ANCHORAGE — A new study presents evidence that salmon populations in Bristol Bay have surged and sagged wildly -- for hundreds of years at a time -- well before the first commercial fishing in Alaska.
The research, published this month in the online edition of the Proceedings of the National Academy of Sciences, summarized data collected and analyzed over the past 15 years by scientists in the U.S., Canada, Norway and China.
Researchers sampled sediment from 23 lakes in Southwestern Alaska, most draining into Bristol Bay, searching for a nitrogen isotope associated with sockeye salmon spawning in freshwater. Salmon absorb the isotope in the ocean, where they do most of their growing. When they die, it's released from their bodies, picked up by plankton in the lake and deposited in lake bottoms as mud, providing a year-by-year indicator of the number of salmon that died in the lake.
Daniel Schindler, a professor of aquatic and fishery sciences at the University of Washington and co-author of the paper, offered the analogy of mixing paint. "Think of the (saltwater) isotope as red paint and the freshwater isotope as white," he said. "Together they make pink. The darker the pink means more salmon that year; a paler pink means fewer."
As expected, the sediments showed a marked decrease in the saltwater isotope beginning 100 years ago, when widespread commercial fishing was established in Bristol Bay. That indicates fewer salmon reached the spawning areas.
"But we were surprised to find that previous returns of salmon to rivers varied just as dramatically," wrote Laura Rogers, lead writer, in a statement accompanying the article.
Though spawning numbers sharply decline after about 1900, Schindler said, it would be a misinterpretation of the findings to say the introduction of commercial fishing in Bristol Bay reduced the abundance of salmon there.
"The total amount of salmon is actually increasing," he said. "But a smaller proportion is making it back to the watersheds. Sixty percent is harvested by the fishery, but they harvest the surplus." Stocks can remain plentiful because a single mating pair of salmon can produce hundreds of offspring.
A key finding of the study is that the spawning populations of any given lake crashed during times when the only human harvest came from a small number of subsistence users.
"That these strong or weak periods could persist for sometimes hundreds of years means we need to reconsider what we think of as 'normal' for salmon stocks," said Rogers, who did research for the paper at UW and now works in Norway.
Another key finding is that strong and weak periods did not occur in all lakes at the same time.
"The downs in one river are often compensated by the ups in other rivers," said Schindler, even in rivers that are fairly close to one another.
What accounts for the difference? Researchers aren't sure.
"We can propose a bunch of theories," Schindler said, "but chances are there are many mechanisms involved:" changes in climate, diseases and spikes in numbers of predators.
The lakes of Southwest Alaska provide an ideal time capsule for such research, Schindler said. "Those Alaska lakes are so clear (free of microbe and plant life) that they don't produce a lot of sediments. You can get 500 to 1,000 years in a two-foot core."
Researchers targeted Southwest Alaska because heavy rain in Southeast Alaska washes away much of the sediment, he said. Historic spawning areas in the Lower 48 have been permanently altered by development.
"The Alaska lakes give you a fresh perspective on what salmon would look like down here if we let them be, which we sure don't," Schindler said. "The systems here are really compromised" by dams, diversions, landfills and fish farming, which he said reduces genetic diversity.
The paper concludes that fishery managers "should be prepared for persistent and potentially abrupt changes in salmon production" and that fishermen be allowed the flexibility to switch from a non-producing to a producing area.
Also, Schindler said, care should be taken to protect the "network of salmon habitat," including places that are not presently sustaining salmon.
"You may have a river that may not look productive for 50 to 100 years, but at some point in the future it could become one of the most important ones in Alaska," he said. "Don't walk away from nonproductive rivers just because they don't have the fish. Protecting only the rivers that have salmon will not be a good long-term strategy for sustaining fisheries."
Reach Mike Dunham at email@example.com or 257-4332.