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Grunion are two fish species of the genus Leuresthes: the California grunion, L. tenuis, and the Gulf grunion, L. sardina. They are sardine-sized Actinopterygii fishes of the New World silverside family Atherinopsidae, found only off the coast of California, USA, and Baja California, Mexico, where the California Grunion are found on the Pacific Ocean coast, and the Gulf Grunion within the Gulf of California. Many people enjoy watching "grunion runs."
Grunion are known for their unusual mating ritual. At semilunar tide, they ride waves up onto sandy beaches where females dig their tails into the sand to lay their eggs. The males then wrap around the female to provide his sperm. For the entire period of incubation, grunion eggs remain hidden in the sand. At the next set of high tides, about 10 or 12 days later, the eggs hatch rapidly when washed out to sea, releasing the larvae into the water.
A related New World Silverside, the Colpichthys ( Colpichthys regis) lives in the Gulf of California. This fish looks similar, and also spawns in the intertidal zone.
Young grunion grow rapidly and are about five inches long by the time they reach one year old and are ready to spawn. Adult fish normally range in size from .
Average body lengths for males and females are , respectively, at the end of one year; at the end of two years; and at the end of three years.
The normal lifespan of the grunion is three to four years. Their growth rate slows after the first spawning and stops completely during the spawning season. Consequently, adult fish grow only during the fall and winter. This growth rate variation causes annuli to form on the scales, which have been used for determining ages.
After spawning, males may return on additional waves to mate again. Spawning may occur from March through August, with occasional extensions into February and September. However, peak spawning is in April, May, and June. Once mature, an individual may spawn during successive spawning periods at about 15-day intervals. Females may spawn up to six times during the season, from about 1,600 to about 3,600 eggs per cluch, with the larger females producing more eggs. The milt from the male might contain as many as one million sperm. Males may spawn repeatedly during a run.
The eggs incubate a few inches deep in the sand above the level of subsequent tides. They are not immersed in seawater, but are kept moist by the residual water in the sand. While incubating, they are subject to predation by shore birds and sand-dwelling invertebrates. Under normal conditions, they do not have an opportunity to hatch until the next tide series high enough reaches them 10 or more days later. Grunion eggs can extend incubation and delay hatching if tides do not reach them for an additional four weeks after this initial hatching time. Most of the eggs will hatch in 10 days if provided with seawater and the agitation of the rising surf. The mechanical action of mixing in the waves is the environmental trigger for hatching. Hatching occurs in less than one minute, with a hatching enzyme that softens the chorion and releases the hatchling from the egg.
The Gulf Grunion, with its smaller eggs, is unique in that it spawns during both night and daytime, depending on when the highest tides occur.
Although some other fish species leave their eggs in locations in intertidal zones (a few, such as plainfin midshipman, may even remain on land with the eggs during low tide) or on plants above the water (), jumping onto land en masse to spawn at the water's edge is unique to the grunion, capelin and grass puffer.
The Gulf Grunion is listed as Near Threatened on the IUCN Red List because of concerns about habitat loss and the lack of any area for future migration.
The periodic appearance of the grunion on Southern California beaches is known as a "grunion run". No fishing license is required for watching the runs. However, to catch, a fishing license is required for persons 16 years and older, and grunion may be taken using bare hands only. No gear of any kind may ever be used to catch grunion, and no holes may be dug in the beach to entrap them. Grunion may be taken on specified dates in March, July and August, but not during the months of April, May or June. There is a limit of 30 grunion per person, but fishermen should take only what they can use, as under Californian law it is unlawful to waste fish. The sport of catching can be combined with "catch and release" as these fish will survive if handled gently and quickly returned to the ocean. With these regulations, the resource may be conserved for future generations to enjoy.
Grunion were mentioned by Spanish explorer Juan RodrÃguez Cabrillo in his ship's log dated around 1542.
Scientists first identified grunion from a market specimen bought in San Francisco in 1860.Ayres, Proc. Calif. Acad. Sci. (Ser. 1) v. 2)
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