Every year the dramatic story of the Pacific salmon is played out in the rivers and coastal lakes of Alaska and the northwest Pacific. Their years-long sojourn in the open sea comes to an end, and they return to the area where their own lives began. There, they spawn and die in one of nature’s most moving pageants.
It is the final act for the salmon, and there is a broadly-based assumption that it has always been like that. Unless humans interfere by building dams or overfishing, so it has been thought, the salmon runs have remained relatively constant over the course of centuries.
But it turns out that that’s not true, according to Bruce Finney, who has clawed his way through the nearly impenetrable brush along the shoreline of Alaska and the Pacific Northwest to reach more than 50 lakes. Finney, associate professor of marine science at the University of Alaska, Fairbanks, set out with colleagues from Queens University in Ontario and the University of Toronto to see if the story of the salmon, which has reached nearly mythological proportions, is as consistent as had been thought.
What they found was quite startling, and it poses new problems for those who struggle to manage fishery stocks throughout the region.
Sleuthing for Signs of Salmon
During the last three centuries, the research shows, salmon runs have gone through many boom and bust cycles long before there was a strong commercial fishery. In warmer years, the runs tended to be much stronger, and in cooler years, there were far fewer salmon returning to spawn. The findings indicate that climate variability plays a critical role in salmon abundance. It has been that way for at least 300 years, and probably much longer.
“We got pretty strong signals,” Finney says.
They arrived at their conclusions after several years of scientific sleuthing in some of the most spectacular regions on the continent. The challenge was to document salmon runs long before people began recording how many fish were caught, or how many returned to spawn.
To do that they concentrated on one of five species of Pacific salmon, the sockeye. Although most salmon spawn in rivers, the sockeye makes its way up coastal rivers to inland lakes, which turned out to be ideal natural labs for the scientists.
After spawning, the salmon die and their decaying carcasses release various nutrients that enrich the waters of the lakes. Nitrogen is taken up by algae, some of which is eaten by zooplankton which in turn are eaten by juvenile salmon the following spring. The rest of the algae dies and settles to the bottom of the lake.
“The dead salmon fertilize the lakes with their own bodies, and everything else living in the lake benefits from this nutrient source,” says researcher Irene Gregory-Eaves of Queen’s University.
“When there is a strong salmon run, there are a lot of nutrients being released, and this, in turn, stimulates production of the lake’s algae.”
The result is a yearly layering of the lake bottom that is rich with nitrogen if there were lots of salmon, and depleted if the run was low.
The researchers took cores from the bottom of the lakes, and then used records such as tree rings, or ash laid down by known volcanic eruptions, to date the layers. To be sure they weren’t being fooled by mother nature, the researchers also took cores from other nearby lakes that are so isolated even the salmon couldn’t reach them.