Dworshak Dam Impact Assessment and Fishery Investigation and Trout, Bass and Forage Species: Combined Project Completion Report.

Abstract

Dworshak Reservoir pool recommendations contained herein are independent of conditions for upstream or downstream anadromous fish migration and of any other purposes not specifically stated. Nothing in this report shall limit or restrict future water rights claims or flow recommendations made by the Nez perce Tribe for any purposes. The authors also recognize that the needs of the resident fish studied may conflict with the needs of downstream anadromous fish. This report makes no attempt to resolve the conflicts or recommend priorities among alternative uses of Dworshak Reservoir water. The kokanee fishery declined during our study from a harvest of 206,000 kokanee and a catch rate of 1.5 fish/hour in 1988 to 95,000 kokanee at a catch rate of 0.8 fish/hour in 1990. Limited surveys in 1991 indicated a further decline in catch rates to 0.5 fish/hour. The Nez Perce Tribe (NPT) and the Idaho Department of Fish andChanges in the kokanee fishery reflected population changes in the reservoir.Mid-water trawling in 1989 indicated the reservoir contained 13 kokanee of harvestable size per acre but that dropped to 2 kokanee per acre in 1991. Angler satisfaction also declined throughout the study from 37% rating the fishing as poor in 1988, to 55% giving it this rating in 1990.Nearly all anglers cited low numbers of fish caught as the reason for the rating.Management goals should therefore be towards more numerous but slightly smaller kokanee to maximize angler satisfaction.Dworshak kokanee have exceedingly low annual survival rates; much lower than other kokanee populations.Over 80% of yearling kokanee die before recruiting to the fishery the following year.Losses of kokanee through Dworshak Dam appear responsible for the high mortality rates. As many as 83,000 to 235,000 kokanee of a single age group were estimated to be lost annually.The resulting low densities reduced catch rates from 1.5 to 0.5 fish/hour and likely reduced fishing effort by 66%. Low kokanee density has, however, triggered good growth rates. Kokanee averaged 11 inches by July of their third summer in the reservoir. This compares to 7.5 inches in Coeur d'Alene Lake, and 8.3 inches in Lake Pend Oreille. Kokanee in these other lakes live to be 1 to 3 years older than Dworshak Reservoir kokanee and so ultimately make up some of the difference in length.During our study, Dworshak Reservoir was much more nutrient poor than when it was first filled in 1971. We would have expected nutrient input from inundated vegetation and soils to have stabilized, however, our results indicated the reservoir may still be declining in nutrient status.Near-shore habitat has also been altered since initial inundation. Vegetation along the shoreline has been eliminated due to fluctuating water levels and wave action. Cover and food production for littoral fish species have been adversely affected by this change. Abundance of redside shiners, an important forage species, peaked only a few years following initial impoundment. This species was in decline prior to the reservoirwide...

Resident Fish Planning: Dworshak Reservoir, Lake Roosevelt and Lake Pend Oreille.

“…interest in the sections below. Additional information on the reservoir and its fish populations can be found in Maiolie et al (1992).…”

“…No specific strategies have been developed to protect westslope cutthroat trout genetic integrity. The problem of potential conflicts between stocking rainbow trout and the management objective for westslope cutthroat trout has not been addressed; however, a genetic inventory of cutthroat in the North Fork Clear-water drainage is needed and recommended (Maiolie et al 1992).…”

“…However, late spawning kokanee were primarily shore spawners and have been voir. Spawning habitat does not appear to be fully seeded even in years when the population density is within the target management range (Maiolie et. al.…”

“…Habitat requirements by life stage are shown in Figure 6. Smallmouth bass spawn in shallow (0 to 1 S m), warm water when shoreline water temperatures reach WC, which is about June 1 in Dworshak Reservoir (Maiolie et al 1992). Spawning takes place over a period of about 1 month, and if a first spawning fails to hatch, smallmouth bass will frequently spawn a second time.…”