Fisheries & Aquaculture News

Salmon Carcasses Promote Tree Growth in Alaska

The life of a sockeye salmon (Oncorhynchus nerka) is one which can be compared to that of a character in a Shakespearean novel.

Every July, thousands of individuals make their way from the open waters of the Pacific Ocean to their natal breeding grounds in the freshwater rivers of the north Pacific. Here, they battle shallow streams and bears, all for the chance to spawn before they die. And now, a new study published in the journal Ecology by scientists at the University of Washington has found that the carcasses of dead salmon have been fertilizing the white spruce trees along the banks of the creek, providing them with nutrients they wouldn’t otherwise obtain.

Over the past twenty years, the researchers have been studying how predation from bears has affected sockeye populations in Hansen Creek, southwest Alaska. When the study on sockeye populations began in 1997, Tom Quinn, a professor in the UW School of Aquatic and Fishery Sciences, decided that all participating researchers should throw sockeye carcasses to the left side of the stream so that they don’t double count the same individual when carrying out surveys.

Hansen creek is home to one of the densest congregations of sockeye populations in Alaska, with approximately 11,000 individuals arriving here each year to spawn. “At some point they just go for it,” Quinn said. “They are basically swimming over what’s little more than wet rocks, powering through the mouth of the stream and up the creek.”

"Tossing the carcasses to the left side started out just as a convenience to keep from counting the same fish twice. I thought at some point in the future, it would be kind of cool to see it if had an effect," said Quinn, the paper's lead author.

Twenty years later, Quinn and colleagues found that as a result of nearly 220,000 carcasses being thrown onto the left side of the bank, white spruce trees grew faster and contained higher levels of nitrogen (an element important for growth in most vegetation) in the needles of the trees than those on the other side.

The researchers were able to tell that the fertilized trees grew faster by taking core samples from the trunks of specimens on both sides of the stream and examining their growth rings. They examined the growth rings during the 20-year study period (1997 to 2016) as well as for the 20-year span before the study began (1977 to 1996). The first 20 years served as a control for the field experiment because during that period the trees on both sides were growing under similar densities of salmon carcasses.

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 those trees started out shorter and were growing more slowly before the study began than their counterparts on the other side.

Long-term data sets for sockeye salmon don’t exist anywhere else and Quinn concluded by saying “This study contributes to our understanding of the role of salmon in the ecosystem, but it also illustrates the importance of patient, careful, long-term research, and the educational benefits that result from such research in a university".

By Ellis Moloney

Quinn, T., Helfield, J., Austin, C., Hovel, R. and Bunn, A. (2018). A multi decade experiment shows that fertilization by salmon carcasses enhanced tree growth in the riparian zone. Ecology.

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