Runnin’ With the Grunion : The fish’s mass arrival at the beach to deposit and fertilize eggs provides a spectacle for visitors.

What flings itself on Southern California beaches each summer, numbers in the tens of thousands and has a skin color that resembles egg whites mixed with milk?

Guess again if you answered “tourists.”

A six-inch shallow-water fish called grunion is the correct answer. Unlike tourists, grunion leave behind millions of offspring during their beach outings, which last from March through September. Their twice-monthly mating ritual has become a popular Southern California rite of summer.

Grunion runs can be spotted on half a dozen Southern California beaches, including Redondo Beach and Alamitos Bay in Long Beach. Among the most predictable spots is the one-mile stretch of Cabrillo Beach in San Pedro. The beachside Cabrillo Marine Museum offers evening educational programs a few hours before grunion run.

Program scheduling is no problem since grunion are more predictable in their appearance than the swallows’ return to San Juan Capistrano each March 19. The museum, in fact, is able to forecast grunion runs down to the minute.

Grunion, whose habitat is one mile offshore, ride in ocean waves every two weeks during full and new moons, when high tides crest. Their impeccable timing kicks in as a single male grunion chooses a wave that follows the turn of the tide. This choice ensures that eggs deposited on the beach will not wash out to sea by oncoming high waves. Like lemmings operating in reverse, the rest of the school follows the single fish en masse toward shore.

“They know more about tides than we do,” said John Olguin, former director of the Cabrillo Marine Museum. They probably have internal clocks or biological mechanisms that trigger the urge to head toward land when ocean currents change, he said.

Because of their translucent, milk-like coloring, it’s difficult to spot grunion in the water. Once they reach shore, you will observe first a few, and within half an hour, tens of thousands of fish will be hurling themselves onto the sand--a mosaic of glimmering, wriggling shapes.

Females drill their tails into the sand to lay eggs, with just their gills and heads poking up. As many as 3,000 eggs are deposited by a single mature grunion, which won’t live past three years. Up to eight male grunion wiggle around each female, releasing milt that fertilizes the orange eggs buried in the sand--safe from marauding sea gulls.

Listen closely and you may hear a squeaking sound emitted by the females. The sound is how the grunion received its name--from the Spanish word grunon, meaning “grunter.” Females usually remain onshore longer than males--up to 20 minutes before catching a wave back home.

Eggs reach maturity in 10 days to two weeks--just in time for the next high tide to signal the tiny fish to release an enzyme that dissolves its egg wall. They pop out like coiled springs, ready to ride the next wave out to feast on plankton.

Olguin has been holding grunion-run programs at Cabrillo Beach since 1950. “I haven’t missed one,” he says of the festive event, which can attract up to 2,000 people. Olguin once held grunion fries on the beach--although the fish are not as tasty as the silver-colored smelt, which run en masse in cold northern waters. Grunion are a bit bony, he says, even when deep-fried in seasoned egg batter.

In March, June and July, grunion may be collected by hand by those possessing a California fishing license. No license is required for those under the age of 16.

The Cabrillo museum’s programs feature a 20-minute film presentation and 30-minute talk. Programs, which are given several times before the grunion run, begin at 9 p.m. in the museum’s auditorium. Bring blankets to sit on at the beach and flashlights. Dress warmly.

WHERE AND WHEN

Location: Cabrillo Marine Museum, 3720 Stephen M. White Road, San Pedro.

Getting there: Travel south on the San Diego Freeway to the Harbor Freeway. Go south on the Harbor, which ends at Gaffey Street in San Pedro. Continue south for two miles on Gaffey Street, then turn left on 22nd Street. Drive two blocks to Pacific Avenue and turn right. Turn left at 36th Street, which becomes Stephen M. White Drive. Follow the road down to the beach. The museum is the modern building on the left just beyond the parking lot.

When: 10:21 p.m. Tuesday to 12:21 a.m. Wednesday, 10:30 p.m. July 1 to 12:30 a.m. July 2, 10:09 p.m. July 15 to 12:09 a.m. July 16. The beach closes at midnight.

Call: (310) 548-7563.