UC San Diego Library – Scripps Digital Collection

The highly specialized spawning habits of the atherine fish Leuresthes tenuis render its life history unique among those of the fishes of this family, in fact of all fishes so far recorded. In southern California, where the species is best known, the periodic spawning runs of the "grunion" are watched for with much interest by those who frequent the long, sandy beaches. The details of this peculiar method of spawning have been accurately worked out by Thompson (1919b). The results of his work may be summarized briefly as follows: These fishes deposit their eggs during high tides in the sand of the beach. In accomplishing this, the fish is carried by the wash of the waves up onto the moist sand, where the female digs in, tail foremost, and there deposits her eggs, which the male simultaneously fertilizes while lying arched around her. According to popular belief, these runs of grunion occur on the high tides of the second, third and fourth nights after the full moon. Thompson made his first observations in April, 1919. The moon was full on the fifteenth; the first fish were taken on the sixteenth, and the last on the eighteenth. Again, in May, the moon was full on the fourteenth, and the first fish observed on the sixteenth. In both cases, the run lasted three days, the sixteenth, seventeenth and eighteenth of the month. The conclusion drawn was that "the spawning run comes shortly after the full of the moon, in other words, after the highest tide of the series, which is really the significant fact." The advantage accrued by the grunion by spawning on these high tides immediately following the full of the moon seems clear. It was demonstrated by Thompson that the females deposited their eggs at the upper edge of the area of sand eroded by that series of tides, and that successively lower tides on the following days actually buried the egg pods more deeply in the sand than the female had been able to deposit them. Here they lie relatively unmolested until two weeks later when, during the next series of high tides, the waves, by renewed erosion, actually dig the eggs out of the sand. Egg pods were collected immediately after a spawning run and taken into the laboratory where the development was watched. It was found that the eggs were ready to hatch in ten days after spawning, but that the fish were not actually liberated until the eggs were agitated and thus freed from the sand. Throughout the summer months of 1919, March to July, the tides associated with the dark phase of the moon were one to two feet higher than those accompanying the full of the moon. With this phenomenon in mind the author concluded that the spawning of L. tenuis on the tides immediately following the full of the moon served to practically assure the liberation of the larvae by the tides accompanying the dark of the moon two weeks later. The possibility of the spawning runs occurring also on the dark of the moon tides was discussed. Eggs spawned at this time might remain in the sand for a period of four weeks, unless the runs occurred on tides late in the series which would be approximately no higher than those of the next full moon series. On June first and second, three days after the dark of the moon, fishes were in fact found spawning on tides of 5.7 feet and 5.0 feet, respectively. The predicted height of the next full moon tide was, on June thirteenth, 6.0 feet, thus high enough to liberate the larvae. Since the fish observed running on these two dates were very few, "the conclusion that the main run does occur during the full of the moon seems therefore entirely probable." The data presented in this paper, however, are not in accordance with this conclusion; this problem will be discussed further under the section dealing with frequency of spawning. The spawning season was considered as beginning in March, and later Thompson (1919a) recorded a small run on July 15 and 16 and August 14. The spawning may thus be said to extend from March to August. This knowledge of how and when L. tenuis spawns opens up several questions concerning the life history of this fish. First, does each fish spawn but once in a season, or does it spawn on each series of favorable tides? Second, what is the age at first maturity, and do the fish spawn more than one season? Third, what is the rate of growth for the species, and does the peculiar spawning habit have any unusual influence on its growth? An attempt has been made to answer the first of these questions by a detailed study of the history of the ova. Their growth has been traced carefully from its onset in January until after the close of the spawning season in August. The second and third problems have been attacked by means of scale studies and of length-frequency data.