Radio Script
1998

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Ancient Salmon Runs
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STORY: For thousands of years, sockeye salmon have returned to spawn in places like Karluk Lake, on Alaska's Kodiak Island. After spawning the salmon die, but with their death comes new life. As the salmon decompose at the bottoms of lakes and streams, their bodies provide essential nutrients sustaining the next generation of salmon.

One of those nutrients--Nitrogen 15--nourishes the growth of plankton, a key food group of young salmon. With each passing year, the uneaten, leftover plankton settles to the lake bottom, taking with it a nitrogen record of the year's salmon return. Now scientists at the University of Alaska Fairbanks have figured out how to use this nitrogen record to reconstruct centuries-old salmon runs. Bruce Finney is a geological oceanographer at the university.

FINNEY: "The more salmon that come in a given year, the more of this nitrogen isotope that comes into the lake. And then that's taken up by the plankton and deposited in the sediments. So that by going down a sediment core and studying how this nitrogen isotope changes with time, it is basically telling you how many salmon have come back into that system with time."

In Karluk Lake, Bruce Finney analyzed nitrogen in layers of sediment that date back some 500 years, to a time when more than a million sockeye salmon returned to the lake. He can easily see years when levels of nitrogen were high--indicating a large salmon run--and periods of low nitrogen--meaning the salmon run was small. But while nitrogen levels are useful for seeing trends in salmon abundance, Bruce Finney went one step further. He translated the nitrogen levels into numbers of actual salmon.

FINNEY: "The technique I'm using is to try to calibrate the upper part of the sediments with the historical record of what's known. For example with Karluk Lake there's a weir and a record going back to the 1920s of how many fish pass that weir every year. And so I try to calibrate the upper part of my sediment core with that to try to equate what I'm measuring in my core to how many fish. And then I can take that and extend it back in time."

His work has attracted the attention of state fishery managers. Dana Schmidt is the state limnologist with the Alaska Department of Fish and Game.

SCHMIDT: "What this tool provides us--or at least has the potential to provide us--is the ability to look back hundreds of years in time prior to the onset of commercial fisheries. It appears at least from the Karluk work that in some of these systems we are going to get a pretty good picture of what's happened in the past."

So far, Bruce Finney has analyzed levels of nitrogen 15 in about a dozen Alaska lakes. In the future, he plans to reconstruct salmon runs back to the last ice age, to learn more about how salmon colonized the North Pacific Ocean.

OUTRO: For Arctic Science Journeys, this is Doug Schneider, reporting from Fairbanks, Alaska.