This Is Why You Can't Miss California's Grunion Run

These fish leave the water to lay their eggs on California beaches—and you can watch it happen.

Every Spring, thousands of tiny grunion fish flop onto the beaches in Southern California to bury their eggs in the sand before catching a wave back out to sea. Known as the Grunion Run, watching the silvery fish glisten on the beach as they spawn has been a long-time California tradition.

The runs take place from March to August on nights following the new and full moons, when the tide is high. The high tides allow the grunion to ride waves onto the beach as far as possible. Once on the beach, the females dig their tails into the sand to lay their eggs. The males then wrap themselves around the ladies to fertilize the eggs.

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When the next wave arrives, the fish catch it back out to sea, leaving the eggs in the sand. “I call them the original surfers,” Karen Martin, a grunion expert from Pepperdine University, told the San Diego Union-Tribune. “They’re the first ones who learned how to ride in on waves.”

The eggs then have around 10 days to incubate before the next high tide arrives. The water from the high tide triggers the hatching and sweeps the baby grunions out to sea.

Watching the tiny fish shimmer and dance in the moonlight is a mesmerizing experience and a long-time California tradition. While some of the biggest runs occur in Southern California, the spawning takes place from Baja all the way up to San Francisco.

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For those who want to see the grunion, The California Department of Fish and Wildlife posts the dates and times of expected runs on their website. But adding to the mystery of the event, it’s impossible to predict on which specific beaches the spawning will happen on any given night.

“They have survived this long in a very highly populated place by being not completely predictable,” Martin told The New York Times.

Both Birch Aquarium in San Diego and Cabrillo Marine Aquarium in Los Angeles host guided grunion runs. While March and April are closed to fishing, those with licenses can catch the grunion by hand during the rest of the spawning season.