Dead Salmon Component Of Healthy Stream Carcasses If Spawned-Out Fish Provide Necessary Food, Fertilizer

It sounds like a paradox: To have a healthy stream where salmon can spawn naturally, you need dead fish.

But a study by Weyerhaeuser Co. fish biologist Bob Bilby shows that the salmon that die and rot in stream beds after spawning play a critical role in feeding newborn fish and even the trees, plants and animals along the stream bank.

The salmons’ spawned-out carcasses, eggs and newly hatched offspring are a prime food source for juvenile salmon and steelhead. Nutrients from their bodies also fertilize surrounding trees and shrubs. Bilby found that 18 percent of the nitrogen in hemlocks, salmonberry and devil’s club in one streamside zone came from salmon, presumably through the wastes or remains of animals that ate the fish.

The rotting fish fertilize the entire food chain in a stream and the land nearby, Bilby found. Microbes feed on the fish, and in turn are eaten by insects and water creatures, forming the foundation of the chain that eventually includes eagles, bears - and people.

Bilby’s work in the Snoqualmie, Chehalis and Willapa river basins has been highly praised by colleagues and champions of naturally spawning salmon.

Nowadays, few salmon are allowed to spawn naturally because of dwindling natural stocks, Bilby said.

“One of the implications of our research is that this robs the streams of nutrients and the ability to support and propagate further generations of fish,” he said.

Fish managers who take spawning fish out of streams are like farmers who never fertilize and eventually exhaust their soil, he said.

“This is the kind of thing people have known for a long time - that salmon carcasses make important contributions to productivity. The problem has been quantifying it, which we have been able to do,” Bilby said.

In large part because of Bilby’s work, state salmon hatcheries this fall will start placing a limited number of salmon carcasses along rivers. Until recently, the Department of Ecology barred the practice, on grounds the dead fish polluted the streams and because some people objected to the smell.

Bilby also said his research suggests that traditional “escapement” goals - estimates of how many returning adults are needed to perpetuate salmon runs - are low. Extra spawners are needed to provide food for salmon eggs and newly hatched fry, he said.

Bilby’s research started in 1991 as a project to trace whether chemical fertilizers were getting into streams. A technique known as stable isotope analysis proved ineffective, however, because the chemical nitrogen’s “signature” was not distinctive enough to track.

His research team, however, found the nitrogen in salmon carcasses was very distinct and could easily be traced as it was taken up by animals and plants.

In the Chehalis basin in 1994 and the Willapa River basin in 1995, he planted 400 salmon carcasses each year. He found that juvenile salmon grew nearly twice as fast as those in streams without planted carcasses. Researchers analyzed food in the stomachs of juvenile fish and frequently found it was the flesh of adult salmon, their eggs or recently hatched fingerlings.

Other studies have made a strong connection between the size of juveniles and survival when they go to sea. Therefore, enriching streams could boost numbers of returning salmon, Bilby says.