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Tossing salmon for science

A decades-long experiment demonstrates how the iconic fish help trees grow.

 

Every year thousands of sockeye salmon meet their end in Hansen Creek, a pebble-strewn tributary of Lake Aleknagik in southwestern Alaska, whether from old age or at the paws and jaws of a brown bear. Either way, they’re almost certainly destined to rot away on the north-facing bank of the stream.

That’s because professors, researchers and students have been systematically tossing their carcasses to that side of the creek for the last 20 years. The scientists count and measure the carcasses and then toss them out of the streambed and up into the forest using wooden poles with metal hooks on the end, called gaffs. In total, they tossed about 295 tons of salmon onto Hansen Creek’s north-facing bank to avoid double counting surveyed fish. In doing so, they have created a unique opportunity to study exactly how salmon fertilize the forest. 

Over the past 20 years, researchers across the Northwest have shown that salmon play an essential role in forests: Trees next to salmon-bearing streams appear to grow better than their salmon-deprived counterparts, and the nutrients salmon bring from the ocean make their way into the needles and wood of trees. But this experiment, described in a recently published paper, led by Tom Quinn, a professor in the School of Aquatic and Fishery Sciences at the University of Washington, proves a basic fact: More salmon means faster growing trees.

Quinn, who has been teaching and researching in southwestern Alaska since the late 1980s, initially studied the relationship between bear predation and salmon populations. Over the years, his work and the work of his collaborators and students has branched out to address other questions, like how climate change and hatchery fish affect the environment. “What’s special here is this is a long-term experiment,” Quinn said. “It’s part of a more holistic study of interactions throughout the ecosystem.”

In the summer of 2016, two decades after the fish-tossing began, Quinn, along with co-authors James Helfield, Catherine Austin, Rachel Hovel and Andrew Bunn, took tree core samples from the spindly white spruce that grow on either side of Hansen Creek. “In the back of my mind I’ve been thinking about the fertilization impact,” Quinn said. It seemed like the right time to finally see what the effect was: “I’m not going to be doing this forever. Twenty years seemed like a good time to analyze the data.” The core samples revealed that the fertilized trees had grown faster in the salmon guts than they had grown in the twenty years prior to the experiment, and were starting to catch up with their taller counterparts on the opposite bank.

[RELATED:https://www.hcn.org/issues/45.20/ecosystems-101-hard-lessons-from-the-mighty-salmon-runs-of-alaskas-bristol-bay]

With control over the conditions, the researchers were able to address some of the criticisms of previous studies. Most studies in the past measured the difference in growth rates of trees above and below waterfalls or between different streams, Quinn said. And in doing so they weren’t able to rule out other potential causes: Water availability, elevation, and other factors impacting soil fertility. Quinn’s somewhat accidental experiment removed many of those factors.

The research reinforced salmon’s importance in the ecosystem, even as their stocks dwindle in many streams up and down the West Coast. This study, Quinn said, “provides perspective on what is lost in other ecosystems.”

Carl Segerstrom is an editorial fellow for High Country News. Email him at [email protected] or submit a letter to the editor