SAN DIEGO -- Stingrays injured dozens of beachgoers in the surf along La Jolla Shores and the Torrey Pines area Wednesday, sending three of them to hospitals, authorities reported.

At least 35 of the painful stings were reported through the day, with a rash of them coming in the mid-afternoon, San Diego lifeguard Lt. Nick Lerma said.

Medics transported three patients -- who reported having trouble breathing after getting stabbed by rays' barbed tails -- to emergency rooms and treated the other victims at the waterfront.

The number of strikes was unusual, though not unprecedented, Lerma said.

"It's not unheard of, but it doesn't happen every day, for sure," he said. "It's certainly the most for this year."

Beachgoers were stung by stingrays similar to the California round ray, which can easily disguise themselves under the sand. They are only about four inches in diameter, but as many found on Wednesday, their barb packs a punch.

To avoid getting stung by a ray, swimmers and surfers are advised to shuffle their feet through the sand while walking in the surf instead of taking normal up-and-down steps, particularly if there have been recent reports of the flat-bodied, long-tailed venomous fish in the area.

According to experts, there are at least 1,500 reported stingray injuries every year in the U.S., although many more go unreported.

A stingray's barb is sharp and has a serrated surface similar to a bread knife.

"It's a little bit like a serrated dagger, very fine, jagged cut [and] covered in nasty toxin," said Nigella Hillgarth of the Birch Aquarium.

Hillgarth believes there may be more stings along local beaches in the days to come. She said the rays are coming in to feed on sand crabs and mollusks and a breeding ground may be developing not far from where the water hits the sand.

Ninety percent of victims are stung in the leg or foot, and there have been very few instances in which humans were killed by stingrays.

The most notable was the death of Steve "The Crocodile Hunter" Irwin, who died when he was stung in the chest.