Swimmers at Santa Cruz County beaches were being warned of great whites Monday after two aggressive sharks were spotted over the weekend, including one that reportedly knocked down a surfer and bit into his board.

A man surfing at Manresa State Beach near Watsonville said a 15-foot shark grabbed his board as he rode Saturday evening about 50 yards from shore, according to state officials.

Since the surfer was attached to his board by a leash, he was pulled underwater for five seconds, officials said.

"The shark broke the board and bit it," explained Joe Connors, a supervising ranger in Santa Cruz County. "Once the shark stopped, the surfer was able to get back on the board and paddle to shore."

The surfer was not injured, Connors said, only startled.

In a second event, a biologist visiting Seabright State Beach in Santa Cruz on Sunday afternoon reported a shark attack on a harbor seal about 50 yards offshore, according to state officials.

"She saw some thrashing about in the water, then saw a harbor seal get thrown up in the air," Connors said.

Officials have not confirmed the type of shark in either case, but both attacks are believed to be the work of great whites.

"Typically those are the kind of sharks that exhibit that type of behavior," Connors said.

Great white sharks commonly comb waters off California, including Santa Cruz, but rarely do they come into contact with humans.

The state has had 81 confirmed, unprovoked white shark attacks - nine of which were fatal - between 1916 and 2013, according to the Florida Museum of Natural History's International Shark Attack File. The nation's total number of attacks during that period is 109.

State officials issued a shark advisory through Tuesday at Seabright beach and nearby Twin Lakes State Beach, warning swimmers of the potential for great white encounters.

Manresa beach was closed to swimming through Thursday because of the attack there.

Officials are asking anyone who might have witnessed either incident to contact California State Parks so that experts can take appropriate steps.

Kurtis Alexander is a San Francisco Chronicle staff writer. E-mail: kalexander@sfchronicle.com Twitter: @kurtisalexander