Sockeye carcasses tossed on shore over two decades spur tree growth

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Story originally appeared in UW News

 

Hansen Creek, a small stream in southwest Alaska, is hard to pick out on a map. It’s just over a mile long and about 4 inches deep. Crossing from one bank to the other takes about five big steps.

Yet this stream is home to one of the most dense sockeye salmon runs in Alaska’s Bristol Bay region. Each summer, about 11,000 fish on average return to this stream, furiously beating their way up the shallow creek to spawn and eventually die.

Kyla Bivens, an undergraduate student, uses a hooked pole to throw a dead sockeye salmon onto the bank of Hansen Creek.Dan DiNicola/University of Washington
Kyla Bivens, an undergraduate student, uses a hooked pole to throw a dead sockeye salmon onto the bank of Hansen Creek.

For the past 20 years, dozens of University of Washington researchers have walked this creek every day during spawning season, counting live salmon and recording information about the fish that died — for a salmon, death is inevitable here, either after spawning or in the paws of a brown bear. After counting a dead fish, researchers throw it on shore to remove the carcass and not double-count it the next day. The data collection is part of a long-term study looking at how bear predation affects sockeye salmon in this region.

When this effort began in the mid-1990s, Tom Quinn, a professor in the UW School of Aquatic and Fishery Sciences, decided that everyone should throw sockeye carcasses to the left side of the stream — facing downstream. They might as well be consistent, he thought, and who knows — maybe someday they could see whether the tossed carcasses had an effect on that side of the stream.

Twenty years later, Quinn and colleagues have found that two decades of carcasses — nearly 600,000 pounds of fish — tossed to the left side of Hansen Creek did have a noticeable effect: White spruce trees on that side of the stream grew faster than their counterparts on the other side.

What’s more, nitrogen derived from salmon was found in high concentration in the needles of the spruce trees on the side of the tossed carcasses.

Essentially, as they report in a paper published October 23 in the journal Ecology, the sockeye carcasses were fertilizing the trees.

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