The California grunion, Leuresthes tenuis, is a small, silvery fish known for its extraordinary method of reproduction. Found along the Pacific coast of North America, this species has gained widespread fame for an unusual biological event known as the “grunion run.” This spectacle involves thousands of fish leaving the ocean to come onto the beach to spawn. This terrestrial spawning behavior is an adaptation linked to the lunar and tidal cycles, making the grunion a unique subject of study and observation.
Identifying the Grunion and Its Coastal Range
The California grunion is a slender, silvery fish belonging to the silverside family, Atherinopsidae. Adults typically measure between five and six inches long, with a maximum length of about seven and a half inches. They possess a greenish-blue back, bright silvery sides and belly, and a distinct silvery-blue stripe running along the length of their body.
Unlike other members of their family, grunion lack teeth, and they feed primarily on tiny plankton and zooplankton in the nearshore environment. This fish is non-migratory and inhabits shallow coastal waters, generally staying within 60 feet of the surface. The species’ range extends from Tomales Bay in Central California down to BahĂa Magdalena in Baja California, Mexico, though they are most common in southern California coastal counties.
The Unique Spawning Ritual
The spawning ritual, or “grunion run,” occurs during the spring and summer months, from March through August. This terrestrial spawning is directly dictated by the highest tides, known as spring tides, which are associated with the new and full moons. Spawning takes place on four consecutive nights following the peak high tide, ensuring the eggs are deposited high on the beach where they will remain undisturbed by subsequent, lower tides.
As the wave breaks, the female grunion rides the water up the sandy beach and twists her body to excavate a nest tail-first into the wet sand. She buries herself up to her pectoral fins, depositing a clutch of between 1,600 and 3,600 eggs several inches below the surface. One or more males wrap around the partially buried female, releasing milt to fertilize the eggs as it flows down her body and into the nest.
The entire process, from stranding to returning to the sea, can be completed in as little as 30 seconds. After fertilization, the males immediately retreat toward the ocean, while the female twists free from the sand to follow the next receding wave back into the water. This careful timing of spawning on a descending series of high tides prevents the developing eggs from being prematurely washed out by subsequent waves.
Survival Strategy: Egg Development in the Sand
Once the adults return to the ocean, the newly fertilized eggs remain buried in the damp sand, where they are protected from aquatic predators and constant wave action. The receding tide covers the eggs, depositing sand over the nest and insulating them above the reach of normal high tides. This incubation period typically lasts for about ten days to two weeks, a duration perfectly aligned with the next cycle of the highest spring tides.
During this time, the embryos develop within a moist, sand-insulated environment that is warmer and provides more oxygen than the water. The eggs are uniquely adapted to delay hatching and can prolong incubation for up to several weeks if necessary. The ultimate trigger for hatching is not simply the return of the high tide, but the mechanical agitation from the incoming surf and its contact with seawater.
When the next series of spring tides arrives, the churning action of the waves erodes the sand, exposing the eggs. Once agitated by the water, the embryos release an enzyme that weakens the egg membrane, and they hatch almost instantly, often within two minutes. The newly hatched fry are then washed out to sea by the receding wave, beginning their life in the nearshore environment.
Predicting and Observing Grunion Runs
The predictable nature of the grunion run, tied to the lunar cycle, allows state agencies like the California Department of Fish and Wildlife (CDFW) to publish annual schedules of spawning events. Grunion runs occur 30 to 60 minutes after the predicted high tide and can last for one to three hours, with the largest numbers of fish appearing later in the run. The best nights for observation are typically the second and third nights of the four-night spawning period.
Anyone 16 years of age or older who wishes to catch grunion must possess a valid California recreational fishing license. Regulations stipulate that grunion can only be taken by hand; nets, shovels, or other implements are prohibited, as is digging holes to entrap the fish. The current daily bag and possession limit is 30 fish per person during the open season. Observers are encouraged to avoid using bright flashlights, which can frighten the fish and disrupt the spawning process.