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Climate and Environment

Thousands Of Fish Are Mating On SoCal Beaches (Yes, It’s The Grunion Run)

Piles of fish on the sand where the water meets. There are people crouching and taking pictures with their phones.
Thousands of grunions on the shore.
(Courtesy Cabrillo Marine Aquarium)
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Grunions are the only fish in the world that come out of the water and burrow into the sand to lay their eggs. Their complicated mating ritual can be watched along Southern California to Baja California beaches at night during high tide.

The yearly event happens between the full and new moons from now through July, according to Jim DePompei, program director of the Cabrillo Marine Aquarium in San Pedro.

“Females will dig down into the sand up to about their gills,” he said. “They'll lay 1,000 to 3,000 eggs. Males will wrap around the females and they'll fertilize the eggs. They'll just drop their milk right down into the sand, fertilize the eggs, and then they go back into the ocean.”

Then about 10 days later, the eggs hatch and the baby fish swim back into the ocean. DePompei said to take a flashlight to see in the dark, and to stay above the tide line to avoid scaring away the grunion.

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The aquarium leads tours of the Grunion Run. DePompei said the fish can be skittish, so it’s best to make sure to stay above the high tide line.

“This may sound silly, but grunion will actually send up scouts onto the beach,” he said. “You'll see a wave and you'll see like a handful of grunion on the beach. What these scouts are doing is they're making sure the beach is safe for the other grunion to come up.”

He said to turn off the flashlight and wait for the scout fish to give the all-clear.

A close-up of the silver grunion fish in the sand.
The grunions’ complicated mating ritual can be watched along Southern California coastline.
( Courtesy Cabrillo Marine Aquarium)

“Within a couple of waves you'll start seeing more fish, and more fish, and more fish, and more fish. And then eventually they'll, what we call, ‘stick,’” he said.

“That means like you'll have several hundred grunion on the beach and that means you're going to have a full on Grunion Run that night.”

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