How researchers flinging salmon inadvertently spurred tree growth

How much salmon would scientists sling if scientists could sling salmon? For one research team, the question isn’t hypothetical, and the answer is … tons.

During 20 years of monitoring salmon populations in one southwest Alaskan stream, ecologists have found and flung a total 267,620 kilograms of dead fish into the forest. Those rotting carcasses leached enough nutrients to speed up tree growth, researchers report October 23 in Ecology.

Some of the fish had died of old age, while many were torn apart by brown bears or gulls, says ecologist Thomas Quinn. He’s been counting salmon, both dead and alive, in Hansen Creek every year since 1997 with a rotating cast of students from the University of Washington in Seattle. For each dead fish, students catalogued the cause of death, then chucked the carcasses to one bank of the river to avoid double-counting.

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That toss is something of an art. The researchers use a gaff, a long pole with a hook on one end. The ideal motion is like a checked swing in baseball, when a batter starts to swing but instead lets the ball pass, Quinn says. If all goes well, the salmon carcass launches off the gaff in a graceful arc and lands far enough away that it won’t be washed back into the stream.

“Some students are clumsy…. Some are naturals,” Quinn says. But with a little practice, everyone makes it work.

Two decades and 217,055 dead salmon later, Quinn and his students changed the balance of nutrients over a two-kilometer stretch of stream: One side received six times more salmon than the other. The idea that salmon might affect plant growth isn’t new, but it’s been difficult to test accurately, Quinn says. Previous efforts have compared trees near streams with and without salmon, or trees above and below a salmon-stopping waterfall. But those sites have differed in other ways that could also affect tree growth.

On both sides of the stream, Quinn’s team measured tree trunk diameters, yearly growth recorded in tree rings, and nitrogen isotopes in the needles of white spruce. The trees on the salmon-fertilized bank grew faster during the 20-year study period than in the prior 20 years. The trees on the other bank didn’t have a similar growth spurt. Plus, more of the nitrogen in the needles of the fish-fed white spruce trees came from marine sources — evidence that the salmon made a difference.

But just looking at the trees, the impact isn’t apparent. “It’s a nice reminder that ecology is about limits,” Quinn says. “No matter how many carcasses you throw … trees are still going to be limited by other things.”