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Fishes of Yosemite National Park (1941, 1948) by Willis A. Evans and Orthello L. Wallis


HATCHERY OPERATIONS

At the hatchery, the eggs are placed into wire-mesh baskets which are lowered into long narrow troughs filled with pure, cold running water. Each basket contains around 20,000 to 25,000 eggs. The hatchery rearing of trout consists mainly in raising them under rigid protection from enemies, disease and other adverse conditions so that a greater number can be assured of reaching maturity than under natural conditions.

Other than the necessary transportation of “green eggs” from the spawning station to the hatchery troughs, all movement is minimized for some time. They become so tender and sensitive when about three days old that even a slight jarring will kill many. Any attempt to ship at this stage would be fatal to all. The eyes of the young developing trout first appear visible through the transparent shell membrane when the eggs are approximately two weeks old in water at a temperature of 54 degrees Fahrenheit. This is known as the “”eyed stage,” at which time the eggs are no longer sensitive and may be transported great distances. For long trips, the eggs are placed in trays in regular refrigerated packing boxes.

Upon hatching, the “”fry,” as they are then called, are able to wriggle through the wire mesh of the egg basket into the trough below. A trout fry is a curious looking creature, with a large yolk sac consisting of stored food attached to the abdomen. During the first stage of its life this yolk sac, which gradually becomes absorbed, furnishes all its food.

With the sac entirely absorbed, the young trout swims up toward the surface of the water as if in search of food. This is the cue for the hatcherymen to begin feeding. At first finely ground beef liver comprises the main diet of these tiny fish in the Happy Isles Hatchery. After a few weeks, canned sardines are added to the liver in increasing amounts until the liver is two-thirds of the mixture. Some hatcheries use rough ocean fish or horsemeat for trout food.

The growth is fairly rapid and in two or three months after hatching, the young trout are perhaps 1 1/2 to 3 inches long and are called “fingerlings.” This is the size at which trout are planted in the waters of Yosemite. Twenty-four hours before planting, feeding of the trout is discontinued; this aids in conditioning them for travel. Clean planting cans and pure water (free from chlorine) are other vital essentials for successful stocking of trout.

To count each trout individually is, of course, impossible. Therefore, the fingerlings are weighed and numbers calculated from the known number present per ounce by actual count. After loading the trout in large fish cans, each containing one to three thousand trout, they are ready to be transported anywhere in the park for planting.



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