Nez Perce Tribe

Department of Fisheries Resources Management


Dworshak Dam Impact Assessment: Trout, Bass & Forage Species

The Nez Perce Tribe (NPT) and the Idaho Department of Fish and Game (IDFG) entered into separate intergovernmental agreements with the Bonneville Power Administration in a cooperative four-year effort to study impacts of Dworshak Dam operation on resident fisheries. The NPT Department of Fisheries Management focused on rainbow trout, smallmouth bass and forage fish and management recommendations, and the IDFG addressed kokanee population dynamics, kokanee entrainment, nutrients and management recommendations.  The 1992 final addressed work performed from 1987 through 1991.

During the course of the study, hybrids of westslope cutthroat trout and rainbow trout were collected in experimental gill nets and by hook-and-line, and the potential impact of stocking rainbow trout into Dworshak Reservoir for fishery mitigation became apparent.  Also, condition factors of smallmouth bass suggested a food limitation for younger age groups of smallmouth bass.

Suggested management recommendations for trout, bass and forage species emanating from this study included:

  • Conduct a pilot test to determine the feasibility of an active revegetation program.
  • Conduct a genetic inventory of cutthroat trout in the North Fork Clearwater drainage to determine the genetic purity of the endemic westslope cutthroat trout and to ascertain the extent of genetic introgression. If rainbow trout are determined to be a source of genetic deterioration for endemic westslope cutthroat trout populations, rainbow trout stocking for resident fish mitigation should be relocated off-site and outside of the North Fork Clearwater Basin. Alternative offsite stocking locations should be selected so as to avoid conflict with wild/natural anadromous and endemic resident fish.
  • Trout stocking in Dworshak Reservoir should be confined to westslope cutthroat trout or bull trout from broodstock endemic to the North Fork Clear-water drainage.
  • If rainbow trout stocking is continued in Dworshak Reservoir then we recommend periodic genetic monitoring of westslope cutthroat trout upstream from Dworshak Dam.
  • Reduce the minimum length limit for smallmouth bass from 305 mm (12 inches) to 254 mm (10 inches) to increase yield and attempt to improve body condition of 254 mm to 305 mm smallmouth bass.
  • Avoid downward fluctuations in pool level from June 1 through August 31 to prevent dewatering smallmouth bass nests.  This measure would need to be reviewed with threatened or endangered anadromous fish requirements.
  • Achieve normal full pool during June, if flood run-off forecasting allows, to avoid rising pool levels and associated temperature depressions in near-shore areas when smallmouth bass are spawning.
  • To promotes terrestrial invertebrates deposition, an important source of food for trout and smallmouth bass, do not initiate reservoir evacuation for winter flood control or hydropower prior to the September 1 date earmarked in the U.S. Army Corps of Engineers' current flood control operating curve. It is recognized that that drawdowns during this period could be required to protect threatened or endangered anadromous fish.

*This project is currently closed and no staff is assigned to it.

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