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Scientists Piece Together Last 2000 Years of Salmon Prodcutivity by Bill Rudolph NW Fishletter, April 30, 2002

A recent article in Nature magazine (April 18, 2002) reports that long-term climate changes lasting for centuries can have huge impacts on salmon populations. Scientists studied core samples from an Alaska lake to measure nutrient levels deposited by returning sockeye salmon, signaled by an isotope of nitrogen picked up in the ocean environment. The scientists found that sockeye population levels, measured by the isotope levels in the sediment layers, had changed drastically over the centuries.

The researchers found that sockeye numbers were high in the oldest sediments that dated back 2,200 years ago and similar to levels from the 1880s when commercial fishing began on the stock, located at Karluk Lake on Kodiak Island.

Around 100 BC, a strong decrease in the isotope was measured, but began to increase about 350 years later, reaching peak abundance around AD 1200 and staying high into the 1900s, when declines in salmon-derived nutrients were attributed to commercial fishing activity and climatic changes.

The changes in salmon abundance seem to have had major effects on human habitation. "These archeological data," say the researchers, "suggest that natural variability in salmon abundance influenced human culture, which differs from what has been observed in the twentieth century, and in other fisheries, where anthropogenic fishing has been an important determinant of coastal marine fish production."

Earlier analysis of sediments from the past 300 years showed that changes in ocean productivity affected fish populations over interdecadal-scale periods, but the long-term analysis, with its peaks in abundance at BC 100 and AD 800-1200, correspond to major changes in ocean-atmosphere circulation in the northeastern Pacific, the scientists say. They also note that decreases in Alaska sockeye productivity correspond to warming waters off southern California, where sardine and anchovy stocks were more abundant during those periods. Patterns from fish bones in southern BC sediments also show that the Pacific Northwest stocks were in synch with the California abundance patterns.

"Our 2,200-year reconstructions of Alaskan sockeye salmon abundances," they say, "demonstrate that an unprecedented shift to a very low productivity regime, lasting centuries, can occur even without the influence of fisheries and other anthropogenic impacts."

The authors say it's critical to develop a better understanding of the links between climate change and ocean ecosystems in order to manage future fisheries with stocks facing additional stress from commercial fishing, habitat degradation and global warming."