Salmon hatcheries deplete wild stocks

Introducing farmed salmon in the rivers doesn't help

 
Published: Tuesday 31 July 2001


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 Nio events -- periods of climatic perturbations fostered by unusual warmth in large areas of the Pacific Ocean.

Populations of wild adults that had struck out for the ocean when near-shore food supplies were low had high rates of mortality. This mortality was aggravated, Levin's team found, when large numbers of hatchery smolts had entered the ocean with the wild fish. In lean years, the more hatchery chinook released, the higher the mortality of wild stocks from that year's smolts. In contrast, scientists detected no adverse effect of hatchery releases on wild smolts entering the Pacific in years with normal food supplies.

Levin notes, however, that El Nios are occurring at greater frequency in recent decades than previously, and global warming may also heat the Pacific. Consequently, the conditions now contributing to poor food availability in near-shore areas may become the norm in future, he cautions.

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