All too little is known at present about the relative importance of natural reproduction in maintaining fish populations in the lakes and streams. It is known, however, that in many of the less accessible areas of the park, natural spawning alone has been responsible for maintaining the trout population after initial stocking. Thus the planting of trout is only warranted in waters which show that the natural reproduction is not capable of replenishing the supply of fish removed by anglers or lost through other causes.

The trout of our mountain areas such as the Sierra Nevada are resident; that is they do not make long migrations to spawning grounds as do salmon and trout of many coastal streams. An interesting thing to note is the fact that the salmon spawn once and die, while trout may spawn for several successive seasons.

The time of spawning among all trout varies with the temperature, season, locality and strain of fish. In Yosemite, the rainbow and golden trout spawn with rising water temperatures from early spring to midsummer depending upon the elevation while brook and brown trout sprawn from late summer to late fall when water temperatures are falling.

The spawning trout tend to move upstream and seek the smaller tributaries, or in the case of those inhabiting lakes, inlets may be utilized. (Outlets and occasionally gravelly lake shores may be used if there is no suitable inlet.)

Eggs of trout a;e laid in nest-like depressions known as “redds,” built by the female in gravel bottomed areas, where currents are fairly swift. In choosing a spawning place, a permanent water supply of even temperature is the most important factor. Digging of the nest is accomplished by vigorous head and tail movements of the female as she lies on her side. The male usually spends his time during this period pugnaciously driving off other males from the nesting area.

When the nest is prepared, both the male and female occupy a position directly above it. At the same moment that the eggs are deposited, sperm or milt is discharged over them by the male. The then fertilized eggs fall into the pit of the nest after which the female covers them by further digging movements that stir up the surrounding gravel. As indicated elsewhere, a high degree of efficiency results from this simple method of fertilization as almost all the eggs become fertile. One female may dig nests and deposit eggs several times within one season’s spawning period. All parental care of the offspring ceases with the covering of the nest.

Before they hatch, the eggs remain in the gravel for several weeks or months depending upon the species and environmental factors. The newly hatched trout are known as “fry” and have a portion of the egg sac still attached. It is from the egg sac that the fry receives its first food. Shortly the sac is absorbed and the fingerling wiggles forth from the gravel and begins to feed upon minute aquatic organisms.

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