A century-old effort to bolster Pacific salmon stocks might be doing more harm than good. A new study suggests that large releases of hatchery steelhead salmon harm wild chinook salmon.
People began breeding and releasing salmon in the Pacific Northwest in the late 19th Century to boost commercial fishing. Hatcheries took off in the 1960s when dam construction on the Snake and Columbia Rivers in Washington State decimated salmon populations. Some scientists have alleged that hatchery salmon--which account for up to 95% of some species of adult salmon in the Columbia River basin--hurt populations of wild salmon. But previous studies have been small and controversial.
To take a more thorough look, researchers studied annual counts of migrating salmon from a dam on the Snake River between 1977 and 1997. These extensive data allowed them to estimate how many of the juvenile salmon that migrated during a given year survived. After adjusting for climate conditions known to affect salmon populations, they found that more hatchery steelhead salmon meant fewer survivors among the chinook. Wild steelhead were unaffected, conservation biologists Phillip Levin and John Williams of the National Marine Fisheries Service in Seattle report in the December issue of Conservation Biology.
The authors speculate that steelhead might outcompete the much smaller chinook juveniles, especially when the two species are carried for several days together in barges to help them migrate around dams. While in the barges, the dominance of steelheads might stress the chinook and deprive them of food, reducing their chance to survive in the ocean. Levin says that this suggests an easy strategy for helping the ailing chinook populations: Carry the two species in separate barges.
The findings provide "the first compelling case" of hatchery fish harming wild fish, according to conservation biologist Peter Kareiva of the Nature Conservancy in Santa Barbara, California. Competition inside the barges is a reasonable explanation for the chinook decline, says ecologist Peter Moyle of the University of California, Davis. "Imagine a hundred grizzly bears descending on a Sunday school picnic and you get some idea of the problem," he says.
Salmon hatchery Q&A from NOAA