The sockeye is Alaska's most important salmon in terms of landed value. Productivity in freshwater systems has been shown to be an important control on sockeye escapement. Information on long-term changes in productivity, nutrient cycling and run size should aid in future fisheries management. Our studies involve two different limnetic systems: Karluk Lake and Fraser Lake on Kodiak Island. Historically, Karluk Lake has had large runs of sockeye salmon. Fraser Lake, the control lake, was not a salmon nursery lake until the 1950s when stocks were established and a fish ladder built for the migration of salmon into the lake. Historical escapement records are closely correlated with
In the marine environment 15N accumulates in the adult salmon tissues because they feed at an elevated trophic level. Atmospheric N2 is the other main source of nitrogen in lake environments introduced by stream runoff and nitrogen fixation. Atmospheric nitrogen by definition has a d15N= 0. Marine derived nitrogen's higher d115N permits its use as a tracer for adult salmon abundance in salmon nursery lakes. Migrating salmon do not take in food or water after entering freshwater (Mathisen, 1988); therefore, the body tissues retain their unique marine isotopic composition. After the salmon spawn and die, marine derived nutrients are taken up by phytoplankton. Zooplankton feeding on phytoplankton package the nutrients into fecal pellets which fall to the lake bottom. The total rain of sediments to the sediments is preserved as a record of lake productivity. The isotopic composition of nitrogen in sediments reflects the abundance of adult spawning salmon (escapement) for that year.
Stable isotope geochemistry can be used to compare marine derived nitrogen in the sediments with historical escapement data. Since the isotopic composition of the sediments closely corresponds to the salmon escapement numbers, a proximal link can be established for isotopic information that precedes the historical record. A longer sediment core from Karluk Lake is being analyzed now to determine prehistoric escapement trends.
Return to global glimpses