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A male sockeye salmon in a stream off the Adams River at Roderick Haig-Brown Provincial Park north of Chase, B.C.JOHN LEHMANN/The Globe and Mail

When bears, wolves and other animals drag salmon carcasses from spawning streams they cause an intricate chain reaction that changes the nature of the surrounding forest, according to new research from Simon Fraser University.

Plant species that efficiently take up nitrogen from the decomposing bodies of salmon flourish - and soon there are more song birds, drawn by the dense growths of wild berry bushes and prolific insect hatches.

The change is so dramatic, according to the research done in one of the largest field studies on salmon in the world, that it is possible to look at the forest in a watershed, and tell how well the associated salmon run is doing.

"Before we started the study I would have thought that was a real long shot. But now I think we could do that," says John Reynolds, who holds the Tom Buell BC Leadership Chair in Salmon Conservation at SFU.

One of the first things Prof. Reynolds would look for as a sign of a healthy salmon stream are thickets of bushes, including salmon berry, which thrive in the soil enriched by nitrogen from dead fish. Not all plants are as efficient at taking up the nitrogen, and these that aren't are pushed out by those that are.

"The shift in dominance of some of these plant species was a lot more dramatic than I frankly had expected. Species like salmon berry it turns out are really well named. They tend to dominate in streams that have a large number of salmon," said Prof. Reynolds, who oversaw the research project which was led by Morgan Hocking, a postdoctoral fellow.

In addition to looking at plant species, Prof. Reynolds said it is important to consider the physical characteristics of a stream as well, because animals avoid fishing in places where getting out of the water with a salmon is difficult because of steep banks.

"If it is a small stream and has shallow banks, then there is a lot better chance that the plants will be effected by the carcasses, because these are more accessible to bears," he said.

Starting in 2007, a team of 10 researchers began examining 50 relatively small watersheds in the Great Bear Rainforest, a vast area of old growth forest on British Columbia's wild Central Coast.

Prof. Reynolds said the area was chosen because there has been little human impact on the watersheds.

"That area is mostly without is the last place that hasn't been logged," he said.

Prof. Reynolds said to gather the massive amount of data they needed, the team was divided into subgroups, each with specific targets.

"It was almost like having a SWAT team. We had a plant crew, we had a stream crew measuring physical characteristics, we had water chemistry work, looking at nutrients in the water. We would beach the boat each day and people would head off in teams of two and three and get to work. It was quite an effort.," he said.

The researchers assessed one or two streams a day, during the summer, and in the fall counted the number of salmon in the small streams, many of which had never been enumerated before. Salmon runs ranged from a few fish to mover than 100,000.

"It's one of the largest field programs in the world in terms of the number of watersheds that we looked at," said Prof. Reynolds.

The research findings support another recent paper, by Prof. Reynolds and Rachel Field, which found that song bird density and diversity is linked to the size of salmon runs.

That paper looked at estuaries and concluded that " breeding birds may benefit from residual salmon-derived nutrients in landscapes adjacent to spawning grounds and that this trend extends . . . well beyond the salmon spawning season."