California Grunion

The California Grunion (Leuresthes tenuis) is a small marine fish native to the nearshore waters of the eastern Pacific, ranging from central California to Baja California, Mexico. It belongs to the New World silverside family Atherinopsidae, within the order Atheriniformes, related to other silversides and true rainbowfishes. The species is best known for its highly unusual spawning behavior, in which adults strand themselves briefly on sandy beaches to lay eggs above the waterline during specific nighttime high tides, a phenomenon that has made it one of the most distinctive and closely studied coastal fish species on the Pacific coast of North America. Populations have experienced some historical declines linked to beach habitat disturbance, prompting regional monitoring and seasonal protective measures.

California Grunion are slender, elongated fish reaching 12-19 cm.

• Body: silvery-green with a bright, continuous silver stripe running the length of each flank
• Fins: two widely separated dorsal fins, typical of the silverside/rainbowfish order; lightly forked tail
• Head: small and pointed, with a relatively large eye
• Shape: streamlined and torpedo-like, built for surf-zone movement It closely resembles other silverside species in general shape but is distinguished by its larger adult size, specific southern California to Baja range, and its unique above-water spawning behavior, which is not shared by related species.

California Grunion inhabit shallow nearshore Pacific waters and sandy surf-zone beaches along the coast from central California to Baja California, Mexico. Outside of spawning events, they remain in schools in the open water just beyond the surf line, favoring sandy-bottomed coastal areas over rocky or vegetated substrate. Water temperatures in their range are temperate to subtropical, and the species is closely tied to specific beach conditions suitable for its distinctive spawning runs, including gently sloping sandy shores. Its dependence on undisturbed sandy beach habitat for reproduction makes coastal development and beach grooming activities a relevant habitat consideration for the species across its range.

California Grunion are schooling fish that spend most of their time in open nearshore water, feeding on zooplankton and small invertebrates suspended in the water column. Their most notable behavior is a highly synchronized spawning run: for several nights following spring high tides, large numbers of adults ride waves onto sandy beaches, where females bury their tails to deposit eggs just below the sand surface while males curl around them to fertilize the eggs, before all individuals return to the water with the next wave. Eggs remain buried in the sand until subsequent high tides erode the beach and allow larvae to hatch and wash into the surf. This tightly timed reproductive strategy is a defining feature of the species' natural history and continues across a multi-month spawning season each year.