This is a story thousands of years in the making. This story is as unique to Southern California as In-and-Out Burger. It’s heard by few and seen by even less, although it is something that takes place right on our doorstep. That is how, late at night on May 23, a group of people were introduced to a diminutive, mysterious fish. “It’s the story of shimmering waves, moonlit sandy beaches and reproductive mayhem. It’s just your typical grunion love story.” The story, as told by Birch Aquarium staff member Lindsay Bradshaw, took place on the sand. The event, attended by 56 people, was one in the aquarium’s series of summer grunion runs. In a unique seasonal phenomenon, hundreds of slender, silvery fish boogie up to the beach for a remarkable mating ritual that only occurs on Southern California’s shorelines. What may appear to be a beachscape of washed up fish squirming from suffocation is, in fact, a natural onshore marvel that breathes life into the tiny sea creatures. Upon closer inspection, the so-called grunion run is an occasion where the tiny female grunion will bury the tail end of her slimy flesh into the sand to lay her eggs, while the male grunion entwines his body around hers to fertilize the eggs a few inches beneath the surface. When the spawning is complete, both catch a receding wave and ride back into the moonlit ocean unscathed. Like the phases of the moon or the cycles of the tides, the timing of grunions’ journeys beyond their sea habitats is so predictable that the date could be marked on a calendar a year in advance — and is, in fact, by the California Department of Fish and Game each year. California grunions (Leuresthes tenuis) — a member of the silverside family — make their runs when the tides are at their highest point, making a human’s venture out to the beach to see the tiny creatures at work a late-night endeavor under a near full or new moon. “Grunions and the tides is something that people have known to be connected for a very long time,” said Bradshaw. “When we have the sun and the moon and the Earth all lined up, you’re going to get a higher high tide than normal. Grunions, as it turns out, only come out after the highest of high tides, so that’s after the full moon and the new moon.” First, the males — or scouts — ride the waves onshore, checking the beach for predators, debris, wind and a gradual slope. A few minutes after returning to sea, the scouts return to the shore accompanied by egg-laden females. And the spawning begins. “It does seem like it would be risky business coming out to the land. There are a lot of predators on land, and they know when the grunions are coming,” said Bradshaw. “But there are a lot more predators in the water than there are on land. Almost anything will eat a fish egg.” By laying their eggs so far up on the beach, grunions eliminate the threat of their eggs getting washed out to sea before they’re ready to hatch. The warmer temperatures and higher oxygen levels onshore are also conducive factors for successful hatchings. Incubation in the sand lasts for two weeks until the next high tide, when the water washes the sand away. The agitation from the rising surf causes the eggs to hatch, and the eyelash-size grunions get swept to sea by the coming tide. If the conditions aren’t quite right, the eggs can wait to hatch until the next moon cycle in another two weeks, or — in extreme cases — up to six weeks total, although the survival rate diminishes. “Grunion runs look like a bunch of chaos,” said Bradshaw. “The actual spawning only takes a couple of minutes to happen, but grunion can be out of the water for up to an hour. As long as their gills stay wet, they’re fine.” After the grunion population took a deep dive due to over fishing in the 1920s, scientists conducted research to determine humans’ effects on the species. “Regular, day-to-day activity on the beach doesn’t seem to affect their hatchery. What does affect the eggs, of course, are things like sandcastle-building and digging holes, but if we think about where kids like to do that, it’s closer down to the shore. It usually doesn’t happen up by the high tide lines,” said Bradshaw. Beach management regulations have been improved over the years thanks to research by scientists like Scripps Institution of Oceanography-based Boyd Walker, who was the first to prove the synchronicity of grunion spawnings’ correlation to the tides. Today, certain grunion fishing regulations are in place to ensure the maintenance of the ecosystem and protection of its grunion population. During peak spawning season, in April and May, grunion fishing is banned, and during the rest of the year, certain regulations must be adhered to by anyone 16 years of age and older. “You can’t use special tools — just a bucket to put them in and your hands. No rods, no scoops, no digging holes to trap them,” said Bradshaw. “And remember, only take what you can use.” The grunion population is back on track since its steep dive nearly a century ago. “The fishery is maintaining itself through the regulations that are in place. We don’t see an increase or decrease in our population,” she said. “A lot of the research that has been happening and general awareness of the grunion has led to advancements in thinking of beaches as ecosystems and not just our playgrounds.” To learn more about the mysterious and elusive grunion, Birch Aquarium offers guided grunion runs several times throughout the season, giving participants the chance to experience the moonlit beach exploration under the expert guidance of aquarium naturalists, along with a rare opportunity to watch grunion eggs hatch. The next and final grunion run will take place on June 6 at 10:45 p.m. For more information or to register, visit aquarium.ucsd.edu or call (858) 534-7336. Tickets are $12 for members and $10 for nonmembers. Fun facts about the little-known grunion: • Local native tribes like the Kumeyaay have used grunion as a food source for thousands of years. They knew when the grunion would come to the beach, making it easy pickings to gather and celebrate the plentiful tide. • Grunion have inspired film, art, literary works and music. What some people know about grunion can be traced back to shows like the “Beverly Hillbillies” and “Three’s Company” or the Frank Zappa tune titled “Grunion Run.” • The California grunion is only found from about Santa Barbara to Baja California, which may attribute to the fact that some people — including Californians — believe grunion to be folklore. • It wasn’t until the mid-1500s that the grunion was given its name by Spanish explorer Juan Cabrillo. The name is loosely translated from the Spanish for ‘grunter.’