Two shark sightings at a pair of North Shore surf spots spooked swimmers yesterday and prompted the posting of several warning signs.

No beaches were closed, and some ignored the warnings.

"If they want to stay, they're going to stay," said city EMS spokesman Bryan Cheplic. "There isn't anything we can do, unfortunately. We can post signs and warn people, but we can't force them to get out of the water."

At 1:30 p.m. yesterday, surfers and lifeguards spotted two 13-foot sharks and two sharks measuring 6 to 8 feet long cruising near Chun's Reef and Piddleys.

At the time of the sighting, about 10 surfers and swimmers were in the water, and half of them swam ashore after lifeguards alerted people with bullhorns.

Shortly after the sighting at Chun's Reef, lifeguards spotted several other sharks near Lanikea, prompting lifeguards to deploy on jet watercraft to chase people out of the water. Most of the people in the water at Lanikea swam ashore after the warning, Cheplic said.

The most recent shark attack occurred March 23, when a 28-year-old visitor from Vancouver, British Columbia, suffered five puncture wounds in her left calf from a shark that bit and released her.

It was the fourth attack in Hawai'i this year and the first reported on O'ahu since Feb. 16, 2005, when a shark estimated to be 8 feet long bit a surfer's board near Sunset Beach. That attack happened at 2:30 in the afternoon in clear water about 10 feet deep.

Reach Peter Boylan at pboylan@honoluluadvertiser.com.