Looking at Salmon Runs in the Days of Columbus
Imagine it's the year 1500 A.D. Alaska is a true wilderness, populated only by scattered bands of Native people. Russian frontiersmen haven't even explored Siberia yet; they won't hit the Alaska coast for another 200 years. On Kodiak Island, 1 to 1.5 million sockeye salmon nose their way up the Karluk River to spawn in Karluk Lake. Now imagine counting those sockeyes. In effect, that's what Bruce Finney and his colleagues at the University of Alaska's Institute of Marine Science did recently.
Finney, an assistant professor of marine science at IMS, tallied the salmon that lived in the lake 500 years ago by using traces of nitrogen found on the lake bottom. When salmon spawn, die, and decompose, their bodies release nutrients, such as nitrogen, that were stored in living flesh and bone. Salmon collect a unique type of nitrogen in their bodies called nitrogen-15, which is easy for researchers to identify in a lab.
After salmon dissolve and release nitrogen-15 into the water, plankton ingest the nitrogen. When plankton die and sink, they leave behind a history of salmon abundance in the layered muck.
Using the nitrogen-15 method, researchers can take an educated stab at how many salmon lived in the lakes before humans began dropping nets over the sides of boats. Scientists also can compare fish abundance with climate records of centuries ago to look for a relationship between fish numbers and climate change.
Last fall, Finney and UAF Graduate Student Susan McNeil used a hand-corer to extract 2.5-inch-wide, 20-inch-long plugs of sediment from the bottom of 200-foot-deep Karluk Lake. Within the 20-inch cores were 500 years of gunk from the lake bottom.
Finney and McNeil dated slices of the core by finding nature's bookmarks in the form of volcanic ash. An ash layer from the 1912 eruption of Mount Katmai on the Alaska Peninsula, for example, is visible as a thin gray layer within all the cores removed from Karluk Lake. The researchers used radio-isotope methods to further hone the year estimate. After dating sections of the core, Finney and McNeil compared current salmon runs and nitrogen-15 levels in Karluk Lake with ancient levels.
When the researchers charted nitrogen-15 levels from 1500 A.D. to the present, they found that a good number of sockeyes (also known as reds) were returning to the lake at about the time Columbus bumped into America. Roughly the same number of salmon returned to the lake each season until the early 1800's. Then, nitrogen-15 in the lake took a nose-dive that Finney said might be attributed to a period of cooler temperatures.
Later, the nitrogen-15 in the lake increased dramatically until about 1900, a rebound of reds Finney interprets as a possible result of warmer temperatures and changes in ocean conditions. The next dive in nitrogen-15 levels began in the mid-1900's and low levels continue today. Some researchers attribute this skid to overfishing in the ocean, where sockeyes live for one to three years before returning to their birthplace.
As salmon return to cigar-shaped, 12-mile-long Karluk Lake this year, their passage up the Karluk River will be counted by biologists who sit in towers above the river. Thanks to the recent work of Finney, McNeil and others, the equivalent of several centuries of fish counting can be performed in a few weeks.
