California Department of Fish and Wildlife

Mokelumne River Fish Hatchery - The Facility

Mokolumne River Hatchery fish ladder Fish Ladder
The hatchery spawning process begins at the entrance of the fish ladder. Salmon and steelhead are attracted to moving water, so they instinctively follow the flowing water of the fish ladder up to the Hatchery Building. A weir, installed across the river during the salmon season and removed during the steelhead season, is used to persuade salmon to swim up the ladder since salmon prefer to spawn in large river beds. On the other hand, a weir is not needed for steelhead since they prefer to spawn in small stream and creek beds, so they naturally travel up the ladder because the ladder mimics a small stream. Once these fish get to the top of the ladder, salmon and steelhead pass through a one way gate which keeps the fish from exiting the gathering tank. Once there are about 200 or more fish in the gathering tank, the next step of the spawning process begins.
weir at hatchery Crowding the Fish
A fish crowder will back up to the far end of the gathering tank, lower a small weir down to the bottom of the tank, and then slowly push the fish towards the Hatchery Building. The fish are pushed through a hatch that opens up to a lift basket.
fish swimming Electro-Anesthesia
When the fish are in the lift basket, they receive an electrical shock that anesthetizes them. This makes the fish easier to handle when being spawned. Once they have been anesthetized, they are lifted up by the lift basket to the sorting table.
Fish on a sorting table Sorting Fish
When sorting fish, hatchery workers separate male fish from female fish and check to see if the females are ready to spawn by pressing on their abdomens. If a female is green (not ready to spawn), her abdomen will be hard, so she will be sent down a tube leading to a holding tank where green fish are held until ready to spawn. If a female is ripe (ready to spawn), her abdomen will be soft meaning her eggs are loose in her egg sac), so she will move on to the next step of the spawning process.
Worker holding fish Spawning Salmon
Salmon only spawn once in their lifetime then soon die afterwards. At the Hatchery, the salmon are killed before being spawned to make work easier and safer for hatchery workers. Spawning starts with one hatchery worker making an incision across the female’s abdomen to collect her eggs in a spawning pan. A second hatchery worker will then squeeze a male’s abdomen to collect his milt (sperm) in the same spawning pan the eggs are in. Both the milt and eggs are mixed together, and the eggs become fertilized.
Fish ready to spawn
Worker holding fish.
Spawning Steelhead
Unlike salmon, steelhead trout do not die after they spawn. In fact, they have the capability of spawning 2-3 times in their lifetime, therefore, steelhead go through a different spawning process called air spawning. Air is injected into the female’s abdomen by a hypodermic needle which causes the eggs to be forced out through the female’s vent into a spawning pan. Then the male’s milt is collected, by squeezing the male’s abdomen, in the same spawning pan filled with eggs. Once they are spawned, steelhead are sent down a tube releasing them back to the river.
Eggs and sperm combined in hatching jars Hatching Jars
After the fertilized eggs absorb water and harden, they are placed in a hatching jar which can hold about 76,000 eggs. The hatching jar acts like a redd (a fish nest) where 54º F water from the Mokelumne River constantly flows around the eggs to provide them oxygen. It takes about 31days for salmon eggs to hatch and about 24 days for steelhead eggs to hatch.
Larval fish
When the eggs hatch, the fish are kept in troughs (pronounced, "troffs") for about 12-13 weeks until they are ready to be transferred to the raceways. Salmon sac-fry take 30 days to absorb their yolks and develop into fingerlings while steelhead only take 22 days. Salmon fingerlings are fed crumb-sized pellets of food for about 4 weeks while steelhead fingerlings are fed the same food for 10 weeks before being transferred from the troughs to the raceways.
Raceway Raceways
Out in the raceways, salmon and steelhead are fed 4-8 times a day, depending on the size of the fish (the smaller the fish the more often they are fed). Salmon are kept in the raceways until they reach 4 inches in length (about 3 months) while steelhead are kept until they reach 8 inches in length about 9 months). Once the fish are eleased, they will swim to the ocean to carry out their life cycle and return to the river in 2-4 years to spawn.
Fish are loaded into oxygenated water tanks carried by large trucks. Loading and Planting
Once the fish are up to size, they are loaded into large, oxygenated, water tanks carried by large trucks and are transported to their planting spot. Salmon are planted at both the Mokelumne River and San Pablo Bay while steelhead are planted only at the Lower Mokelumne River. Once the fish are released, they will swim to the ocean to carry out their life cycle and return to the river in 2-4 years to spawn.