While thousands of salmon and trout return to the Mokelumne River each year, regional and state agencies are working to increase the number of natural fish that swim in the waterways.
Jose Setka, spokesman for the East Bay Municipal Utility District, said his agency and the California Department of Fish and Wildlife, along with the U.S. Fish and Wildlife Service, created hatchery coordination teams in 2012 for several California fish hatcheries on its numerous river systems.
A team was created for the Mokelumne River Fish Hatchery, located just downstream from the Camanche Dam. The team's primary goal, Setka said, is increasing the amount of natural fish in the basin.
The Mokelumne River Basin is home to fall-run Chinook salmon and Central Valley steelhead trout.
Fall-run Chinook salmon migrate from the ocean and begin arriving in the Mokelumne River in late summer and early fall to spawn.
Many salmon enter the Mokelumne River Fish Hatchery, where eggs are collected and incubated.
Other salmon may choose to spawn in gravel beds throughout a 10-mile section of the river just downstream of Camanche Dam, according to EBMUD.
Setka said about 25 percent of both Chinook and steelhead are marked or tagged as hatchery-born.
Determining which are natural and hatchery-raised is difficult, Setka said.
"For every one tagged fish, there are three or four that aren't tagged," he said. "If you caught a couple fish out of the river, you wouldn't be able to tell with the naked eye if they were natural or not."
Setka said hatchery-born fish are genetically no different than wild fish.
Juvenile salmon from the hatchery are released between March and June, and all fish are marked with a fin clip and coded wire tag in the nose.
All hatchery steelhead are released in February and March, and are marked with a fin clip.
Strategies used by EBMUD and partner agencies to boost the numbers of natural-born fish along the Mokelumne include improving habitat by adding gravel to the river, increasing the amount and quality of natural spawning areas and minimizing non-native predator fish in the area.
However, Setka said EBMUD's biggest issue is keeping Mokelumne fish from straying into the Delta and out to the American River.
According to EBMUD's website, experimental pulse flows on the Mokelumne River have been used to mimic storm flows and attract fish upstream over the last four years, in the hopes of reducing straying.
Short-term closures of the Delta Cross Channel when salmon migrate in the fall have helped more native salmon find their way back to the Mokelumne River, according to the district.
With the channel gates closed, a potential route to the Sacramento River is temporarily blocked, keeping fish from straying.
Another strategy is "trapping and trucking," in which the fish are caught and relocated in dry years to other areas of the Mokelumne to spawn in natural environments.
Survival often declines in dry years with warmer ocean temperatures, and increases in wet years with cool ocean temperatures, according to EBMUD.
"In a year like this one, where it's going to be very dry, we're going to have to catch the juvenile fish out of the Delta area, and we're going to need to transport them into our own system with better quality water," Setka said.
The Mokelumne hatchery was built to rear 100,000 juvenile salmon and spawning channels, and has a capacity for up to 15 million Chinook salmon eggs, according to the California Hatchery Review.
Upwards of 12,000 adult salmon and steelhead returned the Mokelumne last year, according to EBMUD statistics.
"Our numbers are above what most of the other hatcheries have seen in terms of adult returns," Setka said. "It's definitely a positive, but there's always room for improvement. And we are always going to have challenges to getting those fish through that Delta."