Reconstructing Salmon Populations

Management of Pacific Salmon has been an issue for years. To determine whether management goals are working, knowledge of historical populations can prove quite useful. In a recent study published in Ecology, Deanne C. Drake, Robert J. Naiman, and James M. Helfield, all of the University of Washington, paired annual tree ring growth with catch data to determine what salmon stocks looked like 200 years ago.

Born in freshwater lakes and rivers, salmon swim to the ocean where they feed and mature. After a few years, they return to the river or lake of their birth to reproduce and die. As their bodies decay, they leave behind ocean nutrients in the freshwater ecosystems.

“There is a great need to understand how many salmon returned to rivers historically,” said Naiman. “Once we knew nutrients from salmon carcasses affected tree growth in certain species, it seemed possible to use tree rings to estimate how many salmon returned to a river in any one year. This was a ’trial’ project to see if the idea would work.”

Previous studies determined the 300-year history of sockeye salmon in the ocean and lakes around Alaska by analyzing nitrogen isotopes in lake sediment records. This method is impossible to use for river habitats, where sediment is constantly washed downstream and out into the ocean.

The researchers examined both cool, dry boreal forests and warm, moist Pacific Northwest rainforests. Drake and Naiman could not find evidence in the tree rings of a relationship between the pacific climate pattern and salmon populations or determine historical salmon populations in the boreal forests. Yet in the rainforests, where they originally did not expect to find a strong correlation, the scientists noticed a pattern in the tree rings which corresponded with the records of salmon abundance along the river’s edge.

Using this information, the scientists set to reconstruct salmon numbers as far back as 1820. They verified their information by comparing records of commercial salmon catch numbers with the tree core samples, and then filled in the gaps for the missing human records of salmon populations.
“These reconstructions follow the general ups and downs of historical catch records, but we need to better understand the relationship between tree-ring growth and salmon before we can say this method is accurate.” said Drake.

Another method which Naiman and others are currently looking into involves the measurement of nitrogen in tree rings. As the salmon decay, a particular type of nitrogen can be detected in soils and leaves near streams. These nitrogen isotopes have been measured in previous studies and found to relate to the massive deaths following spawning salmon.

According to Naiman, “Measuring nitrogen isotopes in tree rings is potentially more accurate, although much harder to determine.”

Ecology is a peer-reviewed journal published twelve times a year by the Ecological Society of America (ESA). Copies of the above article are available free of charge to the press through the Society’s Public Affairs Office. Members of the press may also obtain copies of ESA’s entire family of publications, which includes Ecology, Ecological Applications, and Ecological Monographs. Others interested in copies of articles should contact the Reprint Department at the address in the masthead.

Founded in 1915, the Ecological Society of America (ESA) is a scientific, non-profit, organization with over 7800 members. Through ESA reports, journals, membership research, and expert testimony to Congress, ESA seeks to promote the responsible application of ecological data and principles to the solution of environmental problems. For more information about the Society and its activities, access ESA’s web site at: http://www.esa.org.

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