(Kingston, ON) - Queen's researchers using paleoenvironmental techniques to study West Coast salmon populations over the past 2000 years have found that surprisingly wide fluctuations occurred long before humans began fishing, or concerns were raised about depleted stocks. The researchers say they were shocked to document very low population numbers in the period ca.100 BC to 300 AD, solely as a result of natural fluctuations.
The results of their findings will be published in the April 18 issue of Nature.
"This blows our notion of salmon population dynamics right out of the water!" says paleolimnologist Irene Gregory-Eaves, a doctoral student with Queen's Dept. of Biology and member of the international team of researchers. "We never imagined that we would see such extremely low numbers in salmon, lasting for hundreds of years."
Dr. John Smol, co-head of Queen's Paleoecological Environmental Assessment and Research Laboratory (PEARL), and member of the international team of researchers, agrees. "We expected to see population drops due to human influences, but what was so surprising was the magnitude and duration of the decline from ~100 BC that was maintained until about 300 AD." The numbers then increased very gradually, peaking at about 1200 BC. Since then there have been fluctuations, but nothing to match the magnitude of those earlier, natural changes.
"Salmon are important ecological, economical, and cultural resources in the North Pacific region, and their response to future climatic change is of great uncertainty," says Smol. Human interference - in the form of dams, over-fishing, and hatchery mismanagement - tends to skew historical records of salmon population changes, he continues. "Records of catch abundance are short, and not representative of the natural system."
The research team - comprising Dr. Bruce Finney from the University of Alaska, Dr. Marianne Douglas from the University of Toronto, and Gregory-Eaves and Smol from Queen's - is creating "paleo-maps" that show population changes over time and across geographical regions.
"For this to work in a lake record, you need a fish such as the sockeye salmon from the North Pacific, that comes back up-river to die," says Smol. "Salmon that have lived in the ocean come back to their home lakes with a higher nitrogen isotope signature than the other freshwater nitrogen sources. When the salmon carcasses decompose, nutrients are released from their bodies and analysis of nitrogen isotopes in the sediments provides us with a means of tracking salmon in the freshwater system.
"We have also looked at diatoms, which are single-celled algae that are responsive to changes in nutrients, and leave 'fossils' in the sediments," Smol continues. "Analysis of diatom assemblages tells us about other changes in water quality that can be related to past salmon abundances."
To collect samples for this kind of study, meter-long tubes are lowered into the water, down to the sediments. Sediment cores retrieved from the tubes are "sliced up" and dated, then diatom and isotopic analyses are done. "In this way, the environment can be reconstructed," explains Smol. "We have the time period, the diatom fossils, and the isotopes. Then we put the whole picture together."
The study suggests that climatic change is the primary cause for past fluctuations in salmon populations. The synchronous response of fish populations (including salmon, sardines and anchovies) along the West Coast of North America indicates that dramatic shifts in the North Pacific have occurred over the past 2000 years. That being said, humans have also contributed to the decline of salmon populations, particularly at the southern end of their range, over the past 100 years.
Now that an era of global warming has begun, the fate of fish populations is becoming even more problematic, says Gregory-Eaves. "We've introduced a whole spectrum of stressors, like rapid climate change, dams, fishing, and hatchery policies. The question arises: 'How much stress can the salmon take before they drop out completely?'"
Another interesting find from their study is the response of First Nation populations to salmon abundance. Based on a comparison of their records with archeological data, such as hunting and fishing artifacts equipment, Gregory-Eaves notes a strong relationship between human cultural developments and available resources. When salmon numbers were low, people turned to seal and other foods; and when the salmon returned, they switched back again.
The research team believes their technique for studying fish populations is very powerful, and will be used increasingly to compare "natural" areas with those that have experienced specific interventions such as dams and controlled runs. In the next phase of their investigation, they hope to look at salmon populations from as far back as the formation of these lakes during the last Ice Age - 12,000 years ago. They have also initiated research projects on a far broader geographic scale, from Alaska down to the Pacific Northwestern US states, and inland to regions such as the Yukon.
Journal
Nature