Salmon hatcheries deplete wild stocks

each year, hatcheries release millions of chinook into the River Columbia in the us in a bid by state game managers to save wild stocks of this salmon. The fish there is so beleaguered that many of its populations, threatened with extinction, are protected under the us Endangered Species Act. A new study now offers evidence that hatchery fish may be hastening the wild stocks' demise.

Phillip S Levin and his colleagues with the National Marine Fisheries Service ( nmfs ) in Seattle analysed chinook salmon population data spanning the past quarter century for the river Snake, which feeds into the river Columbia. Some 18 months after the fall spawning of chinook, a river of smolts heads for the ocean, where the young fish will spend the next four or more years. The Seattle-based scientists compared releases of hatchery-reared smolts with data on the number of returning wild adults.

The team also noted fluctuations in food available for the smolts once they had reached the ocean. Measures of the local oysters' plumpness indicate ocean-food resources. Work by others, Levin explains, has shown that this index reflects a year's food availability all the way up the food chain.

Oyster data revealed that for waters around the mouth of the Columbia, none of the past 25 years has provided a feast. All the years had food supplies in the average or poor range. Poor years coincided with El Ni