Photos: Never been to a grunion run? Come along with us

Sex on the beach is best in the dark. Grunion know this. Each spring and summer, a few days after a full moon or new moon, the small, silvery fish wash ashore to spawn by the thousands on Southern California beaches.

Like the swallows’ annual return to San Juan Capistrano or the California gray whale migrations offshore, grunion runs remind us that wildlife survives in a sprawling city of millions. Though unknown to many, grunion runs are a local fixture. Coastal Native American people once relied on them for food. In the 1960s, Frank Zappa wrote and recorded an instrumental song called “Grunion Run� that featured the sounds of a guitar plugged into a fuzzbox.

On a recent night, about 100 visitors to Cabrillo Beach in San Pedro became voyeurs to the mating dance that happens shortly after high tide. It unfolds like clockwork. One moment the beach is empty. Then a wave crashes and deposits the wildly squirming fish on the wet sand to lay and fertilize countless eggs before receding back into the sea.

Beachgoers use flashlights to illuminate grunion spawning at night on Cabrillo Beach in San Pedro.

(Luis Sinco / Los Angeles Times)

Basically, females dig nests with their tails and deposit eggs into the sand. Up to eight males try to mate with each female by curling their bodies around her. Milt flows down the female’s body to fertilize the eggs. Throughout the night more grunion slither up the beach, merging above the surf line.

How the grunion know when to run remains a mystery. It could be the pull of tides or the position of the moon. The eggs develop over 10 days and hatch with the next high tide. Most of the hatchlings are consumed by other fish and shorebirds higher up the food chain.

The fish are unique to Southern California and Northern Baja. They are essentially smelt. Most nights people are legally permitted only to watch the fish. Open nights happen during the season, and people with valid fishing licenses can catch an individual limit of 30 fish by hand. Beach erosion and coastal development threatens the fish’s spawning habitat, though experts say the species currently appears viable.

The next grunion run is expected to happen in San Pedro on June 16. Tickets for guided grunion run programs are available from the Cabrillo Marine Aquarium.�

Thousands of grunion slither around Cabrillo Beach in their nocturnal spawning ritual.

(Luis Sinco / Los Angeles Times)

Flashlights illuminate thousands of grunion spawning on Cabrillo Beach. Each female can lay thousands of eggs, which normally take about 10 days to hatch.

(Luis Sinco / Los Angeles Times)

Thousands of grunion slither around Cabrillo Beach. Females use their tails to dig a nest in the wet sand and deposit thousands of eggs.

(Luis Sinco / Los Angeles Times)

People watch the grunion run on Cabrillo Beach, one of the best places in Southern California to see a run. Guided programs are offered by the Cabrillo Beach Aquarium.

(Luis Sinco / Los Angeles Times)

Beachgoers watch the grunion wash ashore. As the females dig nests, the males try to mate by wrapping their bodies around them.

(Luis Sinco / Los Angeles Times)

Beachgoers watch as the grunion wash ashore. The small, silvery fish are unique to Southern California and Northern Baja.

(Luis Sinco / Los Angeles Times)