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.

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:

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

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.

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):