When it comes to declining stocks of wild Pacific salmon, causes and culprits have been hard to pin down. Scientists and fishery managers are faced with a tangled web of factors, as overfishing, shifts in ocean regimes, dams and habitat loss have all been blamed for declines.

But one team of researchers is showing that some of the best evidence lies buried in the mud, in the decomposed bodies of ancient salmon that have rotted away in the bottom of several Alaskan lakes. Canadian and American scientists have been analyzing sediment cores taken from five lakes, and their work shows that huge swings in salmon populations are nothing new.

"Two thousand years ago, at the time of Christ, there was a dramatic decline in salmon, unlike anything that we have seen in modern fisheries. Humans have undoubtedly affected populations, but when commercial fishing started, salmon numbers were at their highest levels in thousands of years. So we might have had a false impression of what to expect," says John Smol, a biology professor at Queen's University.

Smol was speaking at the annual general meeting of the Canadian Quaternary Association, held in Whitehorse in August, 2001. He is part of a team that has been tracking diatoms, nitrogen isotopes and invertebrates found in the sediments of salmon nursery lakes on Kodiak Island and near Bristol Bay in southwest Alaska.

The researchers have pushed our knowledge of salmon trends way back, and found that climate changes may have influenced salmon numbers long before humans ever began casting their nets into the water.

Most of the research has been on Karluk Lake on Kodiak Island, a major freshwater nursery for sockeye salmon. Commercial fishing began in the area in 1882, and historical data shows that the salmon catches started to decline soon after that time.

While overfishing influenced the twentieth century declines, the scientists' initial round of research showed that salmon numbers also dropped during two cold periods in the early 1800s and the early 1700s. Other sources, such as tree ring analyses and records of sea surface temperatures, confirm that these periods were cooler than normal.

"One of the big problems with salmon work is that all of the present information overlays the period of fishing. There is a lot of blame to go around when it comes to salmon stocks, but only the fossil record allows us to push back in time to see what the populations were like before fishing began."

Salmon have high levels of the stable nitrogen isotope N15 because they feed near the top of the food chain and put on most of their weight in the ocean. By measuring levels of N15 in the lake sediments, the researchers can determine how many salmon returned home to spawn.

They also analyze which species of fossil diatoms are present at different time periods as these small microorganisms indicate water quality. Their initial research on five lakes pushed the fossil record back 300 years, and was published last year in the journal Science. Since then they have managed to establish a 2000-year record on samples from three of the original lakes, showing even more striking cycles over this longer period.

But even though salmon numbers did rebound in the 1900s, the increase did not show up in the sediment samples. That is because once fishing began in earnest, the salmon were no longer contributing N15 to the lake bottom, and this change affected more than the scientific record.

"You hear stories about how people once fertilized their fields with fish," says Smol. "When salmon die the dead carcasses add fertilizer to the lakes. In some lakes more than half of the nutrients come from dead salmon."

The researchers suggest that nutrients from the salmon carcasses help to drive salmon productivity on nutrient-poor lakes such as Karluk. The nutrients are taken up by algae that are then eaten by zooplankton, an important prey of juvenile salmon. The researchers have also been analyzing the fossilized zooplankton found in the sediments, and they match the trends they inferred from other indicators.

Their data shows that these northern salmon runs were largest during times when the climate was warm. And times of more returning salmon meant more carcasses decomposing in the lakes, resulting in higher nutrient levels. Smol says their work could have definite implications for fishery management, suggesting a need for flexible policies that take climate and lake nutrient levels into account.

But he also says there is no guarantee that lakes located further south along the Pacific coast are following the same pattern as the Alaskan lakes, as large-scale regime shifts in the North Pacific Ocean might not follow the same cycles as those found in more southern waters.

Smol and his colleagues would like to sample lakes all along the Pacific coast. With data from a range of locations, they would be able to determine whether different salmon stocks have followed the same cycles.

He also hopes that they can eventually dig even deeper into those lake bottoms, and push their work even further back in time. "The long-term goal is a 12,000 year record, back to the time when salmon first arrived," he says.

Professor Smol can be contacted at smolj@biology.queensu.ca.