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US Fish & Wildlife Service FieldNotes

TOGIAK:New research reveals surprising finds about Bristol Bay sockeye abundance

Region 7, April 26, 2013
Aquatic Suburbia: an abundance of sockeye redds (spawning beds) along one of Togiak's many lake shorelines.
Aquatic Suburbia: an abundance of sockeye redds (spawning beds) along one of Togiak's many lake shorelines. - Photo Credit: n/a
Togiak Refuge or Bust! Salmon, fresh from the ocean, make their way back to their aquatic roots on Togiak Refuge.
Togiak Refuge or Bust! Salmon, fresh from the ocean, make their way back to their aquatic roots on Togiak Refuge. - Photo Credit: n/a
Not exactly restaurant quality: soon these spawned out salmon will possibly contribute to the isotopic nitrogen signature to be found on Togiak Refuge.
Not exactly restaurant quality: soon these spawned out salmon will possibly contribute to the isotopic nitrogen signature to be found on Togiak Refuge. - Photo Credit: n/a

The number of salmon returning to their freshwater roots to spawn has been a subject of curiosity across many Pacific fisheries. Some believe that a century of commercial fishing pressure has potentially impacted salmon populations and the freshwater ecosystem in a negative way. It is certainly true that annual salmon runs can vary wildly from year to year, at times seeming like a boom or bust proposition. Has commercial fishing led to a gradual but steady decline in many salmon fisheries? Perhaps. But the findings of a recent project suggest strongly that long term highs and lows have been a part of the big picture with salmon for far longer than commercial fishing’s mark on the Pacific salmon timeline.

 

This project, led by Lauren Rogers of the University of Washington, and 11 coauthors from the U.S., Canada and China, was recently published in the Proceedings of the National Academy of Sciences (www.pnas.org). The researchers evaluated the isotopic nitrogen signature of sediment core samples from 20 sockeye salmon nursery lakes within 16 major watersheds across southwest Alaska, including eight lakes within the Togiak Refuge. When young sockeye salmon go to the ocean they grow fast and accumulate nutrients that have a specific marine signature. The salmon bring this marine signature back to the freshwater and deposit it in lake sediments after they spawn and die. The sediments on the lake bottom hold a historical record of all the nutrient inputs that have occurred. When there is a higher concentration of marine nitrogen isotope in the lake sediments it indicates that the returning salmon runs during that time period were abundant. When there was little marine nitrogen, the salmon runs had declined. These researchers looked back 500 years through the sediment core record and found that widespread fluctuations have been a part of the equation for far longer than the shadow cast from modern commercial fishing.

Rogers, who is now a post-doctoral researcher with the University of Oslo, Norway, stated that “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.”

As expected, most of the lakes in the study showed declines in marine nitrogen that coincided with the advent of commercial fisheries in the area around 1900. However, results also showed earlier fluctuations in which natural processes had at times reduced salmon densities as much as recent commercial fisheries.

“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.

This research indicates that despite these prolonged periods of low densities, salmon stocks had the capacity to rebuild naturally. To underscore that point is that recent total run size for most sockeye salmon stocks in southwest Alaska has been relatively high, despite intense harvesting over the past century.

Another finding of the study is that salmon populations returning to different rivers in the same region do not all show the same changes through time. According to Daniel Schindler, Professor of Aquatic and Fishery Sciences at the University of Washington, and a co-author of the paper, “It is clear that the salmon returning to different rivers march to the beat of a different – slow – drummer.”

So what does all this mean for fisheries management today? Schindler suggests that a substantial amount can be gleaned from the results.

“The implications for management are profound. 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.”

“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,” Schindler said.

Commercial use of these fisheries will continue to bear scrutiny in the decades to come. However, it now appears that perhaps much more is in play when it comes to these resources than has been previously understood. Climate, changes in food webs, competition, diseases or other factors might all have a role in population fluctuations. How those factors come together to influence the survival and abundance of salmon is still unknown, though the results of this study do shed new light for fisheries experts to consider. The one undeniable fact is that if we maintain the habitat in southwest Alaska, like those protected in Togiak Refuge, we can ensure the annual return of the salmon have a chance.

For more information about this research contact:

Patrick Walsh Patrick_walsh@fws.gov
Dr. Lauren Rogers lauren.rogers@bio.uio.no or
Dr. Dan Schindler deschind@uw.edu

Contact Info: Terry Fuller, 907-842-1063 ext. 8419, terry_fuller@fws.gov