The Paleoproductivity of Sockeye Salmon: Assessment by Sediment Core Analysis
by Susan McNeil, Marine Sciences and Limnology, Institute of Marine Science
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 d15N profiles recorded in the lake sediments.
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.
Figure 1. Karluk Lake. Top figure shows sedimentary d15N vs. downcore depth (cm). Note the general decrease towards the top of the core. An approximate, tentative time-frame can be constructed for this core, recovered in 1994, from the location of an ash layer (indicated) which is likely the Katmai 1912 event. Bottom figure shows the escapement history for this lake since 1900 (ADF&G, written comm., 1994), with the bold line being a five-year running average. Note the significant general decline from the late 1920s to about 1980. Sedimentary d15N appears to correlate well with sockeye escapement.
Figure 2. Fraser Lake. Top figure shows sedimentary d15N vs. downcore depth (cm). Note the sharp increase in the upper few samples. An approximate, tentative time-frame can be constructed for this core, recovered in 1994, from the depth of the Katmai 1912 ash layer (indicated). Bottom figure shows the escapement history for this lake, in which salmon runs were established in the 1950s (Blackett, 1979). Note the strong correspondence between sedimentary d15N and escapement.
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