New research shows that there are so many hatchery-raised Chinook salmon spawning in the Mokelumne River that the wild fish are hardly wild at all. According to the new report, about 10 percent of the fall-run Chinook that spawn in the river are naturally born fish. If, as researchers suspect, this has been happening for years, all of the fish in the river have a parent or recent ancestor that came from the hatchery.
The research team, made up of Laboratory scientist Peter Weber and collaborators, identified hatchery fish using a novel technique, developed in part at the Laboratory, which detects traces of a hatchery diet preserved in the ear bones of adult fish.
Weber, LLNL scientist Ian Hutcheon and collaborators developed the method that was used to track diet.
By analyzing the sulfur isotopes in adult Chinook salmon ear bones, called otoliths, which were deposited during their juvenile years, Weber and colleagues were able to determine whether individuals were produced in hatcheries or naturally in rivers. The result showed that only 10 percent of adults spawning in the river had ear-bone sulfur ratios characteristic of naturally produced salmon.
These bones grow in increments over the life of the fish and incorporate elements from the fish's diet. Hatchery feed is largely derived from marine fish meal, which leaves a sulfur isotope ratio distinctly different from that found in wild fish. This signature from a fish's early diet can be detected even several years after it has left the hatchery.
When the researchers looked at the total fish return to the watershed (in rivers and hatchery), they estimated that approximately 96 percent of returning adults were produced in a hatchery.
"For years the state of the wild population has been an open question," Weber said. "We knew that in many years more than 90 percent of the juvenile fish put into this river were from the hatchery, but there was no consensus on how many hatchery fish came back and spawned in the river. Our work shows that the adult fish that return to the river reflect hatchery practices."
It was the first time biologists were able to quantify the percentage of hatchery fish in this river, most of which are unmarked and therefore undetectable in population surveys. The research shows that wild Chinook salmon are not self-sustaining but rather represent a sink population when hatchery fish are taken into account.
"It looked like a healthy population of fish returning to spawn, but the reality is that without the hatchery fish, the wild stocks are not sustaining themselves," said Rachel Johnson, the lead author and a fishery biologist affiliated with the Institute of Marine Sciences at UC Santa Cruz and with the U.S. Bureau of Reclamation.
The Mokelumne, which is one of California's major salmon producing rivers, flows from the Sierra foothills and Camanche Dam and crosses the Central Valley. It ultimately meets the Sacramento-San Joaquin River Delta, a watery crossroads for the largest run of salmon on the West Coast.
Johnson said the Mokelumne study is a snapshot of what it likely happening in other salmon-spawning rivers.
Other researchers include those from the U.S. Fish and Wildlife Service, East Bay Municipal Utility District, National Marine Fisheries Service and University of California, Los Angeles. The U.S. Fish and Wildlife Service funded the research.
The research appears in the online journal, PLoS ONE .