Evaluating Evidence for Historical Anadromous Salmon Runs in Eklutna Lake, Alaska

We assessed historical presence of sockeye salmon in Eklutna Lake, Alaska, prior to construction of a diversion dam on the downstream Eklutna River in 1929, using nitrogen stable isotopes measured in a lacustrine core 93 cm long. Sediments in the core were dated using varve counts, verified by Pb-210 and Cs-137 measurements. The basal date of the core was AD 1859, and varves became slightly thinner and less distinct after 1929. Sediments were primarily clastic with carbon content below 1%. Nitrogen isotope values were generally low and stable throughout the core, ranging from 1.5% to 2.5%. There is no statistical evidence for a change in isotopic composition after emplacement of the dam. In light of published evidence from oral history, cultural records, and habitat relationships that suggest sockeye salmon could have been present in the lake before 1929, we conducted a simple sensitivity test to assess the possibility that a small salmon run may have gone undetected by our technique. We found that a salmon run of up to 1000/year, and potentially as many as 15 000/year, would be possible without noticeably altering the measured isotopic composition of the sediments in Eklutna Lake. Our results provide no evidence that such runs occurred, but do not preclude the possible existence of a relatively small sockeye fishery in Eklutna Lake before 1929.