Assessment of trophic-level responses and coho salmon (Oncorhynchus kisutch) producti
By G. B. Kyle
Fisheries Research, Volume 20, Issues 2-3, August 1994, Pages 243-261, ISSN 0165-7836, DOI: 10.1016/0165-7836(94)90086-8.
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.