SAN DIEGO (AP) — It’s grunion run time on the Southern California coast.

The small, silvery fish flop onto beaches by the thousands from March through August in a predictable nighttime spawning ritual.

Danny Beckwith of the Birch Aquarium in La Jolla tells The San Diego Union-Tribune grunion runs are triggered when tides are at their highest, at the full and new moon, and continue for four consecutive days.

RELATED LINK: California Grunion Facts and Expected Runs

The fish ride waves onto beaches and the females wriggle into the sand tail-first to lay eggs. Males then curve around the females and release milt, which fertilizes the eggs.

The grunion are then carried back out to sea by successive waves.

Pepperdine University biology professor Karen Martin calls grunion the original surfers.