Just after nightfall in southern California, on sandy stretches of Pacific shoreline, a piece of marine folklore is coming to life.

It's that time of year again. The grunion are running.

Grunion are skinny, silvery little fish only 5 to 6 inches (13 to 15 centimeters) long. They're not harvested commercially, and they'd probably go unnoticed if it weren't for the unusual way they spawn.

But the way they flop, en masse, out of the water and onto the shoreline, dig into the sand to lay eggs, and then scoot back into the surf has become the stuff of legend in southern California.

Natives get a kick out of taking out-of-town guests to see the nighttime spectacle, which occurs each year between March and August on beaches from Point Conception, just north of Santa Barbara, down to Punta Abreojos in Baja California, Mexico. California grunion, members of the Atherinopsidae family, or New World silversides, are found nowhere else in the world.

But how much longer will they be found here? Anecdotal evidence suggests that as hunting has increased, and as development has reduced available spawning grounds, there have been fewer strong, healthy grunion runs in recent years.

"People ask me all the time, 'How are the grunion doing these days?' " says Karen Martin, a biology professor at Pepperdine University and the region's best-known expert on grunion. "I say, 'It's not their best year.' "

See How They Run

The story of "this quirky, kooky fish" is what draws people, says Mike Schaadt, a native southern Californian and the director of the beachfront Cabrillo Marine Aquarium in San Pedro. "It's caught the attention of people in a romantic sense," he said, "and also just how bizarre nature can be."

Schaadt's aquarium takes advantage of the unusual opportunity to unite observers with fish in the wild. Cabrillo Beach, just outside the aquarium's door and in the shadow of the Port of Los Angeles, has long been known as a hot spot where grunion mingle.

Since 1951, the aquarium has been hosting "Meet the Grunion" on high-tide nights when the fish run. The entertainment starts inside, moves out to the beach, and ends—hopefully—with a real-life display of grunion spawning on the sand.

Last Saturday night, as the sun sank into the horizon, families, Cub Scouts, Girl Scouts, school groups, and budding scientists gathered in the aquarium's auditorium to watch a short documentary of the fish's life cycle.

Shot in 1964, the film is quaint by today's high-def standards, but it captures spectators' attention as they learn the details: Grunion ride the surf onto shore at the highest tides, under new and full moons and for three subsequent nights. Females shimmy back and forth to dig their tails into the sand and release a clutch of up to 3,000 eggs. Males, attracted by the movement, circle the females and release milt, which flows down their bodies and fertilizes the eggs.

Grunion only surf high tides, to the highest point of the shoreline, for a reason: The eggs need to incubate undisturbed under the sand for 10 days, where they wait for the next high tide to carry them out to sea. And they've evolved to wait for their ride. If they hatch too soon, they suffocate under the sand.

A year later, they're full-grown fish ready for their own beach rendezvous.

Grunion survive their brief excursion by lowering their heart rates. They conserve energy, take care of business in just a few minutes, and float out on the next wave.

At the Cabrillo Aquarium, visitors learn how to do the "grunion dance"—to twitch and wriggle just as female grunion do when they drill into the sand. They pass around baby-food jars filled with seawater and grunion eggs. They shake the jars, in the same way eggs laid on the beach are agitated by surf, and watch as tiny grunion—mostly eyeballs and tails—spring out of their clear shells. Even the adults are in awe when they see baby grunion jump out of the eggs like popcorn popping.

The kids want to keep the babies, but reluctantly return them to aquarium staff, who grow the young fish for research and release them.

Rules of Engagement

Grunion education hasn't changed much since the 1950s, and neither has grunion hunting. The rules have been the same since 1949, when restrictions limited the open season to March, June, and July. No equipment is allowed—you can only use your bare hands to grab the fish. Anyone 16 and over is required to have a fishing license. And in April and May, the peak spawning time, it's illegal for anyone to catch the fish.

The first rules on grunion hunting were passed in 1927. Before that, when the fish were a means of subsistence for some southern California families, people used bedsheets as a dragnet and harvested as many as they could from the surf.

These days, few of the fish caught during the recreational runs make it to the frying pan. (Garlic and olive oil are a must, although Schaadt says they can't mask the "fishy" taste.)

But one thing that has changed over the decades: the number of grunion hunters. The prominence of SoCal's beach culture—along with the population of San Diego, Orange, and Los Angeles counties—has exploded in the past half century. There simply weren't as many people going to the beach in the 1940s and '50s, says Martin, the Pepperdine biologist who has been studying the endemic fish for 20 years.

At the same time, other pressures on the fish have increased. Waterfront construction has narrowed beaches and armored shorelines with sea walls, reducing the area available for spawning.

Just five years ago researchers saw healthy numbers of grunion on SoCal beaches. But the runs over the past two to three years haven't been very strong. "And since grunion only live two or three years," says Martin, "that's a source of concern."

The Numbers Game

Adding to the mystery of this iconic fish: No one keeps track of them. In fact, scientists only know about them because of their spawning ritual. They aren't snared by hook and line, they don't show up in commercial nets, and they only pop up at night. They're essentially invisible in California's network of recreational and commercial fishery counts.

So populations are hard to gauge, and counting grunion doesn't work. During strong runs, the masses of fish are floppy, slithering chaos. And while females appear only once a run, males ride the waves to shore over and over. Each time they land, they look for another female to sidle up to.

Martin, along with marine conservationist Melissa Studer, helped found Grunion Greeters, a once-robust organization that held workshops, trained a platoon of volunteers, and fanned them out over SoCal beaches to size up grunion runs and report the details. From 2002 to 2010, as many as 600 volunteers a year scanned more than 50 beaches at a time. Data gathered by the group were extensive enough to persuade San Diego and then other California municipalities to change their beach-grooming practices to avoid plowing over incubating eggs.

The grant-dependent program is no longer funded, but volunteers still file scattered reports; about 200 have been received so far this year. Organizers are hesitant to draw conclusions about the overall grunion population from that data, but they say sightings of strong, healthy runs—which were common as recently as 2009—are increasingly rare. This year, only a single one was spotted, near Malibu.

Martin has begun working with the California Department of Fish and Wildlife to determine if more restrictions on grunion hunting are needed. She and Studer are in favor of making June part of the closed season, so catches could only be made in March, July, and August.

"There are few people that need to eat grunion at this point," Martin said. "It's purely a recreational activity. And I think if people can learn to catch and release trout, they can learn to catch and release grunion."

Nor would closing more of the season keep spectators from seeing grunion. Martin argues that enthusiasts would get even more out of the experience if they stood back and watched what happens when these fish toss themselves out of the water.

"It's much more interesting to watch their natural behavior than to watch what they do in a bucket," she said.

View Images Grunions do a unique spawning dance. The females dig into the sand to lay eggs and then scoot back into the surf. PHOTOGRAPH BY MARK CONLIN/ALAMY

Scene and Herd

The thrill of the hunt is what brings late-night crowds to Cabrillo, with buckets and flashlights in hand. When more than a thousand people attended the program at the aquarium last Saturday, they joined at least a thousand spectators already on the beach.

The atmosphere was charged. Sean Vertuno, an 8-year-old redhead with glasses and a Cub Scout T-shirt, was full of questions inside the aquarium. He raised his hand in the crowd and asked: "How do you catch them? I mean, just in your hands? Or do you scoop them up with your arm? Are they wet?"

As smoke from beach campfires floated on the ocean breeze, a pair of black-crowned night herons landed silently and began to investigate the waterline. They saw the fish before the crowd of spectators did, and dipped into the water to pluck out them out and swallow them whole.

In minutes, a few dozen silvery grunion glittered like diamonds on the sand as they flopped in all directions. Aquarium staff directed spectators: "OK! Flashlights on!" An "Ooooooohhhh!" went up from the throng.

Staffers could only hold back the crowds for so long. The spectators inched forward until staff gave them the signal: "Go, go, go!" As bare feet swarmed the surf, eager hands grabbed at every fish that was visible. Squeals rose up in the mad rush.