 Radio Script 2000
Why Salmon Boom, Then Bust __________________ STORY: To arrive at their conclusions, researchers from the University of Alaska Fairbanks first had to reconstruct salmon runs going back hundreds of years, long before anyone kept records on such things. Doing that meant they had to find some kind of "fingerprint" or "signature" left behind by these long-ago salmon runs. Turns out, when salmon die and decompose, they leave behind nutrients such as nitrogen and phosphorous. Researchers had only to measure the amount of the stable nitrogen isotope called N15 to determine the size of past salmon runs. Jon Sweetman is a graduate student on the project. SWEETMAN: "When salmon come into a lake to spawn, the adults decompose and their carcasses have a different signal of nitrogen brought in from feeding in the marine environment. And when they die, this nitrogen gets incorporated into the fresh water and eventually settles into the sediment. By looking at the changes in the stable isotopes of nitrogen, you get an idea of how much salmon returned each year." Sweetman, along with UAF associate professor Dr. Bruce Finney, analyzed nitrogen levels found in sediment layers at the bottom of five sockeye salmon lakes on Alaska's Kodiak Island and near Bristol Bay. They dated the layers using known volcanic eruptions that left ash behind. Finally, they correlated changes in nitrogen levels—meaning the rise and fall of salmon populations—with climate patterns over the last 300 years. Bruce Finney says salmon runs and climate seemed to be almost joined at the hip. FINNEY: "We found that the five salmon records had similar patterns to them. That was consistent with the idea that some large-scale change in the climate of the ocean is driving the populations up and down in sync over decadal time scales." Finney says that as the climate became warmer, salmon returns to the lakes became larger. The reverse was also true: Cold snaps triggered smaller salmon runs. FINNEY: "If you look at the records, there's two features they all show that stick out. One was a period in the early 1800s where they have low values of the nitrogen isotope, which we interpret as being low salmon runs in these systems. It's a pretty strong signal. The second period that we see that's striking is the declines starting around 1950, which we again interpret as low salmon abundance." Nitrogen and other nutrients from decayed salmon are important in other ways. They nourish the growth of algae that are eaten by zooplankton, which in turn is a favorite food of young salmon. Finney and Sweetman say healthy numbers of returning salmon are critical, not just for the offspring they'll produce, but for the nutrients their bodies provide. SWEETMAN: "As you get more nutrients, you get more production of algae and then that goes up the food chain and you see all these things responding to the nutrients, suggesting that there is a tight feedback. And when that's disrupted due to less salmon coming back because of fishing, you get a disruption in the feedback and possibly less salmon being produced." Such disruptions were seen in the lakes researchers studied. Sockeye harvests from the Karluk Lake system, for example, began to decline shortly after commercial fishing began in 1882. By the 1970s, salmon catches that had peaked at nearly four million declined to just 100,000. Researchers say commercial catches hastened the decline by depriving the lake of nutrients needed by the next generation of salmon. To keep such declines from happening again, Sweetman says it's important to learn more about the nutrient needs of salmon-producing lakes and rivers. SWEETMAN: "Each of these lakes need to be individually assessed to look at how important nutrients and climate are, and take this information into better management tools." Sweetman hopes to present his graduate thesis by the end of the year, after which he'll help extend the salmon timeline out to around 2,000 years to see how even larger-scale climate patterns affected ancient salmon runs. OUTRO: This is Arctic Science Journeys Radio, a production of the Alaska Sea Grant Program and the University of Alaska Fairbanks. I'm Doug Schneider.
For more information about this study, visit these web pages: Another ASJ Radio story on this research (1998) Scientists reconstruct 300 years of Alaska sockeye runs (NOAA News Online) Salmon battered by man and nature (CNN.com) Bust and boom: New research links climate to changing salmon populations (abcnews.com) Thanks to the following individuals for help preparing this script: Dr. Bruce Finney Jon Sweetman
Arctic Science Journeys is a radio service highlighting science, culture, and the environment of the circumpolar north. Produced by the Alaska Sea Grant College Program and the University of Alaska Fairbanks.
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Listen to story on RealAudio Photos and graphics from the field Related websites Another ASJ Radio story on this research (1998) Scientists reconstruct 300 years of Alaska sockeye runs (NOAA News Online) Salmon battered by man and nature (CNN.com) Bust and boom: New research links climate to changing salmon populations (abcnews.com)
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