Studies of caged, tagged, and wild fish populations show that reservoir drawdown and dam breaching have had deleterious impacts on fish in the reach of the Clark Fork between Milltown and the Bitterroot River. But these impacts are considerably less downstream of the Bitterroot, and fish populations have increased significantly in the Clark Fork and Blackfoot rivers above the former dam.
Caged Fish--The What and the Why: Caged fish are the proverbial “canary in the coal mine” since they cannot swim to avoid conditions they don’t like. This experiment looked at fish survival and condition in several locations upstream and downstream of Milltown Dam. Fingerling rainbow trout were placed in cages (3 cages per site, 25 fish per cage) at: Turah, Bonner, Milltown, Bitterroot, and Alberton.
Table--Percent Mortality of Caged Fish:
Observations and Conclusions re. Caged Fish Studies (from Fish, Wildlife and Parks):
Tagged Fish (Telemetry)--The What and the Why: Tagged fish are used to track movement and mortality during construction activities (construction causes changes in water quality). Rainbow trout have been tracked through two years of reservoir drawdown in 2006 and 2007 as well as during and after dam breaching in 2008.
Observations and Conclusions re. Telemetry (Tagged Fish) Studies:
2006 and 2007 Reservoir Drawdown:
Population Monitoring--The What and the Why: Fish population was estimated by electro-fishing and counting the number of fish and species in a given reach of river from 2004 to 2008. Populations were assessed in the following reaches: Turah, Johnsrud (Blackfoot), Milltown, Bitterroot, Huson. Overall condition was also assessed on some of the captured fish.
Observations and Conclusions re. Fish Population Monitoring:
1. Between 2004 and 2007, the number of trout per mile changed significantly both upstream and downstream of the dam. Since all sections of the rivers changed in the same way, it’s likely a response to overall climate.
Table--2008 Trout Density (number of fish per mile, 7 inches and larger):
This chart shows trout numbers from 2004 to 2008, but note that this shows the number of trout per kilometer (numbers in the table are per mile - there are 1.6 kilometers per mile): 2004-2008 Trout Numbers
Macroinvertebrates (aquatic insects) took a hit this spring, but the impact was mostly confined to the reach between the dam and the Bitterroot River and does not appear to be significantly worse than in some previous years. Aquatic insects are good indicators of environmental condition because they have limited mobility, they have predictable associations with specific types of habitat, and certain species have specific tolerance levels for different types of pollution. Not only that, but they are an important food source for fish. These charts show results for population, bio-integrity, and species diversity. (Data are from US EPA presentation. The full report from contractor Dan McGuire is not yet available, but will be posted here as soon as it is).
Download Macroinvertebrate Data as PDF