A Massachusetts marine biologist who examined a 13-foot great white shark carcass found ashore near the Rhode Island state line wasn't able to determine how the shark died, state officials said Sunday. 
 
 Researchers investigate the carcass of a great white shark that washed up in Westport, Mass., just over the state line from Little Compton, R.I., Saturday Sept. 1, 2012. A Massachusetts marine biologist... (Associated Press) 
 
 State biologist Greg Skomal performed a necropsy on the 1,500-pound male shark Saturday and found no signs of trauma. A fisherman discovered the carcass Saturday morning on a rocky shoreline in Westport, less than a half-mile from public beaches in Little Compton, R.I. 
 
 "They will have a difficult time, if at all, figuring out how he was killed," said Krista Selmi, a spokeswoman for the Massachusetts Division of Marine Fisheries. 
 
 Little Compton officials closed Goosewing and South Shore beaches after the shark was found. Both beaches reopened Sunday and police sent out a patrol boat to watch for other sharks. 
 
 Selmi said officials plan to leave the shark's body where it is. 
 
 "There's really no means to move an animal of that size," she said. 
 
 The shark appeared to have washed ashore Friday night, Selmi said. The carcass is below the high tide level, and officials hope the tide drags the shark back out to sea. 
 
 Shark sightings have been rare in the area. But further east, there have been numerous reports of sharks off Cape Cod. In July, a man was bitten on his legs by a great white shark while swimming off Ballston Beach in Truro, Mass. ||||| Shark sightings shut down Cape Cod beaches 
 
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 (CBS News) Beach goers in Cape Cod, Mass. found themselves with some unwanted company this Labor Day weekend. 
 
 Two sharks were spotted, prompting officials to raise the red flag, banning swimming at some of the areas most popular beaches. From South Beach in Chatham, to the northern tip of Nauset Beach in Orleans, shark sightings are keeping bathers out of the water. 
 
 One swimmer said, "Someone told me they can't come close to the edge, but I don't believe that anymore, so I'm just staying out." 
 
 The sightings aren't the first of the summer. In July, a man bodyboarding off Truro said he was bitten by a great white shark, the first known attack in those waters in over 75 years. He survived, but the incident led many to question why so many sharks are lurking off the cape. 
 
 Biologist and shark expert Simon Thorrold said the sharks are likely responding to an increased number of mammals - but not of the human variety. 
 
 "I think we're probably seeing more white sharks in the area at the moment," Thorrold said. "And the white sharks have probably been drawn into the area by a really large increase in the number of grey seals that are in the area." 
 
 That suspicion may have been confirmed this week when a family captured dramatic video of a shark attacking a seal off Monomoy Island. 
 
 Seals are a staple of a shark's diet. It is estimated 350,000 of them live along the North-Atlantic coast. They have been protected since the 1970s by the federal Marine Mammal Protection Act. 
 
 With the resident seal population showing no signs of slowing down, beach goers will have to share the shore with the sharks... at least until the great whites migrate to warmer waters this winter. 
 
 For Terrell Brown's full report, click on the video player above.
– A Labor Day disappointment on Cape Cod: Beaches have shut down along much of the coast thanks to shark trouble. Officials have banned swimming at several top sites after a pair of sightings, CBS News reports. The animals seem to be appearing more frequently lately; earlier this summer, a man was reportedly bitten by a great white off Truro for the first time in 75 years. "White sharks have probably been drawn into the area by a really large increase in the number of grey seals that are in the area," says an expert. And the sightings aren't limited to the Cape: A 1,500-pound shark turned up dead this weekend on the Massachusetts side of the Massachusetts-Rhode Island border. A biologist performed a necropsy, but officials aren't sure what killed the creature. They're going to leave it on the rocky shore, hoping the tide might eventually carry it out to sea: "There's really no means to move an animal of that size," a spokeswoman tells the AP.