Surfers enjoying a sizeable south swell and visitors in Hawai'i for the Japanese Golden Week holiday found themselves sharing the beaches with hundreds of box jellyfish yesterday.

About 800 animals were collected at Waikiki and Ala Moana beaches yesterday morning. More than 100 were discovered on the beach at Makaha and more than 25 at Poka'i Bay along the Leeward Coast.

Jellyfish usually appear eight to 12 days after a full moon and their numbers vary from month to month. This week's influx, which is expected to last through today, is considered particularly large, said Emergency Services spokesman Bryan Cheplic.

The large number of jellyfish is in keeping with a pattern observed recently that suggests their annual numbers may continue to increase over the next three years until they reach 10,000, said Dr. Angel Yanagihara of the University of Hawai'i-Manoa's Pacific Biosciences Research Center.

Large numbers of jellyfish tend to appear every 10 years, most likely due to changes in the environment, she said. The last big influx happened in 2001, when about 10,000 animals washed up on Hawai'i shores. Some years see as few as 2,000 jellyfish annually.

"The last two years have seen an uptick again," she said. "A lot of Pacific invertebrates have that cycle. When you look at the big picture, there is a cycle going on."

Yanagihara has been collecting box jellyfish specimens since 1997. She and several student helpers scooped up 445 of the animals with tongs off Waikiki beaches yesterday morning.

The reasons behind the fluctuations and why box jellyfish seem to follow the lunar cycle are still unclear. Scientists have studied ocean temperatures and current patterns to try to explain the phenomenon.

Neighbor Island officials reported no noticeable jellyfish arrivals yesterday. The animals tend to collect around O'ahu shores.

Lifeguards warned hundreds of individuals yesterday and treated 38 stings at Waikiki and Ala Moana beaches, including one person who had to be brought to shore after being stung off Waikiki. No stings were reported along West O'ahu shores.

For Waikiki beachgoers and hotel employees yesterday, the animals were more of an annoyance than anything else.

"I got stung by one yesterday while swimming my morning laps," said John Kelly, who was visiting from Australia. "I went to do five laps and halfway through my first one I felt a sting on the back of my shoulder.

"I didn't seek any medical attention, just took a shower and the pain went away after one hour," he said.

Yuko Johnston, an employee at Ohana Beach Rentals, said her business lost some customers yesterday because of the jellyfish.

"I have seen a few people get stung by them today," she said, adding that "these local guys just rub it off and keep on surfing."

With the particularly large influx and numbers of beachgoers expected to be larger than usual, Cheplic said the situation "could have been a lot worse."

"We are lucky that we have such good swells today," said Cheplic. "The water is circulating and the jellyfish are either being washed up to the beach or washed away," he said.

"A thousand jellyfish versus 38 stings is a credit to our lifeguards being so active," he said.