(CNN) -- The ocean water around Chatham, Massachusetts, is playing host to some unwelcome guests this holiday weekend: sharks.
Only 41 unprovoked shark attacks have been reported in the U.S. since 2008.
At least one large shark sighting was confirmed Friday by Greg Skomal, shark expert for the state's Division of Marine Fisheries.
That sighting occurred off the eastern shore of Monomoy Island, a National Wildlife Refuge off the southern elbow of Cape Cod, according to Lisa Capone, Massachusetts Department of Energy and Environmental Affairs press secretary.
The species, though not confirmed, was "most likely" a great white shark, she said.
The sighting came a day after Skomal reported as many as five large sharks were seen near the island, about a mile or more away from Chatham's Lighthouse Beach, a public swimming area.
Capone said Skomal was searching the area of the island Friday afternoon, though weather conditions were "not ideal," with fog and wind.
Meanwhile, officials were advising residents and visitors to be cautious while visiting the beaches this weekend.
"It is the eve of the last holiday weekend of summer, and we want to make sure folks on the Cape are aware," Capone said.
Chatham's Harbor Master's office issued a public notice after the sightings Thursday, saying the waters surrounding Chatham are "now a year-round home to a few thousand seals." Seals are a main source of food for large sharks, and as recently as August 28, a large shark was seen feeding on the body of a seal near Chatham's waters, the notice said.
It advised people to avoid swimming near seals.
A statement issued Friday by Chatham town officials said "beaches will be closed immediately" if a shark is sighted.
Paul Zuest, general manager of the Chatham Bars Inn, said he and his staff also have been instructing their guests to be cautious. The hotel, about two miles away from where the sharks were spotted, has posted signs providing information on the sightings.
George H. Burgess, director of the Florida Program for Shark Research and curator of the Florida Museum of Natural History's International Shark Attack File, said that on average, sharks have killed four people annually worldwide for the past several years.
"The chances of us as individuals entering the sea and not coming back as a result of a shark attack are slim to none when you consider the millions upon millions of people that go into the sea each year," he said.
The museum's shark attack file lists 41 unprovoked shark attacks in U.S. waters in 2008, with one fatality, in California. Most of the attacks, 32, occurred in Florida.
Massachussetts has recorded only four shark attacks since 1670, two of which have been fatal. The last fatal shark attack in Massachussetts occurred in 1936.