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In the late night hours after new and full moons, something strange hits San Diego’s beaches: fish.

Specifically, grunion, the 5-inch, skinny silver fish with very strange mating habits. In the spring and summer, grunion surf the breaking waves in Southern California and Northern Mexico to get as far up the beach as they can. The female fish then bury themselves in the sand, so only their heads are showing. They lay eggs in this makeshift nest, and then wait for the males to arrive.

Up to eight male fish will swarm a single female and deposit sperm to fertilize the eggs. The males then swim away, while the females wriggle free and catch the next wave back into the ocean.

Grunion wait for late nights after new and full moons to “run” because that’s when tides are highest, said Karen Martin, a biologist at Pepperdine University who heads Grunion Greeters, a group of citizen scientists who collect data during the grunion runs.

The fish work so hard to climb up the beach because the sand away from the ocean provides a good environment for their eggs, Martin said. The baby grunion only incubate for two weeks, and then get roused by the crashing waves of the next high tide.

Because the grunion population is relatively small — much less than one million, Martin said — the California Fish and Game Department has strict rules for how they can be fished. Grunion can be scooped up by the handful, but setting traps, digging holes, or using other gear to catch them is not allowed.

“There is no bag limit, but take only what you can use — it is unlawful to waste fish,” the department warns.

Fishing licenses are also required for everyone older than 16 years old to capture grunion, and even collecting them by hand is forbidden in April and May.

Yet for all their public displays of affection during mating season, little is really known about the little fish. They are being studied now by researchers to see whether they contain clues about the overall health of the ocean and how beach maintenance impacts them. And the grunion’s presence has caused changes in beach grooming practices in Southern California.

San Diego resident Lauren Abrams saw a grunion run on Mission Beach at the end of May and said, “it was like nothing I’d ever seen before.”

“There were hundreds of them, and as we walked around them, they would be flipping up around our ankles,” she said. “You’d see some bury themselves in the sand, and then if you kept watching, you’d see it wriggle back and forth until all of a sudden it launched itself out of the sand.”

To see this unusual sight, head to the beaches beginning Sunday night between 9:30 p.m. to 11:30 p.m. The run will be on for four days before returning for two more stretches in August.

The Department of Fish and Game has the completed schedule, including the best times to see the run.

The site also includes this interesting bit of grunion history:

Grunion food habits are not well known. They have no teeth, so they are presumed to feed on very small organisms. Shore birds, isopods, flies, sand worms, and beetles eat grunion eggs, while humans, larger fish, and other animals prey upon grunion.

Despite local concentrations, grunion are not abundant. The most critical problem facing the grunion resource is the loss of spawning habitat caused by beach erosion, harbor construction, and pollution. By the 1920s, the fishery was showing definite signs of depletion. A regulation was passed in 1927 establishing a closed season of three months, from April through June. The fishery improved, and in 1947 the closure was shortened to April through May. This closure is still in effect to protect grunion during their peak spawning period.

Information on Martin’s Grunion Greeter group, including how to join, is available at grunion.org.

— CLAIRE TRAGESER

Sam Hodgson

Sam Hodgson is a freelance photojournalist and contributor to Voice of San Diego. You can contact him at samhodgsonphoto@gmail.com...

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