A disturbing new study indicates that the survival rates of wild Chinook salmon may be far less than previously thought. The study, which was conducted on California's Mokelumne River, used an innovative test to determine the proportion of wild to hatchery raised fish in an unmarked population. The study's findings reveal that as few as 4 percent of the fish returning to the Mokelumne to spawn are of wild origin.
Researchers collected over 1000 chinook carcasses and tested their otoliths (ear bones) for sulfur isotopes. Testing for the accumulation of certain chemical elements in bone can tell scientists a great deal about the early stages of a fish's life. In this case, the presence of higher amounts of sulfur isotopes allowed researchers to identify differences in the early dietary influences in individual fish, and thus separate hatchery raised fish from those which fed in the wild.