Grunion: bridging land and sea

Grunion are a hotdog-sized fish that spawn late at night on beaches in California and Baja California, Mexico. Though they come on land to reproduce, researchers still do not know where these unique fish go once they leave their spawning beaches. Learn more about grunion, the importance of the moon in their lives, and challenges they have reproducing along an altered coastline below. 

What is a grunion? 

Grunion (Leuresthes tenuis) are slender, silver fish measuring an average of 5 to 6 inches long. Lacking teeth, they feed on tiny plankton suspended in the ocean. Their predators include pelicans and other birds, marine mammals , larger fish, sharks—anything that eats fish. Grunions live for three to four years. 

Typically found off the West coast from Point Conception, California, to Point Abreojos, Baja California Mexico, they have been seen as far north as Tomales Bay, California and as far south as San Juanico Bay, Baja California. About 90% of the population is thought to live off the Southern California coast in San Diego, Los Angeles, and Orange Counties. 

When not on the beach spawning or in shallow waters getting ready to come up, grunions “can’t be found,” says Dr. Karen Martin, grunion expert and Professor of Biology at Pepperdine University. “No one has figured out where they go when they're not on the beach. That's a mystery.”

Moon spawning & lunar cycles

The moon plays a very important role in the life of a grunion. Grunions emerge out of the ocean and onto the beaches to spawn when the tides are at their highest, which occurs twice in the lunar cycle: during the full moon and the new moon. At these times, grunions come up completely out of the water to lay and fertilize their eggs in the sand above the high tide line. This ensures that the eggs will only be submerged during high tides, which is important because it allows them to get enough oxygen. 

Being buried in sand also protects them from aquatic predators that would eat eggs floating in the water. Plus, it’s typically warmer in the sand which allows the eggs to develop faster. The eggs remain below the sand until the next high tide washes them to the surface and into seawater, triggering the embryos inside to hatch.

Grunions spawn according to their local high tides, so in Southern California and Mexico, that will be an hour earlier than it would be in Monterey, for example. 

What’s a grunion run?

Grunion spawning events are also called grunion runs and they occur from February through September, with the heaviest runs usually in April, May, and June. 

On the night of the full and new moons, female grunions follow waves in towards the beach and swim as far up onto the sand as they can. Wriggling and twisting their bodies, the females use their tails to vertically dig themselves into the wet sand, creating a nest. With their heads sticking up out of the sand, the females lay their eggs. 

Male grunions then curl around the half-buried females and secrete their milt, or semen, that runs down the females’ bodies to the eggs. Immediately after fertilizing the eggs, the male grunions untwist themselves and wriggle back to the waves. Female grunions eventually squirm free and return to the water as well. 

A single nest can be fertilized by up to eight male grunions. It can take anywhere from 30 seconds to several minutes for a grunion to come up onto the beach, spawn, and return to the waves. 

Sometimes grunions disperse across the sand and sometimes they clump together in sections. Where they will come up on a beach is also unpredictable.

How to view a grunion run

Grunions are sensitive to light and vibrations on the beach. If there are bright lights that are moving, or they can feel vibrations from people walking on the sand, they may not come up onto the beach to spawn. The best way to view grunions is to: 

• Do your research, check websites like the California Department of Fish and Wildife's Grunion Facts & Expected Runs page or grunion.org to check the best locations and times to view. 
• Check local beach regulations before heading out. 
• Be prepared. It can be cold and damp on the beach at night even in the summer months, so bring appropriate clothing and footwear.  
• Look for darker, calm portions of beach where the grunion may prefer to spawn. Also, be aware of your surroundings, your safety is your biggest priority, do not go to dark areas if they do not seem safe. 
• Keep your voice low while waiting for grunion. 
• Use a red light to navigate the beach at night. The red light will not disturb the fish. 
• If you need to move locations, walk higher up on the beach away from the wet sand. 
• Leave your pets at home.
• Once the grunion start their run, you can approach them more closely and respectfully. Do not touch the fish or interfere with their spawning. 
• Be careful not to step on any grunions!

Cultural significance of grunions

Today, gathering at late-night grunion runs is a popular local activity. Grunions were important to the diets of California Indians, like the Kumeyaay, who lived in coastal regions. “Running Grunion” is the stage name of San Diego-based Abel Silvas, who is a Kumeyaay/Diegeño and Acjachemen/Juaneño historical comedian, mime and storyteller. 

Can I eat grunions?

Yes, but you must have a fishing license to catch them. During the open season, anyone 16+ with a fishing license may catch grunions, but using their hands only—no digging holes in the sand or using nets or traps. While there’s no bag limit, fishermen can take only what they will use. According to California code, it is unlawful to waste fish. To find current grunion regulations, visit the state’s website here.

Conservation challenges 

Grunion numbers have been declining over the past decade due to several key issues. Many grunions are being illegally taken by those without permits. Using nets or other gear and fishing out of season or without a license are not uncommon, says Dr. Martin. 

April, May, and June are the peak spawning months, but since 1949, only April and May are closed to fishing. Protecting the entirety of the peak seasons “could probably ensure the survival of the species,” Dr. Martin says, “and then people could use them for recreational fishing in the other months.”

Habitat loss is another major issue. The natural coastline has been significantly altered by people through much of the grunion’s range. Beaches are disappearing from erosion, real estate development, and harbor construction. Combing or grooming beaches disturbs grunion eggs in the sand and many are washed out to sea before they are mature enough to hatch. Water pollution from urban runoff and the frequency of harmful algal blooms during summer spawning months is also taxing on these small fish. 

Increasing temperatures in the ocean and onland are even more concerning. Air temperature on land is increasing faster than marine temperatures in the ocean. Because grunions breed on land, Dr. Martin is concerned that grunions will have more and more problems in the southern part of their range. Warmer temperatures are already being seen, even during earlier months like April.

Marine Protected Areas (or MPAs) and protected beaches are one way to help grunion. Protected beaches with less light pollution and fewer disturbances offer refuge for grunion trying to spawn along a modified coast.  

Written by Emily Harwitz
Thank you to Dr. Martin from Peppedine University for sharing her expertise.