The Decline of Wild Salmon
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.
The Mokelumne River is one of the major salmon producing rivers for fall-run Chinook salmon in California. Throughout the Central Valley rivers, returning fall-run Chinook salmon numbers have rebounded since a disastrous year in 2007, which led to the unprecedented closure of the commercial salmon fishing season for consecutive years in 2008 and 2009.
In the Mokelumne, the number of returning adult salmon has grown from just 418 in 2008 to more than 18,000 in 2011.
The researchers based their findings on an analysis of ear bones, called otoliths, from fish collected after spawning in fall 2004. Coauthor Peter Weber of Lawrence Livermore National Laboratory led the development of the technique for analyzing chemical signatures in the otoliths. 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 chemical signature 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.
Nearly 12,000 fish returned and spawned in the Mokelumne watershed in 2004. Most were hatchery fish that returned to the hatchery, but about 1,500 fish spawned in the river. The otolith analysis showed that only ten percent of those spawning in the river were produced there, and only 4 percent of the total spawning population were of natural origin.
Maintaining a viable populations of salmon in the wild is a primary goal for many conservation and recovery programs. Yet, the role that immigration of hatchery-produced adults may play in altering population dynamics and fitness of natural populations remains largely unquantified.
The abundance of Chinook salmon spawning in the river was substantially disconnected from the specific survivor and fecundity rates of naturally produced fish owing to immigrants from a hatchery source. Natural productivity is not as great as thought. The potential discrepancy between in-river spawning abundance and natural production may be particularly important in years when natural population abundances are critically low.
For further information: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0028880