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


SOURCES OF EGGS

Trout eggs for artificial propagation must be obtained from one or two sources. They must either be obtained by trapping wild trout of the desired species when the fish are ready to spawn or must be collected from a domesticated brood stock developed for that purpose. Generally speaking, it is easier to use the latter method as the egg supply obtained from wild trout is often rather difficult to obtain and the quantity may vary considerably from year to year. However, in keeping with the National Park Service aim to preserve all fauna in as nearly natural condition as possible, it is generally considered that when possible the eggs from wild stock are more desirable for stocking park waters. Two species, the rainbow and the eastern brook trout, are being raised in the Happy Isles hatchery at present. The eastern brook trout eggs are spawned from hatchery-raised adult stocks at Creede, Colorado,

Formerly the rainbow trout eggs were collected from an egg taking station, established in 1933, at Lake Eleanor in the northern section of the park. Here the wild rainbow stock was captured for artificial spawning. At present, Lake Eleanor is the only body of water within the park boundaries closed to fishing. This action, of course, preserves the breeding stock. It is planned that within the next few years the Lake Eleanor station will be put back into operation again.

Although during the past few seasons, the rainbow trout egg supply has been secured from the Mt. Whitney Hatchery, the methods used in the taking of the spawn at Lake Eleanor will be described as they are characteristic of methods used elsewhere.

During the spring of the year large number of native rainbows run up stream out of Lake Eleanor into the tributaries to spawn. At the mouth of a large tributary, Frog Creek, the National Park Service has constructed a dam with fishways and traps in order to capture the migrating trout for egg collecting purposes.

After a sufficient number of fish have been captured in the live traps, operations begin. The first task is to place the males and females into separate holding tanks or “cars” (long boxes through which water circulates) by netting them out of the trap. Males may be distinguished by their brighter coloration, conformation or feel of the body and the sharply hooked lower jaws. The females must be further subdivided into those which are ready to spawn and those which must be held a few days.

After pans, buckets and other necessary equipment have been assembled, the spawning activities begin. The operator first dons woolen mittens, which not only protect the fish from injury but permit the operator to hold the fish firmly through the natural slime. For this reason, it is necessary to rinse the gloves often. Also, good quality, wool, finger gloves even though wet do provide a certain measure of warmth for the operator. It must be remembered that the spawning takes place in the early spring while snow is still on the ground and the water and air temperatures therefore quite cold.

A large female rainbow is selected. By the method called ‘’stripping,” the abdomen is gently squeezed and the eggs are forced out into a small enamel pan. By a similar procedure the milt or sperm of the male is forced out over the eggs, fertilizing them in about one to one and a half minutes. Usually a gentle stirring of the mass insures a high percentage of fertile eggs. Rainbow trout average 500 to 1400 eggs per female. After the spawning process, the trout are returned to the lake to provide breeding stock for another season.

The trout eggs are thus collected until actual bucketfuls are obtained. (The average-sized pail will hold around 100,000 trout eggs.) Eggs are very tender at first and must be left standing quietly in water for at least three-quarters of an hour before being transported.

The eggs are porous at first but when placed in water they swell slightly and develop a tough outer shell. This period is known to hatchery men as the “freeze” or “water harden” stage. Once past this danger period, eggs may be safely transported back to the hatchery. They are placed in quart jars set in moss for shipment. About 10,000 eggs are contained in one quart.



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