San Felipe Grunion Run

San Felipe, Baja, Mexico

San Felipe Grunion Run

It's better than the a command performance of the Imperial Russian Ballet. It's like watching a Master Silversmith spill his creations on the beach. A thousand little watch mecahnisms ticking and twirling on the damp sand. It's the rarely-seen magical grunion run of San Felipe, Baja, Mexico.

Grunion are small slender fish with bluish green backs, silvery sides and bellies. Their average length is between 5 and 6 inches.

Native Americans called this fish by various names. The Kumeyaay (Diegueno) word for grunion was Hashuupill, which meant fish out of water. The Paipai called it Ha-il or water worm. Early Spanish settlers called this fish grunion, which means grunter. The word was later anglicized into grunion. Grunion are known to make a faint squeaking noise while spawning.

The scientific name for the California grunion is Leuresthes tenuis and they belong to the family Atherinidae, commonly known as silversides. Other more abundant atherinids found in California are the topsmelt, Atherinops affinis, and jacksmelt, Atherinopsis californiensis. Silversides differ from true smelts, family Osmeridae, in that they lack the trout-like adipose fin.

The principal range of the grunion is between Point Conception in southern California and Punta Abreojos in Baja California, Mexico. However, there are small populations both north and south of these points. Occasionally grunion appear in fair numbers as far north as Morro Bay, California, and spawning has been reported as far north as MontereyBay, California.

The grunion that are seen on the beaches here in San Felipe are the gulf grunion (Leuresthes sardina), a relative of the California grunion. They differ from their Californian relatives in that they spawn both day and night, while the California grunion spawns only at night.


The spawning season extends from late February or early March to August or early September, varying slightly in length from year to year. Actual spawning runs are restricted to relatively few hours during this period. Grunion spawns are associated with the highest tides of each full or new moon and then only for a 1 to 3 hour period following high tide.

Grunnion on the beach.
Grunion on the beach.

Spawning runs typically begin with single fish (usually males) swimming in with a wave and occasionally stranding themselves on the beach. Gradually, more and more fish come in with the waves and by swimming against the outflowing wave strand themselves until the beach is covered by a blanket of grunion. Spawning normally starts about 20 minutes after the first fish appear on the beach. Typically a run lasts 1 to 3 hours, but the number of fish on the beach at any given moment can vary from none, to thousands. Peak activity is reached about an hour after the start of the run and lasts from 30 to 60 minutes. Finally, when the tide has dropped a foot or more, the run slackens and then stops as suddenly as it started. No more fish will be seen and they will not appear again until the next the next series of runs.

Observing grunion can be much more interesting than catching them. Females ride a far reaching wave onto the beach accompanied by as many as eight males. If no males are present,a female will return to the ocean with the outflowing wave. In the presence of males, she swims as far up on the beach as possible and literally drills herself into the sand as the wave recedes. This is accomplished by arching her body with the head up, and at the same time vigorously wriggling her tail back and forth. As her tail sinks into the semifluid sand, she twists her body and drills herself downward until she is buried up to the pectoral fins. Occasionally she may bury herself completely. The male (or males) curves around her as he lies on top of the sand, with his vent close to or touching her body. The female continues to twist, emitting her eggs 2 or 3 inches beneath the surface of the sand. Males discharge their milt onto the sand near the female and then immediately start to wriggle towards the water. The milt flows down the body of the female and fertilizes the eggs. The female then frees herself from the sand with a violent jerking motion and returns to the sea with the next wave to reach her. This entire process takes about 30 seconds, but individual fish may remain on the beach for several minutes. Larger females are capable of producing up to 3,000 eggs every 2 weeks. As the mature eggs are deposited in the sand, another group of eggs are developing that will be spawned during the next series of runs. This cycle continues throughout the season. During the early part of the season only older fish spawn, but as the season progresses fish hatched the previous year come into spawning condition and join the runs. Fish of all ages will spawn by April and May.

Fate of the Eggs

The eggs are initially deposited 2 to 3 inches below the surface of the sand by the female. The outgoing tide deposits sand onto the beach covering the eggs to a depth of 8 to 16 inches. Here the eggs remain in the moist sand. They will be ready to hatch in about 10 days, but remain viable until they are freed from the sand by the next series of high tides to reach them. The baby grunion hatch 2 or 3 minutes after the eggs are freed from the sand and are washed out to sea.

Age and Growth

Young grunion grow very rapidly and are about 5 inches long by the time they are 1 year old and ready to spawn. The normal life span is 2 or 3 years, but individuals 4 years old have been found. The maximum size attained is between 6 and 7 inches. The growth rate slows after the first spawning and stops completely during the spawning season, consequently the fish grow only during the fall and winter. This cessation of growth during spawning causes a mark to form on each scale, and the age of the fish can be determined by counting these marks, much like the age of a tree can be determined by counting its “growth” rings. The life history of grunion while at sea is not well known, but these fish apparently spend most of their life close to shore in water 15 to 40 feet deep. Return to Top of Page


Tides are caused by forces exerted on the earth by celestial bodies in direct proportion to their mass. Theoretically all celestial bodies affect the tides but realistically only the sun and moon need be considered. Since the sun has 26 million times the mass of the moon, one might expect it to be the dominant tide producing force. However, the force exerted by a celestial body decreases rapidly as its distance from earth increases (inversely proportional to the cube of the distance). Consequently the sun, being almost 400 times farther from earth than the moon, exerts less than half as much force as the moon. Tidal highs and lows vary according to the relative positions of the sun, earth, and moon. Highest and lowest tides occur when the sun, earth, and moon are most in line, such as during full moon (sun and moon on opposite sides of the earth) and new moon (sun and moon on the same side of the earth). These tides are known as “spring” tides. The tides occurring during the first and last quarters of the moon, when the sun and moon are least in line, are known as “neap” tides and are intermediate in range.

Grunion Behavior in Relation to Tides

Grunion have adapted to tidal cycles in a precise manner. Along the Pacific coast of North America the two daily high tides vary in height, and the higher of the two occurs at night during spring and summer months. Grunion spawn only on these higher tides, and after the tide has started to recede. Since waves tend to erode sand from the beach as the tide rises and deposit sand as the tide falls, it is obvious that if grunion spawn on a rising tide the succeeding waves would wash the eggs out. This danger is eliminated since spawning usually is confined to the falling tide. In addition, grunion nearly always spawn on a descending series of tides when succeeding tides are lower than tides of the previous night. The eggs would be washed out prematurely by succeeding tides if spawned during the ascending tidal series. The eggs mature and are ready to hatch in about 10 days or about the time of the next series of high tides. Thus spawning must take place soon after the highest tide in a series if the eggs are to have adequate time to develop before the next series of high tides. Looking at a tidal cycle, it becomes apparent that there are only 3 to 4 nights following the highest tide that spawning conditions are right, and it is on these nights that grunion spawn.

Internal Clock

How does the grunion know when the time is right to spawn? Evidently some biological mechanism or“internal clock” that can detect some change in the environment sounds an alarm at exactly the right moment. The exact stimulus is not known, but it is suspected that they may be able to detect minute changes in water pressure caused by the rising tides. Without this ability to spawn at precisely the right moment the grunion would not survive.

The Grunion Run of 2010, A Video

For a chart of predicted Southern California Grunion Runs in 2008, click HERE.
WESTWAYS article about the grunion.

The Grunion Greeter Project.

Boyd W. Walker, a modern Ed Ricketts, studied the habits of the grunion in southern California during the '40s. He wrote his theasis on the periodicity of their spawning (you can read it HERE in pdf format). Later, he wrote about the ecology of the Salton Sea, California, in relation to the sportfishery. While on the faculty of UCLA, Walker began his study of the fishes of the Gulf of California (Sea of Cortez). He and his students built a large and important collection of eastern Pacific fishes, much of which has since been transferred to the Los Angeles County Museum of Natural History. This work resulted in the discovery of many new species of fishes and produced large amounts of information about fish distributions, abundances, ecology, and behavior.

For more about Boyd W. Walker, visit: