The 16-year-old was bitten at about 5 p.m. Sunday. He said he was in about 5 feet of water when he felt something biting down on his arm.
Scott Newlin, a marine biologist for the state, said the attacker appeared to be a young sand bar shark probably no more than 3 feet long. He said sharks tend to feed actively in late afternoon, making attacks somewhat more likely then.
"I don't think there is much concern for the public now," Newlin told CapeGazette.com. "But it's best to steer clear of the water especially after dark."
Wayne Kline, chief of enforcement for Delaware State Parks, said the shore would be surveyed by helicopter for signs of dangerous sharks. He said the beach would probably reopen in the afternoon.
Rich King, an experienced surf fisherman, said the shark migration appears to be heavier than normal this year, possibly because ocean waters have been slow to warm. King said he does not believe there is any real danger.
"This is not a Jaws situation," he said. "If it were a big sand shark, it could have taken his arm off."
Shark attacks are rare in the mid-Atlantic region but are reported occasionally. Peter Benchley's novel Jaws was inspired partly by a series of attacks on the Jersey Shore in 1916 that killed four people and left one injured.
King said that sharks are common in the area but being "run over by a cow" is more likely than being attacked by a shark.
"Sharks line our oceans. The Delaware Bay is a nursery for tiger sharks," he said.