In an area like Yosemite Valley the river is so heavily fished that few trout ever become old enough to spawn or lay eggs before being captured. Therefore, to maintain a constant trout population, great numbers must be planted each year. The Valley Floor is stocked with some 250,000 to 350,000 trout annually, depending upon the supply. The purposes of hatcheries are to help sustain fishing in heavily fished waters, to produce superior breeds of trout which have greater vitality and disease resistance, to supplement natural reproduction, and to introduce new species into an area. At present in Yosemite the hatchery operates to supplement natural reproduction and to aid in sustaining fishing in heavily fished waters. According to the National Park Service fish policy, the Yosemite hatchery can no longer stock virgin waters without proper investigation and approval nor can it introduce further new exotic species. Few barren waters remain in the park and it perhaps is desirable to retain a limited number of them at least in their natural state without introduction of fish.

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