Chinook salmon swimming up the Mokelumne River are lured into a trap, killed and gutted of their eggs and sperm, which are blended in jars. State workers rear the hatchlings that emerge from the eggs, and then truck the young fish to a stretch of the San Joaquin River near the Antioch Bridge, where they are poured into the water.
The efforts have succeeded in replenishing depleted salmon populations, but the hatchery-raised fish may actually be weakening the ranks of wild salmon, according to research published Wednesday. Wild salmon have almost entirely disappeared from the river, replaced by their reared-and-released cousins, the scientists discovered.
The hatchery practice is long-running and widespread — the California Department of Fish and Game operates 21 salmon and trout hatcheries, the first of which opened 102 years ago.
Mokelumne River Hatchery, in the Central Valley town of Clements, last year reared and released steelhead trout and more than 5 million salmon, using funds from the sale of commercial fishing licenses and from the East Bay Municipal Utility District to offset the impacts on the species of a dam that it operates.
Researchers analyzed the ear bones of fish to determine whether they were raised in the wild or in a hatchery.
“The ear bone grows with concentric rings, sort of like a pearl in an oyster,” explained Peter Weber, a geochemist at the Lawrence Livermore National Laboratory and co-author of the study. “Those rings can be used to figure out the chronology of the life of the fish.”
The scientists discovered that just 4 percent of the fall-run salmon swimming through the Mokelumne River watershed in 2004 had been reared in the wild. Within the actual river, not including its tributaries, 10 percent were found to be wild. The rest began their lives in a hatchery. The discovery was scheduled to be reported Wednesday in the online journal PLoS ONE.
“It just goes to show that it’s actually not a sustainable population,” said Rachel Johnson, a fishery biologist affiliated with the Institute of Marine Sciences at UC Santa Cruz and with the U.S. Bureau of Reclamation. “The babies that are born in the rivers aren’t surviving enough to come back and replace their parents.”
Johnson, the principal author of the report, said wild-born salmon enjoy greater success as parents in the wild, either because they are better at breeding or because their young are physically and genetically stronger. A salmon population dominated by hatchery-reared specimens could be weakened, she said.
The scientists made the discovery by taking advantage of slight chemical differences in sulfur contained in food eaten by wild and hatchery-raised fish. Fish born in a river eat freshwater food when they are young, while hatcheries generally use feed that comes from the ocean, Weber said. Those differences show up in the ear bones.
The Bay Area and Central Valley mark the southernmost range for Chinook salmon, which hatch in rivers as far north as Alaska before taking to the Pacific Ocean to feed and grow over several years. Under natural conditions, they use scent to help them navigate back to their natal stream to spawn and die, although research indicates that hatchery-raised salmon struggle to locate the rivers into which their parents swarmed.
The local populations of Chinook salmon were decimated in recent decades, leading to the closure of the state’s salmon seasons in 2008 and 2009. Fish numbers have recovered since that time, although they are still far below their natural levels.
Christina Swanson, director of the Natural Resources Defense Council’s science center in San Francisco, said the main reason that the species struggles to survive is the succession of dams that have been built throughout the state’s rivers to trap drinking water and generate hydroelectricity. The dams prevent salmon from reaching their natural spawning grounds.
“For the most part, the declining stocks are a combination of steadily declining freshwater conditions and of course having been blocked from a lot of their habitat,” Swanson said, adding that poor ocean conditions also take a toll.
“Salmon are one of these wonderful species that are entirely dependent upon multiple ecosystems and habitats in order to complete their life cycle,” Swanson said.
The salmon shortage makes life difficult for fishermen, and it also makes it hard for upstream plants and animals to find food.
“Predators and carrion feeders would eat them. The dead bodies would fertilize the shores of the river and be absorbed by the plants and the forests,” Swanson said.