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Department of Fish and Game Marine Resources Leaflet No. 3 by Jerome Spratt
Originally printed 1971. Online version updated April, 2015
Table of Contents
Along Southern California's sandy beaches, from March through September, one of the most remarkable life cycles in the sea is completed: the California grunion come ashore to spawn. The grunion has been known to many Southern Californians for more than 100 years, but there are still those who are skeptical of its existence. To be invited out in the middle of the night to go and watch fish does sound a little ridiculous, but in reality this is the only way to observe this natural phenomenon.
California grunion are small, silvery fish found only along the coast of Southern California and northern Baja California. Most people would be unaware of their existence were it not for the unique spawning behavior of these fish. Unlike other fish, grunion come completely out of the water to lay their eggs in the wet sand of the beach. As if this behavior were not strange enough, grunion make these excursions only on particular nights, and with such regularity that the time of their arrival on the beach can be predicted a year in advance. This phenomenon can be seen on many beaches in Southern California. Shortly after high tide, on certain nights, sections of these beaches are sometimes covered with thousands of grunion depositing their eggs in the sand.
Grunion watching has become a popular sport in Southern California. Since these fish leave the water to deposit their eggs, they may be observed on shore while they are briefly stranded. The common sight of thousands of people lining the more popular beaches in Southern California in anticipation of a grunion run attests to its ever-growing popularity. Often there are more people than fish, so to protect the species, it is important to let them spawn without disturbance. All that is needed to observe grunion is an adventurous spirit and the patience to find a promising beach and then wait for a run to begin.
Grunion are small slender fish with bluish green backs, silvery sides and bellies. Their average length is between 5 and 6 inches. Early Spanish settlers called this fish grunion, which means grunter. This term has been 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 this fish belongs 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 may appear in fair numbers as far north as Morro Bay, California, and spawning has been reported as far north as Monterey Bay, California. Recently, grunion have been observed spawning inside San Francisco Bay. A close relative of the California grunion is the gulf grunion, Leuresthes sardina, which is found in the Gulf of California. This species spawns 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 spawn only on 3 or 4 nights after the highest tide associated with each full or new moon and then only for a 1 to 3 hour period each night following high tide.
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 that night, and they will not appear again until the next night or 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.
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 square 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 (Figure 2). 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 (See Figure). 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 the 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.
Chart: Relationship between the sun, moon, earth, tides and grunion behavior
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.
A valid California recreational fishing license is required to take grunion for anyone age 16 or older. No take is allowed when the season is closed during April and May; however, this is an excellent time for observing runs. Grunion may be taken by hand only—no appliances of any kind may be used, and no holes may be dug in the beach to entrap them. During the open season, there is no limit to the number that may be taken, but grunion (or any fish) should not be wasted per California Code of Regulations Title 14, Section 1.87. Recreational fishing license holders and persons under the age of 16 may handle grunion gently during the open season, and release them back into the water unharmed. Grunion may not be pursued or handled at all during the closed season (April and May).
When to Go
The spawning season extends from March through August, although runs in August are very small and erratic. The California Department of Fish and Wildlife issues schedules of expected grunion runs in advance of each season. These schedules of expected runs are published in newspapers and copies are given to many sporting goods stores throughout Southern California. If these are not available, all a grunion observer needs to make their own predictions is a tide table. Grunion runs may occur anytime from the night of highest tide throughout the descending series of high tides. Runs are most likely to occur on the second, third, fourth, and fifth nights following the night of the new or full moon. Generally, the third and fourth nights are best. The time of the run will be 30 to 60 minutes past high tide and it will last from 1 to 3 hours. The heaviest part of the run usually occurs at least 1 hour after the run starts.
Grunion runs will occur on most Southern California beaches, but may not occur every night on the same beaches and may be limited to small areas of any one beach. The ends of beaches are often the best spots. Some of the beaches in Southern California that are known to have runs are: the beach between Morro Bay and Cayucos, Pismo Beach, Santa Barbara, Malibu, Santa Monica, Venice, Hermosa Beach, Cabrillo Beach, Long Beach, Belmont Shore, Seal Beach, Huntington Beach, Newport Beach, Corona del Mar, Doheny Beach, Del Mar, La Jolla, Mission Beach and the Coronado Strand. The beaches near Ensenada in Baja California also have good runs.
Hints for Successful Grunion Observing
It is best to go to the end of an uncrowded beach. This is not always possible, but the fewer people the better. Flashlights should be used sparingly. Light may scare the fish away and they will not come out of the water. After a wave has receded, flashlights may be used to help locate fish. Finally, plan to stay late. Many people quit an hour after high tide and miss a good run.
Clark, Frances N. 1925. The life history of Leuresthes tenuis, an atherine fish with tide controlled spawning habits. California Department of Fish and Game, Fish Bull., (10) :1-51
Ehrlich, K. F., and D. A. Farris. 1971. Some influences of temperature on the development of the grunion Leuresthes tenuis (Ayres) . Calif. Fish and Game, 57 (1) :58-68.
Fitch, John E., and Robert J. Lavenberg. 1971. Marine Food and Game Fishes of California, Univ. Calif. Press. Berkeley, Los Angeles, London. 179 p.
Idyll, Clarence P. 1969. Grunion, the fish that spawns on land. National Geographic Magazine. 135 (5) :714-723.
Spratt, Jerome D. 1981. California grunion, Leuresthes tenuis, spawn in Monterey Bay, California. Calif. Fish and Game, 67 (2) :134.
Thompson, Will F. 1919. The spawning of the grunion (Leuresthes tenuis). Calif. Fish and Game Comm., Fish Bull., (3):1-27.
Walker, Boyd W. 1952. A guide to the grunion. Calif. Fish and Game, 38 (3) :409-420.
Woodling, Bruce A. 1968. The mysterious, dancing grunion. Sea Frontiers. 14 (3) :131-137.