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Salmon Battered by Man and Nature

by John Roach
Environmental News Network, October 27, 2000
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Humans are a big burden on sockeye salmon populations.Scientists now say natural climate variability also wreaks havoc on the fish. Devoured at the dinner table, blocked by hydroelectric dams and crowded out by urban development, sockeye salmon face a big burden in humans. And now it turns out that natural climate variability also wreaks havoc on the fish, according to research in today’s Science.

“I definitely wouldn’t want people to use climate to take humans off the hook,” said Bruce Finney, a marine scientist at the University of Alaska in Fairbanks. “But climate change is a regulator in the population cycle going up and down.”

Sockeye gain 99 percent of their body mass while at sea on a diet of plankton, squid and small fish. When the fish return to freshwater lakes to spawn and die, their carcasses release the isotope nitrogen-15.

Since few other fish or atmospheric factors add nitrogen-15 to freshwater lake ecosystems, sediment cores from five Alaskan lakes allowed the researchers to reconstruct the species’ population history over the past 300 years.

The history shows several major fluctuations in nitrogen-15 levels. These fluctuations suggest decreases in the sockeye population during the early 18th and 19th centuries, periods that tree-ring data indicates were cooler in Alaska.

Bruce Finney, a marine scientist at the University of Alaska in Fairbanks, used a core sampler to take sediment from lake floors. “It is not thought that the temperature change is the direct cause,” said Finney. “It is thought that as the climate changes the way the ocean works, it changes availability of food for young salmon.”

The fluctuations appear to be tied to a phenomenon known as the Pacific decadal oscillation, which warms waters along the western coast of North America during its positive stage.

The warmer waters that boost sockeye populations in Alaska appear to be devastating to populations in the Pacific Northwest, said Finney.

As profound as the impact of climate variation can be on sockeye populations, the researchers’ data also confirmed fallout from overfishing.

Sockeye carcasses act as fertilizer for lake ecosystems, providing food to juvenile sockeye during the first few years of their lives. In some instances, overfishing has robbed lakes of the nutrients needed by subsequent generations of salmon.

For example, sockeye harvests from the Karluk Lake system began to decline shortly after commercial fishing began in 1882. By the 1970s, salmon harvests that had peaked at nearly 4 million declined to just 100,000.

By letting more salmon reach spawning grounds, and therefore putting more nutrients into the system, lakes and rivers could produce larger salmon runs, researchers say. “Our data suggests that the prolonged 20th century collapse of the Karluk sockeye fishery was driven in part by reductions in the carcass-derived nutrients from overharvest,” write Finney and colleagues.

The researchers hope that fisheries managers will begin to include climate change and the nutrient cycle into their calculations as they determine the maximum number of salmon that fishermen can harvest each year.

“I don’t think there is a recipe, you need to look at each system individually,” said Finney. “I think we need to be as conservative as possible, to do everything possible to reduce stresses on fish.”

Related Link: RealAudio

John Roach
Salmon Battered by Man and Nature
Environmental News Network, October 27, 2000

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