Grunion are 2 species of the genus Leuresthes: the California grunion, L. tenuis, and the Gulf grunion L. sardinas. They are sardine-sized teleost fish in the Atherinopsidae family of New World silversides. They are found only off the coast of California, USA and Baja California, Mexico (found on both the Pacific Ocean and Gulf of California coasts).
Grunion are known for their very unusual mating ritual. At very high tides the females come up on sandy beaches and dig their tails into the sand to lay their eggs. A male then wraps himself around the female to deposit his sperm. For the next ten days the grunion eggs remain hidden in the sand, but at the next set of high tides the eggs hatch and the young grunion are washed out to sea.
There is also a related species in the Gulf of California, the false grunion (Colpichthys regis) that looks very similar and acts similar, but does not have the same breeding method. However, you may see a false grunion on the sand.
Grunion were originally classified in Atherinidae, the family of silversides, but are now classified in the family Atherinopsidae, along with the other New World silversides, including the jacksmelt and topsmelt. The California grunion, Leuresthes tenuis, is found along the Pacific Coast from Point Conception, California, to Point Abreojos, Baja California. They are rarely found from San Francisco on the north to San Juanico Bay, Baja California, on the south. The Gulf grunion, L. sardina, is found along the coast of Baja California in the Gulf of California.
They are small, slender fish with bluish green backs, silvery sides and bellies. Their snouts are bluntly rounded and are VERY slippery. Silversides differ from true smelts, family Osmeridae, in that they lack the trout-like adipose fin. They inhabit the nearshore waters from the surf to a depth of 60 feet (20 m). A description of their essential habitat would be the surf zone off sandy beaches. Marking experiments indicate that they are non-migratory.
Young grunion grow very rapidly and are about five inches long by the time they are one year old and ready to spawn. Grunion adults normally range in size from 5 to 6 inches (13 - 15 cm) with a maximum size recorded at 8.5 inches (La Jolla Ca.,05-11-05(19 cm). Average body lengths for males and females respectively are 4.5 and 5.0 inches (11.5 and 12.7 cm) at the end of one year, 5.5 and 5.8 inches (14.0 and 14.7 cm) at the end of two years, and 5.9 to 6.3 inches (15.0 to 16.0 cm) at the end of three years. The normal life span is two or three years, but individuals four years old have been found. The 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 aging purposes.
Gulf grunion are unique in that they may spawn in the daytime.
California grunion spawn at night on the beach, from two to six nights after the full and new moon, beginning a little after high tide and continuing for several hours. As a wave breaks on the beach, the grunion swim as far up the slope as possible. The female arches her body, keeping her head up, and excavates the semi-fluid sand with her tail. As her tail sinks, the female twists her body and digs tail first until she is buried up to her pectoral fins. After the female is in the nest, up to eight males attempt to mate with her by curving around the female and releasing their milt as she deposits her eggs about four inches below the surface. After spawning, the males immediately retreat toward the ocean. The milt flows down the female’s body until it reaches the eggs and fertilizes them. The female twists free and returns to the sea with the next wave. The whole event can happen in 30 seconds, but some fish remain on the beach for several minutes. (The Gulf grunion spawns during the daytime, and has smaller eggs.)
Spawning may continue from March through August, with possibly an occasional extension into February and September. However, peak spawning is from late March through early June. Once mature, an individual may spawn during successive spawning periods at about 15-day intervals. Most females spawn about six times during the season. Counts of maturing ova to be laid at one spawning ranged from about 1,600 to about 3,600, with the larger females producing more eggs. A female might lay as many as 18,000 eggs over an entire season. The milt from the male might contain as many as one million sperm. Males may participate in several spawnings per run.
The eggs incubate a few inches deep in the sand above the level of subsequent waves. 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 to reach them, in 10 or more days. 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 the seawater and agitation of the rising surf. The mechanical action of the waves is the environmental trigger for hatching, and the rapidity of hatch, in less than one minute, indicates that it is probably not an enzymatic function of softening the chorion, as in some other fishes.
The hatching of grunion eggs can be watched by collecting a cluster of eggs after a grunion run and keeping them in a loosely covered container of damp sand in a cool spot for 10 - 15 days. Then, add one teaspoon of sand and eggs to one cup of sea water and shake gently; the eggs will hatch before your eyes in a few minutes.
Grunion food habits are not well-known. They have no teeth, and feed on very small organisms, such as plankton. In a laboratory setting, grunion eat live brine shrimp. Humans, larger fish, and other animals prey upon grunion. An isopod, two species of flies, sandworms, and a beetle have been found preying on the eggs. Some shorebirds such as egrets and herons prey on grunion when the fish are on shore during spawning. Seagulls, sea lions and sand sharks have also been observed feeding on grunion during a grunion run. The reduction of spawning habitat, due to beach erosion, harbor construction, and pollution is probably the most critical problem facing the grunion resource.
Despite local concentrations, the grunion is not an abundant species. While the population size is not known, all research points to a rather restricted resource that is adequately maintained at current harvest rates under existing regulations.
In the 1920s, the recreational fishery was showing definite signs of depletion, and a regulation was passed in 1927 establishing a closed season of three months, April through June. The fishery improved, and in 1947, the closure was shortened to April through May. This closure is still in effect to protect grunion during the peak spawning period.
A fishing license is required for persons 16 years and older to capture grunion. Grunion may be taken by sport fishermen using their hands only. No appliances of any kind may be used to catch grunion, and no holes may be dug in the beach to entrap them. Grunion may be taken on specified dates between March and the end of August. There is no limit, but take only what you can use as it is unlawful to waste fish. With these regulations, the resource seems to be maintaining itself at a fairly constant level. While the population size is not known, all research points to a rather restricted resource that is appropriately harvested under existing law.
The coastal Indians in California harvested grunion during spawning runs. Archeologists have found fossil grunion otoliths (tiny, bonelike particles or stony platelike structures in the internal ear of lower vertebrates) at various Indian campsites.
Grunion were mentioned by Spanish explorer Juan Cabrillo in his ship's log (circa 1542).
Scientists first identified grunion in San Francisco Bay in 1860 (Ayres, Proc. Calif. Acad. Sci. (Ser. 1) v. 2).
Note: much of this article is from the two public domain references above.