Project/contract history:

     Dworshak Dam was built in 1971 by the Army Corps of Engineers (Figure 2 - see proposal).  This 218.8 m (718 ft) high dam irrevocably blocked the North Fork of the Clearwater River for access to hundreds of miles of tributaries for anadromous fish spawning, and flooded 86.9 km (54 mi) of riverine habitat for resident fishes.  The resident fisheries that were developed in the reservoir were intended to mitigate for some of these losses; however, they were only partial mitigation for the historic losses.  Current fish mitigation is inadequate for the reservoir operations that continue to severely impact native and non-native resident fish in Dworshak Reservoir and the North Fork Clearwater ecosystem.  In addition, the productivity of this ecosystem has been significantly reduced due to the loss of "marine derived nutrients" from anadromous salmon and steelhead that no longer access the drainage (Cederholm et. al. 2000).
     Kokanee are the best-adapted species for this fluctuating reservoir since they occupy the pelagic, offshore areas and spawn in tributary streams.  Their densities have exceeded 100 adult kokanee per hectare, and harvest of kokanee by anglers has exceeded 200,000 fish in some years.  In addition, kokanee are likely functioning in a similar role as historic anadromous fish runs by providing an important prey source for other resident fish, including threatened bull trout.  They also contribute to the productivity of the reservoir and its tributaries.
     Although kokanee may serve as a surrogate to the ecosystem, bull trout and other resident fish may still be limited by reductions in available forage, aquatic macroinvertebrate biomass and taxonomic richness, and reduced growth rates due to loss of anadromous fish production and the nutrients that large anadromous carcasses provide (Clearwater Subbasin Plan, section 8.3.1, pg. 342).  Declining reservoir productivity, limited food supply and nutrient levels has been suggested by Berry-Martin (1975) and Bennett (1997), as a critical limiting factor to stable fish populations in Dworshak Reservoir.  Rieman and Meyers (1992) clearly demonstrated the positive relationship of reservoir productivity and kokanee yield.  Bennett (1997) suggested this factor should be addressed before emphasis is placed on intensive fisheries management.
     The addition of nutrients to increase fish abundance is not a new science.  It is consistently utilized in aquaculture to increase fish biomass and has also proven to be successful in recovering wild fish populations as well.  Several nutrient enhancement programs in Canada have resulted in increased production of zooplankton and kokanee populations including experiments in Kootenai Lake (Stockner 1995, Ashley et al. 1999) and Arrow Reservoir, BC (Pieters et al. 2003).
    

FY 07 SOW summary:    

     The IDFG fish management objective for kokanee in Dworshak Reservoir is to maintain densities of 30 to 50 adult kokanee per hectare on an annual basis, at catch rates of at least 0.7 fish/hr, at a 30 cm size (Idaho Department of Fish and Game 2001b).  This project addresses this objective through increasing and stabilizing kokanee populations, thereby providing more forage and nutrients, and further enhancing ecosystem productivity to benefit all resident fishes.
     The U.S. Army Corps of Engineers Walla Walla District recently contracted Dr. John Stockner to evaluate the current state of the reservoir and then developed a prescription for a 5-year nutrient enhancement experiment on Dworshak Reservoir (Stockner and Brandt 2005).  USACE Dworshak Project personnel will be purchasing the needed fertilizer and equipment and performing the nutrient applications.  Our project staff will work cooperatively with both Dr. Stockner and the U.S. Army Corps of Engineers to assess the effectiveness of nutrient additions to increase reservoir productivity and enhance kokanee size and/or abundance.