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'Salmon signature' used to study historic Alaska fish returns

Hannah HeimbuchThe Arctic Sounder
Sockeye Salmon in Bristol Bay, Naknek flickr / echoforsberg

Alaskan fishermen are no strangers to the roller coaster flux of salmon returning to home spawning grounds. Most fishermen have experienced or at least witnessed both banner years and disaster years, riding out the consequences of a life lived according to marine resources.

Modern biologists have come to recognize these strong and weak variances as cycles that span not years but decades. Now, studies coming out of the University of Washington (UW) are indicating recognizable cycles that cross hundreds of years in their rotation.

"We've been able to reconstruct what salmon runs looked like before the start of commercial fishing," said Lauren Rogers in a Science Codex release. Rogers, lead author of a recently released paper on the subject, studied salmon cycles while earning her doctoral certificate at UW. "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."

This could imply that human custodians need to start rethinking how to interpret salmon statistics and predictions, and especially what is seen as normal, Rogers said.

The researchers studied 500-year records of southwest salmon, finding significant variation in return long before human harvest patterns entered the picture.

That variation was also found to be very system specific, and not region-wide.

"Surprisingly, salmon populations in the same regions do not all show the same changes though time," said Daniel Schindler in the release. Schindler is the co-author of the paper, and a professor of aquatic and fishery sciences at UW. "It is clear that the salmon returning to different rivers march to the beat of a different - slow - drummer."

So what does this mean for Alaska's fishing future?

"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 regimes."

Schindler went on to encourage a more aggressive system of harvesting along rivers during productive times, with guidelines shifting specific to individual rivers, not entire regions.

"It is not realistic to assume that all rivers in a region will perform well or poorly all the time," he said.

The researchers defied the constraints of ancient fish history by examining sediment cores from nursery lakes. They pulled cores from 20 sockeye nursery lakes from 16 southwestern Alaska watersheds - including Bristol Bay habitats. Salmon accumulate nitrogen while feeding and growing in the open ocean, and leave behind an isotopic signature of that nitrogen in lake sediment after they spawn and die.

High nitrogen layers indicated high numbers of returning salmon, and vice-versa.

Researchers named climate shift, food web changes, disease and a number of other factors as contributors to long-term variance. That being said, the scientists note that the centuries-long cycles their study revealed cannot be clearly explained at this time.

While they did mark a decline in return around the turn of the 20th century, when commercial fishing took root in Alaska, other instances of decline are noted prior to that.

"We expected to detect a signal of commercial fishing," Rogers said. "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."

The new research revealed interesting patterns and unexpected new questions, but it also highlighted a marked resiliency in one of Alaska's favorite fishes.

From the research paper: "Interestingly these same fluctuations also highlight that salmon stocks have the capacity to rebuild 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."

This article originally appeared in The Bristol Bay Times and is reprinted here with permission. Hannah Heimbuch can be reached at hheimbuch(at)reportalaska.com.