Contract Description:

This contracting period represents the fourth year of a nutrient supplementation project on Dworshak Reservoir.  This project is a joint venture between the U.S. Army Corps of Engineers (USACE) and the Idaho Department of Fish and Game (IDFG).  The USACE will supplement the reservoir with nitrogen to promote an improved nitrogen to phosphorus  ratio and increased foodweb efficiency.  The IDFG will be responsible for collecting limnological samples, which will be used for in-season management of nutrient supplementation and for evaluating project effectiveness.  The USACE has contracted TG-Ecologic to interpret the results of limnological sampling for recommending in-season adjustments to the amount of fertilizer to be applied.  Fish monitoring will also be conducted by the IDFG and, combined with limnological data, will allow for an overall assessment of project effectiveness.

Both limnological and fish sampling during this contracting period will be similar to the last contracting period.  During the last contracting period, sampling was scaled down somewhat, including less sampling of the hypolimnion and eliminating zooplankton sampling below 10 meters.  This sampling scheme has provided adequate data to address the goals of the project.  Limnological sampling will be further evaluated to determine how much it can be scaled back as the project transitions from an experimental phase to an operations phase.  Future changes to the sampling scheme will focus on key parameters to be monitored and times of year which prove to be most useful in actively managing the nutrient applications.  We will also seek input in adjusting the sampling scheme to improve monitoring of downstream water quality.

The project has progressed well during the first three seasons.  Nitrogen supplementation has resulted in increased densities of picoplankton and edible plankton.  Although kokanee were not necessarily longer than typically expected during years of similar densities (although this analysis is preliminary), they were considerably heavier.  This has lead to positive feedback from anglers despite lower than average kokanee abundance.  While undesired blooms of inedible diatoms and blue-green algae did occur, we gained greater understanding of how the reservoir responds to nutrient supplementation.  We expect effects of the supplementation to be incremental, with benefits seen at a higher trophic level in each subsequent year.  With this in mind, any noticeable benefits to the kokanee population this early in the project should be viewed as encouraging.  We expect a high density of adult kokanee during this contracting period, which will allow us to evaluate kokanee growth for a wide range of fish densities since the supplementation project began. A thorough analysis of the effects of supplementation is underway and will be included in future reports.