over the globe. It is pretty much a protected species. And it is wonderful they are coming back. We know they are in our waters. What are the chances of someone being bit by one? Common sense has to dictate. Would I go swimming at South Beach at night? No.

“If you are in Africa and your outhouse is 500 feet from the camp, would you walk out there at 2 o’clock at night in lion country? No. Use common sense.”

Dr. Greg Skomal, a shark specialist for the state Division of Marine Fisheries who lives and works in Oak Bluffs, said: “When people ask me about swimming in the ocean. I tell them to take into consideration you are a stranger going into a strange land which we don’t fully understand.

“The probability hinges on the number we have documented which is very low,” he said. “There are inherent dangers when a land animal goes into the ocean. When you get into your car there are inherent risks more probable than a shark attack. Believe it or not, sitting under a coconut tree is more dangerous.”

Mr. Skomal received his doctorate in shark and other marine animal research two years ago. He has committed his career to studying sharks and other marine life. In just the last year he has traveled to Britain, the Caribbean and a few years ago he went up to the Canadian arctic to study sharks.

Most of his work concerns the movement and health of living sharks, not dead ones that wash up on the beach, though he did inspect the great white female shark that came ashore Monday on Nantucket.

“We know from our studies in the ocean that a great white shark can span thousands of miles, moving from one continent to another. They know no borders. We don’t know all about it, but my goal is to find out. I am tagging animals. I am interested in how great white sharks fit into the whole.”

Anthony Wood, an oceanographer who works with the National Oceanic and Atmospheric Administration out of the School of Marine Science and Technology at the University of Massachusetts, said: “I guess I don’t understand why people are surprised to know that sharks are in the ocean.”

Mr. Wood recalled when a 1,700-pound female great white shark got stuck in a small pond on Naushon Island in the fall of 2004.

“There was a media frenzy. People are always interested in shark sightings. People should be aware of an ecosystem works. There are top predators. It is not something we should be scared of. It is not just great white sharks in the ocean. There are all sorts of sharks around the Vineyard.”

“Seventy-one per cent of the earth is covered with ocean. That is a huge environment. The Vineyard is but a pinprick of the whole ocean.”

Mr. Wood credits the fictional movie Jaws for feeding the excitement over shark sightings: “I would bet that 98 per cent of the population has seen that movie, that was filmed on Martha’s Vineyard.

“Nobody seems to be concerned about sharks as a whole,” he said. “When you take an apex predator out of the ecosystem things can go down the drain. I like to think of the Bering Sea, where without the otter, the sea urchins got out of control and resulted in the kelp fields being decimated.

“Otters are keystone species,” Mr. Wood said. “With regard to sharks, there is a recent study which suggests that without the large sharks there can be serious problems. How important is it whether they eat us or not? They are there for a reason.”