At one time, the American River provided approximately 100 miles of stream in which salmon and steelhead could spawn and rear. When the Folsom-Nimbus project was completed in 1958, most of the spawning and rearing areas were cut off. Nimbus Salmon and Steelhead Hatchery was constructed to replace the salmon and steelhead runs that were blocked by Nimbus and Folsom dams.
Annual counts of salmon spawning in the American River by the California Department of Fish and Game (CDFG) provided justification for construction of this hatchery.
In cooperation with the CDFG and U.S. Fish and Wildlife Service, the U.S. Bureau of Reclamation designed and built the hatchery, and currently pays the CDFG to operate and maintain it.
The presence of this hatchery is an example of the multiple-use concept of water resource development. The primary purposes of the Folsom Dam project, of which Nimbus is an integral part, are flood control, municipal water supply, power generation and irrigation. In developing the American River for these important purposes, planners included Nimbus Hatchery to mitigate for project impacts on salmon and steelhead populations.
Fish are sorted and spawned on a table in the spawning building at the south end of the holding pond. In the case of salmon, fish ready for spawning are killed. Eggs are more easily and quickly taken when the body cavity is opened by an incision. (All Pacific salmon die after spawning, so killing the fish is not wasteful.)
Sperm or "milt" is squeezed from the male salmon and mixed with the freshly taken eggs to fertilize them. A typical salmon has about 5,000 eggs. Female salmon that are not sexually mature and males not needed for fertilizing eggs are returned to the pond to be sorted the next spawning day.
In these twelve ponds, salmon and steelhead are raised for release into the Sacramento River system. Chinook (king) salmon usually are planted at about six months of age when they are four to six inches long. Each year, nearly four million salmon of this size are trucked and planted in the Sacramento River-San Joaquin Estuary.
Steelhead remain in the ponds about a year until they are eight to twelve inches in length. All steelhead yearlings are normally planted in the Sacramento River in January and February near Rio Vista.
The fish rack or weir stops the upstream migration of fish because of the limited spawning area between this spot and Nimbus Dam. The rack also guides fish to the ladder entrance.
It is installed in the American River about the middle of September and remains until the run is over. Because the eggs will die before they hatch if the water temperature is too high, the ladder is not opened until the water temperature has cooled to below 60 degrees, usually by the first week of November.
Salmon and steelhead jump over twenty steps to the top of the fish ladder. From here, they swim into the holding pond through a v-shaped weir that allows passage into the pond but not out. In some years, more than 10,000 salmon will climb the ladder and enter the holding pond.
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