Dead men may tell no tales--but dead salmon do. Evidence of ancient salmon runs buried in lake mud has allowed researchers to reconstruct the number of sockeye salmon in Alaska over the last 2200 years. The results suggest that long-term population swings were bigger in the past, and that a salmon boom starting in A.D. 1200 may have fueled a cultural shift in the local human population.
Born in lakes and streams, salmon spend most of their lives at sea before returning home to spawn and die. The decaying bodies leave a lasting record in the sediment: Salmon carcasses have a high ratio of the heavy nitrogen isotope 15N. In addition, certain species of algal diatoms thrive when many salmon return. As the glasslike skeletons of diatoms settle into the lakebed's sediment, they also create a year-by-year record of the size of the salmon return.
To access these records, paleolimnologist Irene Gregory-Eaves of Queen's University in Kingston, Ontario, geological oceanographer Bruce Finney of the University of Alaska, Fairbanks, and colleagues collected meter-long sediment cores from three lakes on Kodiak Island in Alaska. They analyzed the mud for the dominant diatom species and the nitrogen isotope signature. The team found that salmon populations dropped steadily from 100 B.C. to A.D. 800 and exploded between A.D. 1200 and 1900. They think these variations stem from fluctuations in the regional climate; studies have shown that changes in ocean temperature reshuffle the plankton ecology, which alters the availability of food for the fish. Also around 1200, human populations in Kodiak started growing. People began to use tools to catch fish rather than hunt marine mammals, suggesting that they learned to reap the salmon bounty, the team says.
Gregory-Eaves notes that past salmon runs varied from fewer than 200,000 to about 3.5 million. That's a far bigger spread than the two- to threefold changes seen between decades in the last century, says fisheries scientist Ray Hilborn of University of Washington, Seattle. He adds that it's important to consider the effect of climate when researchers and policy-makers set limits on modern fisheries; for instance, a climate-prompted rebound in salmon population could mask low population levels that are still unhealthy, he says.