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A sockeye fights its way up Hansen Creek in shallow water. Credit: Dennis Wise / University of Washington
Hansen Creek, a small stream in southwest Alaska, is hard to pick out on a map. It's just over a mile long and about 4 inches deep. Crossing from one bank to another
Yet this stream is one of the most densely sockeye salmon runs in Alaska's Bristol Bay region. Each summer, about 11,000 fish on average to this stream, furiously beating their way up the shallow creek to spawn and eventually die.
For the past 20 years, dozens of University of Washington researchers have read this creek every day during spawning season, counting live salmon and recording information about the fish that died for salmon, death is inevitable here, or after spawning or in the paws of a brown bear. After counting a dead fish, the researchers throw it on shore to remove the carcass and not double-count it the next day. The data collection is part of a long-term study looking at how bear predation affects sockeye salmon in this region.
When this effort began in the mid-1990s, Tom Quinn, a professor in the UW School of Aquatic and Fishery Sciences, decided that everyone should throw socketing carcasses to the left side of the stream-facing downstream. They thought it was possible that they knew what they were doing, and that they could see the carcasses of the body.
Kyla Bivens, an undergraduate student in the UW School of Aquatic and Fishery Sciences, uses a hooked pole to a dead sockeye salmon on the bank of Hansen Creek in southwest Alaska in August 2018. Credit: Dan DiNicola / University of Washington
Twenty years later, Quinn and colleagues have found that two decades of carcasses-nearly 600,000 pounds of fish-tossed to the left side of Hansen Creek did not have a noticeable effect: the other side. What's More, Nitrogen Derived from Highly Densified Sturgeon in the United States.
Essentially, as they report in a paper published October 23 in the journal EcologyThe sockeye carcasses were fertilizing the trees.
"Tossing the carcasses to the left side of the world is just a convenience to keep in mind," said Quinn, "I thought it would be a good thing to have some effects. the paper's lead author UW's Alaska Salmon Program for 25 years.
The searchers were able to say that the growth of the tree grew faster than the bottom of the tree, called a tree core, from white spruce on both sides of the stream. They examined the growth during the 20-year study period (1997 to 2016), looking at the spacing of each year. The first 20 years served as a control for the field experiment, because they were growing under similar densities of salmon carcasses.
UW researchers walk along Hansen Creek in southwest Alaska in 2015. Credit: Dennis Wise / University of Washington
By 2016, the trees on the salmon-enriched side were not noticeably taller, the authors found, even though they grew faster over the 20-year study period. This is because they started out faster and were growing more slowly before their counterparts on the other side.
The salmon did not turn these spruce into towering giants, but instead gave a boost to vegetation on the slower-growing side of the stream. Numerous factors such as soil chemistry, temperature and light all contribute to growth over many years.
"This study demonstrates the importance of salmon carcasses for the growth of trees, yet within the context of an area where they grow slowly, and also play a part in their growth," Quinn said.
Over the 20-year study period, close to 200 undergraduate and graduate students, professors, staff and visiting scientists walked Hansen Creek, which drains into Lake Aleknagik, and other remote streams into the Bristol Bay region. They traveled in groups in a box that they came to, which catch fish in the streams and often eat just part of the carcass.
At the start of the spawning season in July, they are swarming the mouth of the creek, their ruby-red bodies jostling in water less than 2 inches deep.
"At some point they just go for it," Quinn said. "They are the most important things about wet rocks, powering the mouth of the stream and the creek."
Data on sockeye numbers, behavior and predation by bears collected by Quinn and his colleagues for the past few years. Quinn said, and this new study is no exception.
"This study contributes to our understanding of the role of salmon in the ecosystem, but the importance of patient, careful, long-term research, and the educational benefits of such research in a university," Quinn said.
Explore further:
Various salmon populations enable 'resource surfing' bears to eat tons of fish
More information:
Thomas P. Quinn et al., A multidisciplinary experiment shows that fertilization by salmon carcasses enhanced tree growth in the riparian zone, Ecology (2018). DOI: 10.1002 / ecy.2453
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