The microfossils in the lake core show that algae are very sensitive to the nutrients released to the water from the adult salmon carcasses. The algae are then eaten by tiny invertebrates called cladocerans, which are the main source of food for newly-hatched salmon. Thus, fewer adults returning the lakes probably means even fewer surviving juveniles, Finney and his colleagues think.
"People have speculated since the 1920's that this effect might occur, but now we can verify it with our paleoenvironmental techniques. We can ask, when nutrients increased in the lakes, did that actually do anything?" Finney said.
The nature of the climate changes underlying the salmon trends are unclear right now. Finney said many scientists believe the process occurs in the ocean, rather than in individual lakes, and may involve changes in salmon food supply. This effect might be focused in coastal waters, which young salmon inhabit during the most precarious stage of their development.
"I could see a situation in which, if you knew that in a few years things would be bad for the fish in the oceans, you'd adjust other aspects of your management approaches on land," Finney said.
It also appears that climate has a stronger influence upon salmon numbers in lakes with dense populations (holding as much as 30,000 fish per square kilometer). Lakes with smaller concentrations of fish (ranging down to 5,000 in the same area) haven't varied as much in their nitrogen levels, the researchers found.
The other authors of the Science study are Irene Gregory-Eaves and John P. Smol, of Queen's University, in Kingston, Ontario; Jon Sweetman, of the University of Alaska, in Fairbanks, Alaska; and Marianne S. V. Douglas, of the University of Toronto, in Toronto, Onta
Contact: Ginger Pinholster
American Association for the Advancement of Science