Pacific salmon populations have plummeted recently, due to overfishing, dams, and disease. Now it turns out that a more natural force can also drive salmon numbers: climate. Using a new technique, paleoceanographers have traced the ups and downs of salmon populations in Alaskan lakes for hundreds of years.
Near Kodiak Island and Bristol Bay on Alaska's southern coast, among other sites along the Pacific Coast, sockeye salmon hatch and spend their youth in lakes. Then, after 1 to 3 years, they migrate to the Pacific. The sockeyes feast on plankton, squid, and small fish, eventually doubling their initial body weight. At the ripe old age of 5 or 6, the salmon return home, spawn, and die.
The sockeyes' seafood diet makes them easy to track, according to paleoceanographer Bruce Finney of the University of Alaska, Fairbanks, and colleagues. As they eat, the fish take on a high ratio of a nitrogen isotope called nitrogen-15. Later, when the salmon die in their freshwater homes, the isotope is consumed by microorganisms and eventually settles to the bottom of the lake. Finney's team took core samples from several Alaskan salmon nursery lakes and measured levels of nitrogen-15 going back 3 centuries. They correlated the isotope levels with tree-ring climate data and found that warm waters were associated with thriving salmon populations, they report in the 27 October issue of Science.
"This paper is terribly important, because it's the first solid proof that salmon abundance fluctuates naturally," says ecologist Richard Beamish of Canada's Department of Fisheries and Oceans. Although many researchers have assumed that climate variability affects Pacific salmon, precise data have been scarce. Now, Beamish says, fisheries managers might take climate into account when they set harvest limits or evaluate population swings.