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Dworshak Dam Resident Fish Mitigation

Data Categories Size at age
Age and growth
Population estimates
Dates of Data 2003 TO 2017
Data Set Status In work
Data Set Update Schedule Annually
Date Data Set Published on StreamNet Data Store 01-16-2018
Project Name & Number 2007-003-00
Purpose of Data Set The goal of this project is to improve resident fisheries in Dworshak Reservoir as partial mitigation for losses from the construction of Dworshak Dam and continuing impacts from ongoing dam operations. Dworshak Dam was built in 1971 by the U.S. Army Corps of Engineers (USACE). 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 production 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 salmonids that no longer access the drainage.
Kokanee are the best-adapted species for this fluctuating reservoir since they occupy the pelagic zone and spawn in tributary streams. Kokanee densities have exceeded 100 adults per hectare, and angler harvest has exceeded 200,000 fish in some years. In addition, kokanee function similarly to historical 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 for anadromous fishes in 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 provided (Clearwater Subbasin Plan, section 8.3.1, pg. 342). A limited food supply, due to declining reservoir productivity and nutrient levels, has been suggested as a critical limiting factor to stable fish populations in Dworshak Reservoir.
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 and catch rates of at least 0.7 fish/hr, at an average length of at least 25 cm. This project addresses this objective through supplementing the reservoir with nutrients in an effort to increase the efficiency of the food web. This will result in more desirable phytoplankton community (i.e. edible taxa) and increased zooplankton abundance, which should, in turn, provide more forage for kokanee. While kokanee will be the primary species benefiting from this project, it will also benefit other resident fish throughout the entire ecosystem. An improved kokanee population provides forage for the reservoir’s bull trout and smallmouth bass. Also, having 300,000+ adult kokanee migrate up tributary streams and die each fall will add nutrients to these stream systems, thereby enhancing fluvial fish populations above the reservoir.
This project will be conducted jointly with the USACE. The USACE Walla Walla District recently contracted Dr. John Stockner to evaluate the current state of the reservoir and develop a prescription for a 5-year nutrient enhancement experiment. The USACE will be purchasing the needed fertilizer and equipment and performing the nutrient applications, while IDFG 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 or abundance.
In order to assess the effects of the nutrient supplementation, IDFG will monitor reservoir limnology at eight limnological stations; seven throughout Dworshak Reservoir and a single station in the North Fork Clearwater River below Dworshak Dam (NFC). Nutrient treatments will occur in the main reservoir and the North Fork Clearwater Arm, therefore stations in these areas will represent the treatment area. Since the Elk Creek Arm and Little North Fork Arm will not receive any nutrient supplementation, EC-6 and LNF-3 will serve as controls. A detailed description of the monitoring efforts and study area can be found in the QAPP attached to this project in Pisces.

Figure 1. Map of Dworshak Reservoir, major tributaries, reservoir sections, and limnological sampling stations.
Summary / Abstract This data set consists of several types of data collected on kokanee from Dworshak Reservoir. It includes biological data collected from fish captured in a mid-water trawl, along with population estimates derived from the combined use of acoustic and trawl data. It also includes associated measures of the population calculated from these, including biomass, production and mortality by weight. It further includes annual growth data obtained from length back calculations using scales. In addition to the data collected from fish in the reservoir, it includes counts of spawning fish in index tributaries above in the reservoir, and biological data collected from spawning fish.
Broad Biological Groups fish
Taxa Kokanee Oncorhynchus nerka (Walbaum, 1792)
Location Dworshak Reservoir
North Fork Clearwater River
NPCC Subbasins (2001 Subbasins) Provincial (Mountain Snake/Clearwater)
Dams Dworshak Dam
Keywords Dworshak
Lead Person and Organization That Created the Data Set Idaho Department of Fish and Game (IDFG)
Other Participating Organizations Funding provided by Bonneville Power Administration
Contact Person for Questions About the Data Name:  Sean Wilson
Organization:  Idaho Department of Fish & Game
Address:  3316 16th St
Lewiston, ID  83501
Phone:  2087504244
Broad Category of Methods Field
Data Collection Methods mid-water trawl
spawner counts
File Formats MS Excel 2016
Data structure description See data set files.
URL where updated data may be available
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File Size: 8.7 MB