CLIMATE CHANGE, FISHING ALTER SALMON ABUNDANCE
SCIENTISTS RECONSTRUCT 300 YEARS OF ALASKA SOCKEYE RUNS

October 26, 2000 — Clues left by decaying salmon at the bottom of five Alaska lakes point to climate change and over-fishing as two causes of the state's boom and bust salmon runs, according to a study by Alaskan and Canadian researchers to be published October 27 in the journal Science. The research was funded primarily by the University of Alaska Sea Grant College Program in Fairbanks, Alaska, and NOAA's Auke Bay Fisheries Lab in Juneau, Alaska.

"The lakes we studied on Kodiak Island and near Bristol Bay often had similar patterns of salmon abundance that corresponded to climate changes over the last three centuries," said Dr. Bruce Finney, the study's lead researcher and associate professor of marine science at the University of Alaska Fairbanks, Institute of Marine Science. "That's consistent with the belief that climatic and oceanographic changes are driving the sockeye populations up and down in sync over decades-long time scales."

Commercial fishing was found to have negative impacts on Kodiak Island salmon runs, the study also found.

Researchers determined the size of sockeye salmon runs going back 300 years by measuring levels of the stable nitrogen isotope, N15, in lake bottom sediments. As each generation of salmon returned home to spawn and die, their decomposing bodies left behind a nitrogen fingerprint in the sediment. Scientists took core samples and measured the nitrogen isotope in the sediment layers. From that, they calculated the relative size of salmon runs over time. Markers, such as ash from known volcanic eruptions, helped scientists affix a date to the layers.

"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," said Dr. John Smol, a co-researcher from Queen's University in Kingston, Ontario. "This provides us with a means of tracking the salmon nitrogen in the freshwater system through an examination of the nitrogen isotopes in the sediments."

Scientists then were able to link abrupt changes in the size of sockeye runs with large-scale climate shifts in the North Pacific Ocean, as indicated by sea surface temperature records and tree ring analysis. In general, sockeye runs were larger during periods of warm climates, and smaller during cold periods.

"We got some pretty strong signals," said Finney. Finney and his colleagues from Queen's University and the University of Toronto noted that salmon runs were low during the early 1700s and again in the early 1800s. "These periods were particularly cold, some of the coldest in the past several centuries, and we found that salmon runs were also quite low," said Finney.

Salmon runs became larger during warm periods, such as the late 1700s, mid 1800s, and early 1900s. However, their ability to measure changes during more recent decades was hampered by the influence of commercial fishing.

"Even though the catch went up in the late 1970s, and reached records in the 1990s, we don't see that in the core samples," said Finney. "That's because the management philosophy is to manage for a constant, optimal numbers of spawning salmon in each system. So, essentially they harvest the excess salmon, and the nitrogen levels in the sediment remained relatively constant."

Nitrogen, phosphorous, and other important nutrients from salmon carcasses help ensure the survival of young salmon in Alaska's otherwise nutrient-poor lakes. The nutrients are taken up by algae that are eaten by zooplankton, an important prey of juvenile salmon. The algae eventually die and settle to the lake bottom, carrying with them a nitrogen archive that can be used to calculate the size of the year's salmon run.

"The dead salmon fertilize the lakes with their own bodies, and everything else living in the lake benefits from this nutrient source," said Irene Gregory-Eaves of Queen's University. "When there is a strong salmon run, there are a lot of nutrients being released, and this, in turn, stimulates production of the lake's algae."

The study found that the 300-year record showed these nutrient links to be important to salmon survival, and are amplified by the effects of climatic change. Finney says salmon runs were largest in years of favorable climate and increased production of algae and zooplankton.

Commercial fishing, on the other hand, was found to have negative long-term effects on the size of Kodiak Island's sockeye runs. Harvests of sockeye salmon from the Karluk Lake system, for example, began to decline shortly after commercial fishing began in 1882. By the 1970s, salmon catches that had peaked at nearly four million fish declined to just 100,000. Finney says the decline was hastened by commercial catches, which deprived the lake of nutrients needed by subsequent generations of salmon.

"Our data suggest that the prolonged 20th century collapse of the Karluk sockeye fishery was driven in part by reductions in carcass-derived nutrients from over harvest," the scientists said in their report.

The findings have important implications for commercial fisheries management in Alaska. Traditionally, fisheries managers assumed the environment was constant when they calculated the maximum number of salmon fishermen could harvest. The study suggests a need for new, flexible management policies that take climate and lake nutrient levels into account.

"We found good evidence for a positive feedback loop between salmon-derived nutrients, climate, and larger salmon runs," said Finney. "Higher levels of nutrients support more juvenile salmon. More younger salmon survive, and more salmon eventually come back and die, releasing more nutrients. This positive feedback can increase the carrying capacity of the freshwater system."

While salmon runs in lakes on Kodiak Island seemed to have a built-in dependence on nutrients from decaying salmon, lakes studied near Bristol Bay continued to produce large numbers of sockeyes despite large catches by commercial fishermen.

"The Bristol Bay lake systems seem to be less affected by the boost of nutrients from decomposing salmon," Finney said. "One possible reason may be that the boost is small compared to nutrients coming from other sources."

Lakes studied for this research were: Karluk, Frazer, Red, and Akalura on Kodiak Island, and Ugashik, Becharof, and Tazimina near Bristol Bay on the Alaska Peninsula.

The Alaska Sea Grant College Program is a marine research, education and outreach service headquartered at the University of Alaska Fairbanks, School of Fisheries and Ocean Sciences. Alaska Sea Grant is one of 30 Sea Grant programs in the coastal and Great Lake states that is funded by NOAA in partnership with the states and private industry.

Relevant Web Sites
NOAA's Auke Bay Fisheries Lab

National Sea Grant College Program

University of Alaska Sea Grant College Program

University of Alaska Fairbanks, Institute of Marine Science

Photos

Media Contact:
Doug Schneider, University of Alaska Sea Grant College Program, (907) 474-7449 or Ben Sherman, Media Center for National Sea Grant College Program, (202) 662-7095

 

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