Fairbanks, Alaska -- Fishermen accustomed to the quick boom then bust of Alaska's salmon runs may wish they were born in a different time-about 800 years ago to be exact.
In a study published in the April 18 issue of the journal Nature, Alaska and Canadian scientists say sockeye salmon runs to the state once rode waves of abundance lasting centuries. The most recent long-term boom began in the year 1200 and lasted until the turn of the last century, according to the report.
"Looking back over the past 2,000 years of salmon runs, there were periods of high abundance lasting hundreds of years, not just the decades we see today," said Bruce Finney, the study's lead author and a marine scientist at the University of Alaska Fairbanks, Institute of Marine Science.
Of course what went up, eventually came down--hard. In the case of salmon, Finney says the busts lasted as long as the booms.
"There were periods of time lasting hundreds of years when salmon runs were far less abundant as well," Finney said.
To reach their conclusions, Finney and scientists from Queen's University in Ontario, Canada, studied cores of mud taken from the bottom of two lakes on Alaska's Kodiak Island. The mud contained traces of the stable nitrogen isotope, N15, released into the lakes by salmon that died and decayed after spawning. Nitrogen is a nutrient critical to northern freshwater systems, helping to nourish the growth of algae called diatoms. Once the diatoms die, they fall to the lake bottom and become a kind of historical record of N15 over time. Researchers dated the mud layers using carbon dating techniques and known events such as volcanic eruptions that left ash in the mud. They then analyzed the concentration of nitrogen in the diatoms and mud to estimate the relative size of ancient salmon runs to the lakes going back some 2,000 years.
"If you have high levels of specific types of diatoms and high levels of Nitrogen-15 present in the mud dated at certain times, the reasoning is that more salmon returned to the system at those times," said John Smol, a co-author of the study and a biologist at Queens University.
Using these techniques, the researchers say salmon runs rose and fell on time-scales unheard of today. Beginning 800 years ago, at about the year 1200, for example, Finney said salmon numbers increased markedly and stayed relatively high through to the beginning of last century, when modern commercial fishing began.
"The change was abrupt but sustained itself for hundreds of years," Finney said.
Researchers say natural climate change most likely altered salmon abundance in the centuries before commercial fishing and global industrialization. That's contrary to salmon declines during the most recent century, which researchers say were caused by over fishing, pollution and habitat destruction.
"What we can show is that there were dramatic changes in salmon populations long before humans entered the picture in any appreciable way. The lesson here is that salmon runs are very sensitive to changes in their environment, Smol said.
Researchers also found evidence of low salmon runs lasting hundreds of years. One of the most stark salmon busts occurred from 100 BC to 800 AD.
"Salmon runs at the time of Christ were far lower than anything we have seen up through modern times," said researcher Irene Gregory-Eaves, a co-author of the study and graduate student at Queens University. There were perhaps only a few hundred thousand salmon returning to those lakes at that time instead of the millions of salmon we see in later periods."
Finney said the increase in salmon abundance that occurred beginning in 1200 AD was most likely caused by a change in climate that triggered glacial advance in Alaska and brought drought conditions to the Midwest, which led to more favorable conditions for salmon in the North Pacific.
The researchers also found evidence that while salmon runs flourished in Alaska, other fish species declined in waters farther south. The finding lends support to the belief that fish stocks in Alaska rise and fall in trends opposite those of species along the U.S. west coast.
Their research was funded by the National Science Foundation, the Alaska Sea Grant Program at the University of Alaska Fairbanks, and the National Oceanic and Atmospheric Administration's Auke Bay Laboratory in Juneau, Alaska.
The scientists also say their data supports the belief that salmon upswings influenced the development of Alaska's coastal Native cultures. Finney said evidence found at archeological sites on Kodiak Island shows the island's indigenous groups further developed fishing gear and began using salmon more frequently from 800 AD through 1200 AD, a time when salmon were more abundant.
"One of our ideas was that since Native people along the coast depended on salmon, that changes in salmon influenced cultural changes," Finney said. "These abrupt changes in salmon abundance coincide with archeological evidence. In times of low abundance they depended more on marine mammals. In time of high abundance, the archeological evidence shows a stark increase in reliance on salmon."
Smol says understanding how the environment affected salmon runs of the past may help researchers predict how salmon may respond to future climate shifts.
"These records give us a glimpse into how salmon might do in future global warming because there are other periods in the past, at least in Alaska, that were as warm or warmer than today," Smol said.
The study's scientists aren't new to the study of ancient salmon runs. Two years ago, the team published in the journal Science the results of sediment core analysis of salmon runs going back some 300 years. That work demonstrated the reliance of freshwater ecosystems on nutrients released by decomposing salmon. Their next project is to reconstruct salmon runs going back to the end of the last major ice age, some 15,000 years ago, a time when salmon are thought to have colonized North Pacific watersheds as glaciers retreated.
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