For 20 years, scientists have been chucking fish into the forest. Here's why

Originally published on November 10, 2018.

A long term study in Alaska has demonstrated the importance of salmon as a fertilizer for the forests of the Pacific Northwest — and it involved throwing salmon into trees.

Every year, tens of thousands of sockeye salmon make their way from the ocean 400 kilometers upriver to Hansen Creek, an ankle-deep stream in southwestern Alaska, where they spawn, and die.

And for the past 20 years, Dr. Tom Quinn and his students have been throwing these throngs of dead salmon onto the north bank of the river.

"It started out as a study of bear predation," Quinn told Quirks & Quarks host Bob McDonald. Brown bears were often seen in the area feasting on the fish.

"With only a very vague sense of where it was all going to lead, I told the students to make sure all the carcasses were thrown to one side of the stream and we made that the official protocol and kept it up."

Fish in the forest

It turned into an unprecedented, 20-year study of the effects of fish on the forest.

"That tremendous amount of nutrients that the salmon have acquired while feeding out in the ocean, is then distributed to a wide variety of predators, scavengers, decomposing animals, it goes directly or indirectly to insects, birds, mammals, and in fact the trees themselves," said Quinn, a professor in the University of Washington's School of Aquatic and Fishery Sciences.

The point of the long-term experiment was to reproduce the effect of all those scavengers moving dead fish into the forest, but add in a bit of experimental control, by moving fish to only one side of the creek.

His team were able to quantify that effect, first by looking at the needles of trees, to confirm they were taking up nitrogen and carbon isotopes from the fish. Then, they collected cores from the trees on both sides of the river to compare growth. They found that the trees on the north side of the river grew more slowly than the south side before the experiment began. But with the fish acting as fertilizer, the trees started growing faster.

"Obviously the salmon carcasses are not the only thing that differs between the left and the right side of the stream and essentially the carcass has allowed the trees on the slower growing side to catch up."

Salmon stuck in trees

20-year studies like this one are exceedingly rare, and without the long duration of the project, he might not have been able to see the results so clearly. Plus, Quinn says, with a long term project like this one, he was able to share the experience with dozens of students.

'We tend to use long poles with a little metal spike on the end which gives you a little bit of leverage," said Quinn.

"Some of the people tend to want to make a competition and throw it farther or get them to stick in the trees. I don't mind that because it achieves that goal and keeps the students happy. ... I absolutely guarantee that they will always remember it."