Grunion - Natural History

Natural History

Grunion were originally classified as part of the Atherinidae family of silversides but are now classified in the family Atherinopsidae, along with 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 Punta Abreojos, Baja California Sur. They are rarely found between San Francisco in the north and San Juanico Bay, Baja California Sur, in 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 slippery. Silversides differ from true smelts of the family Osmeridae in that they lack the trout-like adipose fin. Inhabiting the nearshore waters from the surf to a depth of 60 feet (18 m), marking experiments indicate that they are non-migratory.

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 6 to 7 inches (15 – 17 cm) with a maximum recorded size of 8.5 inches (220 mm) (La Jolla Ca.,05-11-05(19 cm).

Average body lengths for males and females are 4.5 and 5.0 inches (11.5 and 12.7 cm) respectively 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 of the grunion is three to four years although individuals up to five years old have been found. 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 aging purposes. The Gulf grunion, with its smaller eggs, is unique in that it may spawn during daytime.

California grunion spawn on beaches from two to six nights after the full and new moon beginning soon 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 whilst 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. Spawning may continue from March through August, with occasional extensions 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 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 the waves is the environmental trigger for hatching. The rapidity of hatching, which occurs in less than one minute, indicates that it is probably not an enzymatic function of a softening of the chorion as in some other fishes.

Grunion feeding 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 believed to be the most critical problem facing the grunion species.