The “grunion run” is a biological spectacle specific to the Pacific coast, where thousands of small, silvery fish emerge from the ocean to spawn on the sandy beach. This mass, coordinated onshore movement is a unique reproductive strategy, allowing the fish to deposit their eggs in a location protected from aquatic predators. It is one of the few instances globally where a fish species intentionally leaves the water entirely for reproduction. This phenomenon is tightly synchronized with the moon and tides, turning specific stretches of shoreline into a temporary nursery.
Defining the Grunion
The fish responsible for this unusual event is the California Grunion (Leuresthes tenuis). These small, slender fish belong to the silverside family (Atherinopsidae) and often reach five to seven inches at maturity. They have an iridescent appearance, characterized by a bluish-green back and a prominent silvery band along their sides. The California Grunion’s natural habitat is restricted to the shallow coastal waters of the eastern Pacific, ranging from Point Conception in Southern California down to Punta Abreojos in Baja California, Mexico. They feed primarily on tiny crustaceans and other zooplankton near the water’s surface.
The Mechanics of the Grunion Run
A grunion run is the collective, synchronized movement of these fish onto the beach to spawn. This event is precisely timed by environmental cues, occurring only during the highest tides (spring tides), which follow a new or full moon. The spawning season lasts from March through August, with the heaviest activity typically falling between late March and early June.
Timing and Tides
The fish emerge from the surf only on the three or four nights immediately following the peak high tide associated with the lunar cycle. The timing is important because the fish must spawn on a descending series of tides, ensuring succeeding high tides will be lower than the previous ones. This keeps the newly laid eggs undisturbed and above the water line for incubation. Runs generally occur at night, lasting for one to three hours, with fish riding the crest of a wave as far up the sand as possible.
The Grunion Spawning Process
Once on the beach, the female grunion initiates the reproductive process by using her tail to excavate a nest in the wet sand. She twists and wriggles, burying herself tail-first until she is approximately half-submerged, with only her head exposed. In this vertical position, the female deposits a cluster of eggs two to three inches below the surface.
Multiple males, sometimes up to eight, then curve around the partially buried female to release their milt. The milt flows down the female’s body and into the sand, fertilizing the eggs in the nest. The entire process takes as little as 30 seconds before the fish free themselves and catch the next wave back to the ocean.
The eggs incubate in the moist, warm sand, protected from aquatic predators above the normal high-tide mark. This incubation period lasts approximately 10 to 14 days, aligning with the next series of high tides. When the next spring tide arrives, the surf washes over the buried eggs. The mechanical agitation of the water triggers the embryos to hatch rapidly, usually within minutes. The newly hatched larval grunion are then washed out to sea by the receding waves.
Viewing and Catching Rules
The best time to observe a run is typically one to three hours after the predicted peak high tide, usually beginning around 10 p.m. or later. Grunion are sensitive to light and vibration, so observers should use minimal light, such as a red filter, and keep noise and movement to a minimum to avoid scaring the fish away.
Regulations for Collection
Regulations govern the collection of grunion, which is a popular recreational activity in California. A valid California recreational fishing license is mandatory for anyone 16 years of age or older who intends to catch them.
Current regulations prohibit taking grunion during their peak spawning period, with a closed season running from April 1 through June 30. During the open season, grunion may be taken by hand only; no nets, hooks, or digging implements are permitted. A daily bag and possession limit of 30 grunion per person is enforced.