Sex on the beach: a beloved California fish wriggles ashore to spawn

·8 min read

On certain nights on a quiet California beach, thousands of small, silvery fish gather in the moonlight to perform a unique mating ritual.

Known as the “grunion run”, the spectacle is one of the lesser known natural wonders of the US west coast. Grunion are a rare fish species that come ashore to spawn, and during the months of April to August they cover beaches from Baja California to Santa Barbara like a glittering carpet, wriggling in the sand to lay and fertilize eggs just after the highest tide of a full or new moon.

The grunion run has fascinated scientists and locals alike for decades. But its future could be imperiled by the climate crisis – including warmer land and water temperatures and increasingly acidic oceans – as well as human activities such as fishing. Experts wonder how much time is left to unravel the mysteries that still remain about the grunion’s life.

On a recent night near Topanga, the beach was silent save for the rhythmic crash of waves and the dull roar of cars whizzing down the Pacific Coast Highway. Under a dark sky with the big dipper just beginning to peek out, Pepperdine University biologist Karen Martin and her students stood ready and waiting.

Martin, who has been studying grunion for years and is considered a foremost expert on their behavior, sets some rules. One, don’t touch the fish. Two, don’t shine lights on them until the frenzy has started. Three, don’t wander too far away on the darkened beach, which is a critical fish habitat.

Finally, Martin wishes them well with a special phrase. “May the fish be with us.”

She points out the “scout fish”, the grunions that scientists believe arrive on the beach first to make sure it’s safe. Students use night-vision goggles to spot their wiggling bodies flipping around, avoiding using light sources. The air is thick with anticipation – how much fish sex will there be?

The first hour starts slow, with only a small cluster on the beach. But then, something shifts. More and more fish arrive and dig themselves into the soft sand, with only their flippers and eyes poking out. Males flopped around, wrapping their bodies around the females. The orgy has begun.

Grunion runs are graded on a scale of 1 to 5, depending on the number of fish that arrive, the length of the run and how long they mate for. Martin was visibly relieved as she held up a laminated copy of the Walker Scale for Monitoring Grunion Runs and agreed this was a W4 – a big night for the little fish.

But runs of four and above are growing less frequent. There have been no W5 evenings this year, and few in the past few years – a troubling sign.

A mysterious fish with spectacular behavior

If it were not for their magnificent spawning behavior, the unassuming grunion might have gone largely unnoticed.

Beach spawning fish are rare – just a handful of fish around the world come out of water to lay eggs on the sand. And beach spawning fish near major cities are even more unusual. 90% of the grunion population lives off the coast of three densely-populated counties: San Diego, Orange county and Los Angeles. Even so, grunion weren not really studied until 1927, when the state created the first rules to protect them.

The fish are part of southern California lore, a beloved footnote to beach culture. The run even appeared in the 1991 teen comedy “Don’t Tell Mom the Babysitter’s Dead”. Sometimes crowds gather to watch the spectacle. But often, seeing grunion involves patience and a lot of time. The conditions for a successful grunion run have to be just right: it depends on the moon and tides, a lack of predators and the right coarseness of sand.

After the eggs hatch and they swim into the ocean, little is known about their three or four-year lifespan. They play an important part in the food chain for larger fish, as well as seals, sea lions, sharks, dolphins and squid at sea, and on shore for herons, egrets and other birds.

More is known about their time on land. Grunion lay 3,000 tiny neon-orange eggs in a bunch, buried six inches in the sand before they surf back out to sea. If all goes well, the eggs wait for an environmental trigger – usually another high tide that occurs two weeks later, between the new and full moon – when the larvae break free of their eggs and swim out into the ocean.

There is no way to know if the fish return to the same beach where they were born, says Martin, because so many of them get eaten by birds or larger fish. She has plans for a tagging program that could help answer that question, but funding to study the humble, six-inch long fish has not been easy.

To answer some of the persistent scientific questions of the species, Martin has called upon beach-goers that can add to scientific knowledge.

Along with marine conservationist Melissa Studer, Martin started a program called Grunion Greeters in 2000, unleashing an army of volunteer “citizen scientists” to help study the fish and their habitat. Since then, more than 5,000 fish-watchers have participated, collecting data on 50 beaches up and down the coast of southern California.

It’s an addictive habit, says Mimi DiMatteo, a longtime greeter, who gently steered a group of teenagers away from touching the fish during the recent grunion run. “There’s something so invigorating about being on the beach at night.”

Under threat

The grunion are facing threats on several fronts.

While they are not commercially fished, they make easy pickings during a grunion run, when people collect them for bait, or a snack that can be dredged in flour, fried and eaten whole.

While grunion fishing dates back decades, officials have recently begun to observe people taking an excessive amount of fish, and available data appears to show their numbers are decreasing.

This year, the state has prohibited people from taking any fish in April, May and June, the peak time of the grunion run – adding June to the prohibition for the first time since 1949.

“We’re trying to curtail that activity to protect the species and hopefully they will recover,” says Armand Barilotti, an environmental scientist with the California department of fish and wildlife.

Other threats include beach erosion – as the fish need a good runway of sand for their spawning activities – and climate crisis. Grunion eggs cannot survive at high temperatures, making them vulnerable to warming temperatures on land. Another population, the Gulf grunion living at the top of the Gulf of Mexico, is getting smaller because it has no place to move for cooler temperatures, Martin says.

Even in California, the population is shifting northward: grunion are now seen in San Francisco bay, where they have not been in the past. Reports from Baja of grunion runs are getting more and more scarce.

Darren Johnson, a biologist at California State University at Long Beach, has been doing studies on climate crisis’ impact on the species. He has found the effects of changing ocean chemistry – more carbon dioxide in the water makes it more acidic – are having an impact on the larval fish.

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The fact that grunion spawn on land makes them a good species for understanding such effects, he says. “Unlike fish you have to search for, you can just see them on shore, collect them and take the larvae back to the lab.” That makes grunion part lab rat, part sentinel to understand how ocean acidification is affecting fish in the area.

Johnson collected hundreds of eggs from the beach and tested their survival in different levels of high carbon-dioxide conditions (when there is more carbon in the atmosphere, the ocean becomes slightly more acidic). Overall, ocean acidification meant fewer grunion survived from the larval stage to adulthood. But there are genetic variations that allow some fish to thrive even in changing ocean conditions. “What we find is that within the population, there is a lot of variation of sensitivity to changes in seawater chemistry,” Johnson says. “So some groups of families can tolerate changes to seawater chemistry more than others.”

Really big spawning events – where the entire beach is covered with fish – have been rare in the past few years, which is one reason to close the fishery to human catch in June. “Giving them another month will be huge,” says Johnson. “Most of the spawning happens in April, May and June, so with an additional month of a closed season, it means more input back into the population – more time to make more babies.”

Back on the beach, the evening stretched close to midnight. What began as a small run of grunion crescendoed into an avalanche, their silvery bodies catching the light from cameras and buildings around the bay. The sound of the waves punctuated by the slapping of their bodies in wet sand, and an occasional squeal of glee from Martin’s students. Thousands of fish piled on top of each other, leaving little room to walk.

DiMatteo, the grunion greeter, pointed to the marks left behind when they wiggled back into the Pacific ocean – they looked like snake tracks, or ancient hieroglyphs.

Despite the heating oceans and other troubles facing the grunion, the night’s turnout left Martin optimistic. She stood smiling into the darkness watching fish flop out of water to lay their eggs. “A good night,” Martin said quietly.

The humble grunion’s future remains uncertain, but one thing is sure: there are people out here looking out for them.