Page 815 - The Toxicology of Fishes

Mining Impacts on Fish in the Clark Fork River, Montana: A Field Ecotoxicology Case Study 795

surveys of wild fish, evidence of exposure in the wild, evidence of toxicity from foods from the water
body, and biochemical signs of harm in survivors. It is unlikely that any single experiment or measurement
can fully explain the metal effects. Fish kills and effects on young trout held in situ were discussed
earlier. Other lines of evidence supporting metal effects also have been systematically explored in the
Clark Fork.

Abundance and Diversity of Trout Are Reduced
Trout are native to the upper Clark Fork River, as elsewhere in the Rocky Mountains of Montana. A
history of depauperate populations provides the first-order suggestion that mine wastes may have affected
trout populations in the Clark Fork River. For nearly a century, the Upper Clark Fork contained no trout
because of the hazardous materials released by mining, milling, and smelting operations (Johnson and
Schmidt, 1988). Even though fish began to reappear in the Clark Fork River in the 1950s, periodic kills
were indicative of continued stress. Fish returned after such episodes, probably because of immigration
from tributaries, but effects on abundances were likely.
Modern populations of wild trout are less abundant and less diverse than expected, but identifying
the cause is challenging. Historical data do not adequately define baseline conditions. A substantial
amount of time has passed since the release of hazardous substances began. Possible control areas
upstream from impact areas do not exist because the headwaters of Silver Bow Creek and the Clark
Fork River have been contaminated with hazardous substances. Finally, separation of mining impacts
from the influences of other stressors can be problematic. Despite these complexities, a consistent body
of evidence points to metal contamination as a major cause of the disturbed fish populations in the Clark
Fork River.
With the advancement of electrofishing gear, the State of Montana began conducting trout population
estimates in the Clark Fork River in the late 1960s, using the adjusted Petersen mark–recapture method
(Chapman, 1951). The Peterson method makes a number of assumptions, most of which were likely
violated in the Clark Fork River (Hillman, 1991). The population estimates, therefore, may overestimate
true population sizes (systematic error); however, trends in numbers may be valid if there is continuity
in sampling protocols. Estimates from the two most common upstream sites (immediately below the
ponds and below Deer Lodge) rarely recaptured small brown trout, so the population numbers are for
brown trout larger than 6 inches. Sampling was sporadic spatially and temporally; data are most complete
for spring sampling, so those are reported here.
Figure 19.11 shows the pond anomaly in fish abundance, in context. There are no fish upstream of
the ponds. High abundances occur in one very short reach of the river, but below the pond anomaly
trout abundance is low throughout the Clark Fork. The Deer Lodge site averaged 225 trout per mile
from 1967 to 1998. The highest density of brown trout was 356 trout per mile in 1987; 89 trout per
mile were found in 1989. During spring 1987, the State of Montana conducted population estimates
from the settling ponds to Milltown Dam, at 80 contiguous sections, which they combined into 31
sections to improve density calculations. Trout numbers were less than 500 trout per mile near Deer
Lodge and less than 100 trout per mile between Flint Creek and Rock Creek. Trout numbered less than
500 trout per mile downstream below the confluence of Rock Creek.
Trout numbers for the Clark Fork are considerably lower than in its tributaries (Knudson, 1984),
although physical conditions differ among the streams. The Blackfoot River has the highest trout popu-
lations in the upper Clark Fork system, numbering about 2500 trout per mile. Warm Springs Creek and
Rock Creek average about 1500 trout per mile. Flint Creek and the Little Blackfoot River support about
1000 trout per mile. Knudson (1991) compared numbers of trout in the Clark Fork River with other
Montana streams, including the Madison River downstream from Ennis which has 4600 trout per mile.
Clark Fork tributaries and other trout streams in Montana also contain a greater diversity of trout species
than the upper Clark Fork River; for example, the Blackfoot River, Rock Creek, and the Little Blackfoot
River support populations of brown, rainbow, cutthroat, bull, and brook trout (Hillman and Chapman, 1995;
Hillman et al., 1995; Knudson, 1984). Willow Creek supports brook and cutthroat trout, while German
Gulch Creek supports cutthroat, brook, and brown trout (Camp, Dresser, and McKee, Inc., 1991). Lower
river tributaries, such as the Bitterroot, Flathead, and St. Regis rivers, support brown, rainbow, brook,
cutthroat, and bull trout. Outside the Clark Fork watershed, the Madison, Gallatin, Yellowstone, and