It's grunion run time on the Southern California coast

This is the time of year when thousands of shimmering, silver fish flop onto Southern California beaches in a strange nocturnal mating dance.

Grunion runs, among California’s most stunning natural phenomena, start this month. Some of the early rounds of beach-going fish are expected to make their appearances on shore between Saturday and Tuesday.

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“In terms of what triggers the spawning, what the grunion are waiting for is when the tides are at their highest, at the full and new moon,” said Danny Beckwith, an education specialist at Birch Aquarium at Scripps in La Jolla.

For four days after a high tide, grunions surf to shore on breaking waves. Females wriggle into the sand tail-first and bury their eggs; then males curve around them to fertilize the eggs. As the next round of waves wash in, the fish float back to sea.

“I call them the original surfers,” said Karen Martin, a professor of biology at Pepperdine University, who studies grunions. “They’re the first ones who learned how to ride in on waves.”

The eggs incubate in the sand, where warmer temperatures and higher oxygen levels hasten their development, Martin said. To keep hatchlings from being crushed in their sandy burrows, grunions have evolved an environmentally cued hatching system that’s triggered only when the eggs wash out to sea.

Anywhere from dozens to thousands of fish may show up during spawning season, and lucky late-night beachgoers can view the ritual between March and August each year. Grunion fishing is allowed, but in April and May -- peak spawning season — the fish are off limits to anglers.

“April and May are closed season,” Martin said. “People can’t catch the fish, bother or disturb them on shore. ... That’s the period when the best runs will occur, with the highest number of fish on the beach. That’s to give them a chance to reproduce without human interference.”

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During March, and starting again in June, anglers can fish for grunions, but only with their hands. No nets, buckets or other gear is allowed, according to the California Department of Fish and Wildlife, which publishes schedules of expected grunion runs, along with other fish facts. People over 16 need a fishing license to participate, and while there’s no bag limit, it is illegal to waste them.

Humans aren’t the only ones hunting grunion, though. On land, predators ranging from raccoons to rats to shorebirds dine on the snack-sized fish. In the ocean, they’re prey for young white sharks and marine mammals. Some bigger fish will even follow them ashore.

“Shovel-nosed guitar fish will ride in, surf the wave, and be munching on the grunion, and then wash back,” said Melissa Studer, project director for Grunion Greeters, a citizen science group that works with Martin and other researchers to track spawning events.

Volunteers with Grunion Greeters patrol Southern California beaches for grunion runs and report their observations on the website, grunion.org. The six-inch-long, silvery-blue fish hold a special allure, she said.

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“They’re somewhat mysterious,” Studer said. “It’s wildlife, so it’s fairly unpredictable, it’s a fish out of water, so right there that’s intriguing. ... To be able to see the life cycle, see the spawning, the fish coming out of the water, the males curling around them, I think, it’s rewarding for people to see that. And a lot of volunteers are motivated by participating in real science and their protection.”

The fish can be found as far south as Baja, and recent warmer waters have allowed the fish to colonize areas up to San Francisco and Tomales Bay, Martin said.

Southern California, however, is ground zero for grunion runs. Experts say there’s no predicting where they’ll turn up on a given night, but note that grunions prefer wide, flat, sandy beaches, with little noise or activity.

If you go on your own, tread carefully, Beckwith said, as sound and vibrations can deter grunions from spawning. Dim your flashlights, or use red lenses to keep the lighting low. Ultimately, grunion sightings depend on being in the right place at the right time, experts said.

“You have to be persistent and lucky,” Martin said. “That’s the best solution.”

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