Salmon Runs Boom, Go Bust Over Centuries

Salmon runs are notoriously variable: strong one year, and
weak the next. New research shows that the same may be true from one century to
the next.
Scientists in the past 20 years have recognized that salmon
stocks vary not only year to year, but also on decades-long time cycles. One
example is the 30-year to 80-year booms and busts in salmon runs in Alaska and
on the West Coast driven by the climate pattern known as the Pacific Decadal
Now work led by University of Washington researchers reveals
those decadal cycles may overlay even more important, centuries-long
conditions, or regimes, that influence fish productivity. Cycles lasting up to
200 years were found while examining 500-year records of salmon abundance in
Southwest Alaska. Natural variations in the abundance of spawning salmon are as
large those due to human harvest.

“We’ve been able to reconstruct what salmon runs looked like
before the start of commercial fishing. But rather than finding a flat baseline
– some sort of long-term average run size – we’ve found that salmon runs
fluctuated hugely, even before commercial fishing started.

That these strong or
weak periods could persist for sometimes hundreds of years means we need to
reconsider what we think of as ‘normal’ for salmon stocks,” said Lauren Rogers,
who did this work while earning her doctorate in aquatic and fishery sciences
at the UW and is now a post-doctoral researcher with the University of Oslo,

Rogers is the lead author of a paper on the findings in the
Jan. 14 online early edition of the Proceedings of the National Academy of
“Surprisingly, salmon populations in the same regions do not
all show the same changes through time. It is clear that the salmon returning
to different rivers march to the beat of a different – slow – drummer,” said
Daniel Schindler, UW professor of aquatic and fishery sciences and co-author of
the paper.

“The implications for management are profound,” Schindler
said. “While it is convenient to assume that ecosystems have a constant static
capacity for producing fish, or any natural resource, our data demonstrate
clearly that capacity is anything but stationary. Thus, management must be
ready to reduce harvesting when ecosystems become unexpectedly less productive
and allow increased harvesting when ecosystems shift to more productive
“Management should also allow, and probably even encourage,
fishers to move among rivers to exploit salmon populations that are
particularly productive. It is not realistic to assume that all rivers in a
region will perform equally well or poorly all the time,” he said.
The researchers examined sediment cores collected from 20
sockeye nursery lakes within 16 major watersheds in southwestern Alaska,
including those of Bristol Bay. The scientists homed in on the isotopic
signature of nitrogen that salmon accumulate in the ocean and leave behind in
lake sediments when they die: When there was a lot of such nitrogen in the
sediments, it meant returning runs during that time period were abundant; when
there was little, runs had declined.
Climate is not the only reason for long-term changes in
salmon abundance. Changes in food webs, diseases or other factors might be
involved; however, at present, there are no clear explanations for the factors
that cause the long-term variability observed in this study.
Most, but not all, of the lakes examined showed declines in
the kind of nitrogen the scientists were tracking beginning around 1900, once
commercial fisheries had developed. However, earlier fluctuations showed that
natural processes had at times reduced salmon densities as much as recent
commercial fisheries, the co-authors said.
“We expected to detect a signal of commercial fishing –
fisheries remove a lot of the salmon, and thus salmon nitrogen, that would have
otherwise ended up in the sediments. But we were surprised to find that
previous returns of salmon to rivers varied just as dramatically,” Rogers said.

As the paper said, “Interestingly these same fluctuations
also highlight that salmon stocks have the capacity to rebuild naturally
following prolonged periods with low densities, suggesting a strong resilience
of salmon to natural and anthropogenic depletion processes. Indeed, total
salmon production (catch plus escapements) has been relatively high in recent
years for most sockeye salmon stocks in southwestern Alaska, despite a century
of intense harvesting.”

Other co-authors are Peter Lisi and Gordon Holtgrieve with
the UW, Peter Leavitt and Lynda Bunting with University of Regina, Canada,
Bruce Finney with Idaho State University, Daniel Selbie with Fisheries and
Oceans Canada, Canada, Guangjie Chen with Yunnan Normal University, China,
Irene Gregory-Eaves with McGill University, Canada, and Mark Lisac and Patrick
Walsh with Togiak National Wildlife Refuge, Alaska.
Funding for the project was provided by the Gordon and Betty
Moore Foundation, the National Science Foundation, the U.S. Fish and Wildlife
Service and the Natural Sciences and Engineering Research Council of Canada.