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.”

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