A great white shark was reported twice in Vineyard waters this past week, the most recent off Menemsha Beach on Wednesday evening.

Capt. Buddy Vanderhoop of Aquinnah called the Gazette Wednesday night to say a great white shark had been seen 50 feet off the Menemsha beach at about 6:30 p.m. He said the animal was finning and moving up the beach. Later it moved toward the Brickyard.

Capt. Scott McDowell of Chilmark was out fishing on a charter on Sunday afternoon when, he said, a great white shark came out of the water three times near his new boat.

Great white sharks are not common in Vineyard waters.

Greg Skomal of Oak Bluffs, a leading state fisheries biologist who specializes in studying sharks, especially in New England waters, said the spottings definitely are worth investigating.

“I didn’t see the fish, but based on the reports it sure narrows down the possibilities,” Mr. Skomal said. “Given the knowledge of these fishermen — they are both veterans — I have no reason to doubt what they say.”

Mr. Vanderhoop and Mr. McDowell’s reports are similar. They both describe an animal that is around 15 feet in length with a wide girth.

Mr. Vanderhoop said: “I see great white sharks about two or three times a year. It is not uncommon. It is pretty uncommon to see it right off the bathing beach. It was probably 14 feet between the tail and the dorsal fin, so it was probably 18 feet long and weighted between 3,000 and 4,000 pounds.”

Mr. McDowell’s encounter was quite different but it could have been the same animal. Like Mr. Vanderhoop, Mr. McDowell is a charter fisherman.

Mr. McDowell was returning after an afternoon of fishing with his first charter of the season. He was in his newly acquired fishing boat Lauren C, a 35-foot Duffy fishing boat.

“We had just been fishing for striped bass off Gay Head,” he said. “I saw something very large come out of the water [at 4:30 p.m.] I didn’t know what it was. It was 30 yards away. It was 15 feet long and white.”

He spoke to the two men aboard his boat. “I said, ‘God, look at something breach.’ Then right next to the boat a very large animal came out of the water. I thought it was a porpoise, and then lo and behold. There was no doubt about it. It spun. It did a 180-degree spin and you could see it. That was a shark and it was white. What was most striking was that it was really fat.

“Then it breached a third time,” Mr. McDowell said. Mr. McDowell said he could see the Gay Head Light, and was near Pilot’s Landing.

Mr. Vanderhoop said he heard on his VHF radio that there was a shark spotted earlier in the week.

Mr. Skomal, who works for the state Division of Marine Fisheries, said he is aware of other sightings, but is most anxious to get a report that includes a photograph. He is working on a research project on the movement of sharks in state waters. As part of that project, he is studying basking sharks and all the other sharks.

“The process is a multi-year study and we are looking at the movement and dynamics of sharks in New England waters,” Mr. Skomal said. He is preparing his work for state and federal fishery agencies as well as the public.

There is plenty of reason for sharks to be visiting Island waters. Basking sharks come for the food and so do the flesh-eating sharks. Great white sharks are known to feed on seals, and seals are now congregating around Noman’s Land and off Chappaquiddick. Freelance photographer Dick Sherman took an aerial shot of Skiff’s Island this week covered with seals. Skiff’s is southeast of Wasque. The photograph is published on the Commentary Page in today’s edition.

Mr. Vanderhoop said he has seen many seals on Noman’s Land. He estimated there may be thousands. “It is a dinner plate for sharks,” Mr. Vanderhoop said.

Charlie Blair, the Edgartown harbor master, is also concerned about the possible draw that seals will have on sharks visiting his end of the Vineyard and the impact it could have on public safety.

“There is an abundant supply of food and their population is expanding. Whenever there are seals and sea lions, whenever there is a population expanding, the predators find out about it,” said Mr. Blair, who has long experience on the waterfront.

Mr. Blair said he is concerned about the potential danger to surfers.

“Surfers run the biggest risk. They wear black wet suits and they can easily be mistaken. They are right on the break of the wave where there is poor visibility. I don’t think sharks purposely hunt humans,” he said.

Last August a large shark was spotted close to South Beach at Katama. The animal was spotted by a pilot flying overhead and reported later to park and recreation staff overseeing the beach. It never caused trouble, but it created a stir among the public safety officials.

“It increased our concern,” Mr. Blair said. “How do we react? That is the big question. If you have a sighting, how do you react? Certainly you try and get the swimmers out of the water.”

Not every shark seen in Vineyard waters is harmful. The basking shark, which feeds on plankton and often grows to a huge size, poses no danger to humans.

Basking sharks have been known to show up in Menemsha Bight. Mr. Skomal said anyone who does not know the difference should be careful.

“Be cognizant of the subtle differences between these animals,” he said.

Basking sharks also breach. On Tuesday just before lunch, another charter fishing captain was fishing off Gay Head with a party and observed one.

Capt. Terry Nugent of the fishing boat Riptide of Bourne, made the sighting at the 31 green can.

“We saw a large shark leap out of the water, 16 to 18 feet in length. We were 200 yards away. It came clear out of the water, twisted in the air and landed with an enormous splash. The first thing I said to my clients was that is a basking shark. I have video of basking sharks breaching going back to 2005,” he said.

Mr. Nugent said the lighting was favorable, allowing him to tell the difference. Unlike the great white shark, basking sharks do not have a white belly.

Mr. Skomal is flying to England in a week to tag and film basking sharks in England as part of an international study.

He would like to hear from anyone who sights a shark, or any large animal of any kind, as he is trying to further the study of such animals in area waters. His telephone number is 508-693-4372.