SAN PEDRO, Calif. — If the tide is high, the weather is warm, the clock is approaching midnight and the beach you’re standing on is in Southern California, it’s a given that romance is in the air — or the water. In these parts, it’s a time for grunion love.

The California grunion does something no other fish on the planet is known to do. It surfs a wave right out of its world and into ours. Then it plops itself down on the sand to lay and fertilize its eggs before waiting patiently for another big wave to carry it home.

Sometimes, before it hitches a ride back to the ocean, someone like 13-year-old Judy Feng will catch it — or at least try to (they’re slippery).

“At first I was trying to get it, but it was all slippery and I dropped it,” a delighted Feng said during a recent midnight run at Cabrillo Beach in San Pedro, where more than 2,000 people lined the shore for a good half-mile in search of the legendary but elusive aquatic wonder, known scientifically as leuresthes tenuis.

“I was going to let it go, but then my friend said, ‘It’s at my feet, don’t let it get away!’ And I grabbed it again,” she squealed, still bouncing up and down with excitement as she held her catch in a water-filled plastic bag.

Feng had officially become a grunion hunter, a distinction as uniquely Californian as the fish she was now holding at eye level. The California grunion is found nowhere else but along a thousand-mile stretch of coastline extending from central California to the southern half of Baja California, Mexico.

It’s along that region’s scores of flat, sandy beaches that people line up every spring and summer by the tens of thousands to watch the female grunion burrow tail first into the sand as the males wrap themselves around it.

“When they flop up against your feet, it sounds sort of like this,” says veteran grunion watcher Mimi DiMatteo, popping her cheeks with her fingers and making a noise that sounds something like “whuppa, whuppa, whuppa.”

Although many come just to watch the grunion spawn, in these hard economic times some people are trekking to the beach to bring home fish to eat. Veteran grunion watcher Chris Lindeman says he counted more than 200 people on a recent night who said that was their plan.

Like DiMatteo, Lindeman is a “Grunion Greeter,” one of some 550 volunteers who go out and count the fish during their runs from March through August to help marine biologists make sure that their numbers remain strong, and that people aren’t taking them without fishing licenses or breaking state law by using anything other than their hands to catch them.

For those who plan on taking the fish home to eat, veteran grunion hunter Matt Christopherson warns that one taste of the skinny, crunchy little silver fish is often enough to last a lifetime.

“If you want to eat fish, go to the store and get a salmon steak — it’s so much better,” says Christopherson, who puts on grunion lectures from time to time at the Cabrillo Marine Aquarium on nights when runs take place on the beach outside.

Most of the people who attended a recent one said they planned to throw their grunion back.

First they would have to grab them, however, and to facilitate that task, some had arrived dressed like beach-bum versions of coal miners, wearing flashlight-attached hard hats. Not that the lights seemed to give them much of an advantage.

Grunion, which measure only 6 or 7 inches long, are naturally endowed with the ability to flip and flop just out of the reach of hundreds of outstretched hands.

“There’s one,” would come a typical excited shout from the crowd as midnight approached and the tide reached its high-water mark.

On at least one occasion, the shout was followed quickly by the words, “No, it’s only a piece of seaweed.”

But the warning had come too late. A huge rogue wave had followed the first one in, and it sent hard hats and grunion buckets floating as people turned to flee and crashed clumsily into one another in the dark.

When the wave did recede it left behind the best catch of the night, nearly a dozen flip-flopping fish.

Overall, the turnout of fish during the two-hour run was one of the lightest DiMatteo has seen in seven visits to the beach this season. Other times, she said, fish have arrived by the thousands, briefly turning large patches of sand into shimmering silver.

But those runs, she added, usually come on dark, secluded beaches, and not on pieces of sand where people are running into each other, turning on bright lights and shouting, “There’s one!”