Grunions are small, silvery fish found along the coasts of Southern California and Baja California, known for their unique spawning behavior where they come entirely out of the water onto sandy beaches. This event, often called a “grunion run,” allows them to lay and fertilize their eggs in the sand, a departure from the underwater spawning common to most fish. Their predictable appearances during specific moon phases and tides have made them a fascinating natural spectacle for many observers.
Biological Profile
Grunions, specifically the California grunion (Leuresthes tenuis), are slender fish with bluish-green backs and silvery sides and bellies. They measure between 5 and 6 inches in length. These fish possess blunt, rounded snouts and lack teeth as adults.
Their natural habitat spans the nearshore waters of the Pacific Ocean, from Point Conception in California to Point Abreojos in Baja California. They inhabit shallow waters, often staying close to the surface. Lacking teeth, grunions feed on very small organisms like zooplankton and tiny crustaceans. Grunions live for about three to four years.
The Phenomenon of Grunion Runs
A “grunion run” is the spawning behavior where these fish intentionally leave the ocean to reproduce on sandy beaches. This synchronized event occurs at night during spring and summer months, for four consecutive nights following the full and new moons. Spawning commences shortly after the high tide, continuing for several hours.
As waves break on the shore, female grunions ride them as far up the beach as possible. A female then arches her body, excavating the wet sand with her tail to create a nest, burying herself halfway with her head exposed. She deposits her eggs, between 1,600 and 3,600 per spawn, into this nest. Males curve around the female to release milt, fertilizing the eggs as it flows down her body.
After fertilization, males immediately return to the water, while the female twists free and is carried back by the next wave. The entire spawning process for an individual fish can take as little as 30 seconds. The eggs remain buried in the moist sand, safe from aquatic predators, and develop for about 10 days. They hatch during the next series of high tides when agitated by the surf, washing the larvae back into the sea.
Engaging with Grunion Runs
Observing a grunion run is common in Southern California. Grunion runs occur from March through August, with peak activity from late March to early June. The California Department of Fish and Wildlife publishes a schedule of predicted runs.
When observing, visit less crowded sections of beaches and use flashlights sparingly, as bright lights can deter the fish. Grunion runs are best viewed after high tide, with the most significant activity during the second and third nights of a four-night spawning period. If planning to catch grunions during the open season (July 1 through March 31), a valid California sport fishing license is required for individuals 16 years and older. Grunions may only be taken by hand; the use of nets, traps, or digging holes to entrap them is prohibited. During the closed season, from April 1 through June 30, observation is permitted, but no grunions may be taken or disturbed to protect their peak spawning period.