Assessment of trophic-level responses and coho salmon ( Oncorhynchus kisutch) production following nutrient treatment (1981–1986) of Bear Lake, Alaska

Abstract

Bear Lake, located on the Kenai Peninsula in south-central Alaska, was treated with nutrients (mainly nitrogen) during 1981–1986 to increase coho salmon ( Oncorhynchus kisutch) smolt production through stimulating zooplankton, a known food source for juvenile coho salmon in Bear Lake. Nutrient additions increased seasonal mean total nitrogen concentrations by 145% and more than doubled the ratio of inorganic nitrogen to total phosphorus. Consistently higher seasonal mean chlorophyll a concentrations were not detected, but a significant increase in zooplankton density was detected during the period when juvenile coho salmon were absent (June–July), but not when juveniles were present (August–October) in the limnetic area of the lake. After treatment, a 25% average increase in the efficiency of Bear Lake to produce coho salmon smolt biomass was observed, and the composition of fingerlings that migrated as age-1 smolts increased by 30%. I believe the finding of no significant change in zooplankton density after nutrient treatment during August–October was due to expanded predation by juvenile coho salmon. From results of this nutrient enrichment experiment, it is clear that for a full understanding of trophic efficiency, further research is needed to integrate trophic-level dynamics with energy flow.