In recent
years scientists have recognized that, along with commercial fishing and other
human activities, climate is a significant factor in the survival of adult salmon.
Indeed, climate is one of many variables fish managers are trying to incorporate
into their models for predicting how many salmon will make it back from the
ocean to their natal rivers and lakes to spawn. The most reliable method to
date, is to count how many salmon return after spending only a year in the ocean.
If conditions are good, the siblings of those first-year salmon will spend a
longer time in the ocean and also have a strong survival rate. “Climate and ocean conditions together form the backdrop on which the salmon can thrive or not thrive depending on the stage salmon are in, and all other things fall on that potential,” says Oregon State climatologist George Taylor.
Bruce Finney (pictured) and colleagues use lake sediment cores to identify the productivity of sockeye salmon during the last 2,000 years. Photo by Jim Larson, U.S. Fish & Wildlife Service
Until now, swings in salmon population linked to climate were known to last only decades, not centuries. But the longest track record for salmon went back only 300 years. Bruce Finney of the University of Alaska in Fairbanks and colleagues had set that record two years ago using lake sediment cores. Finney had followed the unique life cycle of sockeye salmon. Unlike other salmon species, sockeye return to lakes rather than rivers to spawn and die. There, the sediment collects over time without washing away. By taking deeper lake sediment cores, the team have extended their knowledge of salmon populations back to 2,200 years ago. They reported their findings in the April 18 Nature. About a meter in length, the cores gave the team a new perspective on the highs and lows of sockeye salmon productivity in the northeastern Pacific Ocean.
Locked in the sediments of Alaska’s Karluk and Akalura lakes are chemical and biological indicators that reflect past abundances of sockeye salmon spawners. Sockeye will sometimes spend up to four years in the Pacific Ocean feeding on krill, squid and other fish. With their diet comes a body mass loaded with nitrogen-15, a stable nitrogen isotope. The fluctuations in the number of salmon returning to the nursery lakes left signals the scientists read like tree rings in the freshwater environment. After spawning a new brood, the adult salmon would die and litter the lake bottom with nitrogen-15 and other nutrients that would provide sustenance for zooplankton and eutrophic diatoms, a group of algae characteristic of a high- nutrient environment. These in turn provided baby food for the newly hatched salmon waiting to continue the cycle with their own trip to the ocean. By contrast, a neighboring lake without salmon showed consistently low levels of nitrogen-15 and oligotrophic diatom species, typically found in low-nutrient lakes.
Finney and his colleagues analyzed every half centimeter of the cores for nitrogen-15 and diatom species, scooping from the mud the equivalent of what would fall during a 20-year period of deposition. “It was a great surprise to see the dynamics have changed quite a lot,” says Marianne Douglas of the University of Toronto in Ontario, Canada. She was shocked to see salmon abundance fall to such dramatic lows without human influence. “It’s the first time we’ve seen good hard data that huge dynamics in the population occurred without being altered by human activities.”
They found that the population of sockeye salmon in Karluk lake around 200 B.C. was about as high as it was when commercial fishing began at the Karluk estuary in 1882: about 3 million. But beginning around 100 B.C. the return of salmon to the lake steadily decreased, hitting a record low of about 100,000 or 200,000 fish around A.D. 250. The population slowly crept back up, but remained well below the average mean for another 550 years. Not until A.D. 1200 did the salmon reach the 3 million mark again. From then until the start of commercial fishing, the return of salmon to Karluk lake remained relatively high. Over the last 100 years the numbers have again decreased dramatically; the authors attribute this dip to both commercial fishing and climatic changes.
But the climatic influence that may have caused the centuries-long, low productivity 2,000 years ago is still not clear. The authors note that around 100 B.C. a warming of marine waters occurred in the Santa Barbara basin in California, presumably lowering ocean productivity in the north. The rebound of salmon productivity in A.D. 1200 corresponded to a period of glacial advances in southern Alaska and the Canadian Rockies. “More work is needed to try and delineate what is the relationship between climate and salmon beyond just colder waters lower productivity in the Gulf of Alaska. If the ocean is too warm that’s not too good either,” says Irene Gregory-Eaves of Queen’s University in Kingston, Ontario, a co-author of the Nature report. “If we can nail that down in a more definitive way and get together with those working on climate projections then we can better understand the response of salmon to climate changes in the future.”
The concern is if a climatic shift topples the natural productivity levels at a time when human impacts have already brought the salmon population to another low. Adds Finney: “The global warming projections indicate that they may see changes that could be more extreme and sudden than in the past.”
Christina Reed
