The Chinook salmon, Oncorhynchus tshawytscha, is the largest species in the pacific (Oncorhynchus) salmon family. Other commonly used names for the species include King salmon, Quinnat salmon, Spring salmon and Tyee salmon. Chinook are an anadromous fish native to the north Pacific Ocean and the river systems of western North America ranging from California to Alaska. Scientists have found that only about ten percent of the fall-run Chinook salmon spawning in California’s Mokelumne River are naturally produced wild salmon. A massive influx of hatchery-raised fish that return to spawn in the wild is masking the fact that too few wild fish are returning to sustain a natural population in the river. The study, published in the online journal PLoS ONE, highlights the danger of relying on ordinary census techniques to evaluate the health of wild salmon populations and their habitats. Most hatchery fish in California are unmarked and therefore undetectable in population surveys. For this study, the researchers were able to identify hatchery fish by using a novel technique to detect traces of a hatchery diet preserved in the ear bones of adult fish.