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Sockeye carcasses thrown ashore over two decades spur tree growth



A sockeye battles up the shallow waters of Hansen Creek. Credit: Dennis Wise / University of Washington

Hansen Creek, a small stream in southwestern Alaska, is hard to spot on a map. It is just over a mile long and about 4 inches deep. The crossing from one bank to the other takes about five steps.

However, this creek hosts one of the densest salmon salmon in Alaska's Bristol Bay region. Every summer, an average of about 1

1,000 fish return to the creek and beat wildly up the stream to spawn and eventually die.

Over the past 20 years, scores of researchers from the University of Washington have been running this creek every day during spawning season, counting live salmon and recording information about the fish that died – for a salmon, death is here either after spawning or in the brown paw's paws inevitably. After counting a dead fish, the researchers throw it ashore to remove the carcass and not count the next day. The data collection is part of a long-term study investigating how bear hunt affects red salmon in this region.

When these efforts began in the mid-1990s, Tom Quinn, a professor at the UW School of Aquatic and Fishery Sciences, decided that everyone should throw sockeye carcasses on the left side of the downstream stream. They might as well be consistent, he thought, and who knows – maybe one day they might see if the thrown cadavers had any effect on this side of the stream.

Kyla Bivens, a student at the UW School of Aquatic and Fishery Sciences, uses a hook to deliver dead red salmon to Hansen Creek in southwest Alaska in August 2018 toss. Credit: Dan DiNicola / University of Washington

Twenty years later, Quinn and his colleagues found that two decades of carcasses – nearly 600,000 pounds of fish – had a notable effect on the left side of Hansen Creek: white spruces on this side of the creek grew faster than their counterparts on the other Page. In the spruce needles on the side of the thrown carcasses was also found nitrogen in high concentration, which was obtained from salmon.

In essence, as described in an October 23, in the journal Ecology the red-eye carcasses began to fertilize the trees.

"Throwing the carcasses on the left side started with the convenience of counting the same fish twice, I thought at some point in the future it would be so cool to see it if it had an effect," said Quinn, the newspaper's lead author, who has been teaching and directing research projects in the Alaska Salmon Program at UW for 25 years.

Researchers were able to determine that the fertilized trees grew faster a deep slice of the stems, called tree core, of white spruce on both sides of the creek. They studied the growth rings during the 20-year study period (1997 to 2016) as well as for the 20 years prior to the study (1977 to 1996), considering the distance of the rings each year. The first 20 years served as control for the field experiment, as at that time the trees grew on both sides under similar densities of salmon bodies.

UW researchers 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 significantly larger, the authors found, although they grew faster during the 20-year study period. This is because these trees started earlier and grew more slowly before the study began than their counterparts on the other side.

The salmon did not turn this spruce into huge giants, but gave the vegetation on the slower a thrust – growing side of the stream. Numerous factors such as soil chemistry, temperature and light contribute to the growth of trees over many years.

"This study shows the importance of salmon bodies for the growth of trees, but in the context of an area where trees are growing very slowly, and where climate and other factors also play a role in their growth," said Quinn.

During the 20-year study period, nearly 200 students and professors, professors, staff and visiting scholars went to Hansen Creek, which drained in Lake Aleknagik and other remote streams in the Bristol Bay region. They traveled in groups if they came across bears, catching fish in the rivers and often eating only part of the carcass.


At the beginning of the spawning season in July, it is common to see up to several thousand red-eye swarms. The ruby-red bodies crowd into water that is less than 2 inches deep.

"At some point, they're just up," Quinn said. "They are basically swimming over something that is little more than wet rock, through the mouth of the stream and up the stream."

Data on the number of sock eyes, the behavior, and the predation of bears that Quinn and his colleagues have collected for decades are unique. Long-term records of this detail for red salmon are nowhere else. Numerous studies have emerged from this valuable data, says Quinn, and this new study is no exception.

"This study contributes to our understanding of the role of salmon in the ecosystem, but also highlights the importance of patients to long-term research and the pedagogical benefits that result from such research at a university, "said Quinn.


Further information:
Diverse salmon populations allow bears that surf resources to eat tons of fish

Further information:
Thomas P. Quinn et al., An experiment with multiple fenders shows that salmon-engrafted fertilization promotes tree growth in the riparian zone Ecology (2018). DOI: 10.1002 / ecy.2453

Citation in Journal:
ecology

Provided by:
University of Washington


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