Mitigation for the Construction and Operation of Libby Dam

differences occur they usually indicate a shift in the trophic composition of the community.
Thus these changes may indicate changes in community function as well as community
structure. In addition, collectors often benefit from the addition of fine particulate organic
material; which may come from animal grazing, sewage particulates or some forms of
sedimentation. The increase in collectors was also accompanied by a decrease in the
Shredders and Predators (Table 4 and a slight (not significant) decrease in scrapers.
Shredders are the specialists that typify mountain streams. That is, they represent a
functional feeding group that should be abundant at reaches higher in the river continuum
(e.g., Vannote et al. 1980). Their decline may be due to the sites failing to retain coarse
particulate organic material, such as leaves, bark, pinecones, smaller limbs, or other coarse
detritus. These items make up the principle forage for shredders and the shredder-aided
breakdown of course detritus is one of the key functional characteristics of mountain streams.
Predators are common in many stream types and are not more important in mountain
streams than in any other stream system. The combination of declining shredders and
predators in a mountain streams can be disconcerting for resource managers because many of
taxa in these groups are large stoneflies; which are excellent forage for fish. In addition,
many are cool stenotherms; requiring consistently cool temperatures and high oxygen
concentrations for success. Thus declines in these groups can also suggest situations that may
become stressful for salmonids.
However, a closer look at these data indicates that these changes in function are not
outside of range of normal levels for Mountain streams. Shredders are usually aggregated on
deposits of organic detritus, and therefore have a wide range of mean abundances usually
from about 3-25%. Predators usually comprise about 10 – 30. So the values are within the
expected range. More importantly the changes are largely due to the increase in the number
of collectors—which were probably under represented in the year 2000 sampling. That is, the
combined abundance of collector-filterers and collector gatherers is usually much higher than
the 11% reported for the year 2000. The collectors were higher in 2003 (23%), which caused
a significant decrease in other functional feeding groups because of the fixed-count
subsampling procedure used.
For comparison with regional streams, we ran the metrics through the Montana
Mountains metric battery proposed by Marshall and Kerans (2003). The evaluation
compares 6 metrics to two regional deviation thresholds (1 moderate, 1 extreme). If the
metric is beyond the extreme deviation threshold, there is a 1% chance of type-1 and type-2
error and the metric scores 2 deviation points. If a metric is between the two thresholds it is
moderately deviant from the reference condition, there is a 15% chance of error, and the
metric scores 1 deviation point. If the metric is not beyond the moderate deviation threshold
it is not different from the values attained from the states reference sites and scores zero
deviation points. The deviation points for all 6 metrics are summed and the stream is
classified according to the total sum. The classification scores are 0-3- Normal; 4-7
moderate deviant; 8-11 extreme deviant; 12 non-functioning. In both 2000 and 2003, the
stream scored 3; normal (Figure 19). This needs to be cautiously interpreted, because the
mesh size and sampling methods were different from the criteria used to develop the MT
DEQ reference criteria.
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