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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.

Aquatic Life

Montana Fish, Wildlife and Parks is following a three-pronged approach to tracking the health of the fishery during the Milltown project, including:

Caged fish to determine health and mortality
Telemetry (tagged fish) to determine movement and mortality
Fish population monitoring

All fisheries data courtesy of David Schmetterling of Montana Fish, Wildlife and Parks

Aquatic Insects

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:

Year Blackfoot Turah
Milltown Activity
Reservoir drawdown
Reservoir drawdown
Dam breaching

Observations and Conclusions re. Caged Fish Studies (from Fish, Wildlife and Parks):

1. Reservoir drawdown was a source of significant stress to fish, but it was not a source of acute mortality or toxicity alone
2. The cumulative effects of sediments, sediment composition (organic matter with associated bacteria and pathogens), and unprecedented high water temperatures in 2006 and 2007 all contributed to the near total mortality of caged fish while the reservoir was drawn down
3. Mortality in the Bitterroot in 2008 was 32%, and mortality at Alberton was similar to mortality in the Bitterroot, but results were confounded by vandalism of the Alberton cages
4. Harmful effects were mainly between Milltown and the Bitterroot
5. Combined mortality of fish upstream of the dam (Blackfoot and Turah) was less than downstream at Milltown, but not by much

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:
1. Fish did not travel significantly up- or downstream, even when water quality was less than ideal
2. Significant mortality downstream of the dam compared to upstream
3. Less mortality downstream of the Bitterroot
2008 Dam Breaching:
1. Fish traveled significantly more than during the previous two years in the reach between Milltown and the Bitterroot
2. Tagged fish downstream of the dam traveled more than tagged fish upstream of the dam
3. Less movement was observed downstream of the Bitterroot
4. Mortality below the dam was less than previous years, but more than in the control reach
5. Fish passage past the dam site was impeded for about one week during peak flows.  The rest of the time, fish were able to swim past the dam and through the bypass channel.

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.
2. Large increases in number of trout per mile in 2006 (1st year of reservoir drawdown)
3. Significant declines in all reaches in 2007 (2nd year of reservoir drawdown)
4. In 2008, there was a slight increase in the Bitterroot, a slight decrease in the Huson reach, a significantly greater increase at Turah above the dam (50% more), and significantly fewer trout per mile between Milltown and the Bitterroot (60% less)
5. Changes in the Milltown section were a combination of mortality and movement.
6. Trout condition was worse in the Milltown reach than upstream of the dam or in the Bitterroot, but condition of fish improved below the Bitterroot in the Huson section
7. What about the northern pike that used to thrive in Milltown Reservoir?  Without flat water, their preferred habitat is gone. In 2004-2007, there were about 80 northern pike per mile.  In 2008, the population was too small to get an estimate.

Table--2008 Trout Density (number of fish per mile, 7 inches and larger):

Location Fish per mile
Standard deviation
Turah 378 62
Johnsrud 545 41
Milltown 123 17
Huson 206 32
Bitterroot 356 54


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

Aquatic Insects

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