Salmon hatcheries can deplete wild stocks

Each year, hatcheries release millions of chinook into the Columbia River system 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 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 analyzed chinook-population data spanning the past quarter century for the Snake River, which feeds into the 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 4 or more years.

The Seattle 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 reach 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, the NMFS 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 decades, he cautions.

Levin’s group reports its findings in the June 7 Proceedings of the Royal Society B.

The widely varying year-to-year numbers of chinook released by Snake River hatcheries made the new analysis possible, Levin explains. The release totals trace to political decisions, he notes, not to estimates of the environment’s capacity to support salmon.

Today, Columbia River chinook adult stocks are so depleted that the northwest states permit little fishing of them. Yet, thanks to hatcheries, “there are more juvenile fish coming down the Columbia River than there have ever been,” notes Ray Hilborn, a population ecologist at the University of Washington in Seattle. Moreover, he notes hatchery-reared smolts, owing to their coddling, tend to enter the river bigger than their wild brethren–with bigger appetites. What’s happening, he says, is that hatchery fish are replacing wild salmon.

That’s not what was supposed to happen, says Jim Lichatowich, a consulting salmon biologist in Oregon. “The Endangered Species Act says that [wild populations] have to be sustainable in their natural environment,” he explains. The act also seeks to preserve local wild populations, not to replace them with hatchery-reared emigrants.

The new report illustrates the flawed logic in attempting “to overcome declining wild populations by filling the system to overcapacity with hatchery fish,” Lichatowich argues.

Indeed, until this report by Levin’s team, discussions of the environment’s carrying capacity for wild chinook focused on competition for food in rivers, adds Michael Schiewe, director of fish ecology for NMFS in Seattle. “Most people thought that in the ocean there’d be no problem,” he says–its resources seemed limitless.

This is just one more piece of evidence “making it abundantly clear the ocean is not unlimited,” says Brian Riddell of the Canadian Department of Fisheries and Oceans’ Pacific Biological Station in Nanaimo, British Columbia.

The bottom line, Lichatowich says, is that over their 125-year history, salmon hatcheries have shown that “they cannot maintain the supply of salmon in the face of shrinking habitat.” Though overfishing contributed to the initial depletion of chinook, he says, “what’s keeping the salmon populations low right now is habitat”–rivers cut off by dams, drained periodically by irrigators, and contaminated with pollutants.


Janet Raloff

Janet Raloff is the editor of Science News for Students, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer.

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