The family Hydropsychidae dominates the caddisfly fauna of the Yellowstone 

 River. Representatives (13 species) of this family are all net spinners and 

 include the genera Cheumatopsyche , Hydropsyche, and Avatopsyche. One species, 

 Hydropsy ohe corbeti, was not known to be present in the United States until 

 collected in the Yellowstone River. 



BOTTOM FAUNA POPULATION 



Bottom samples taken during the fall of 1974 were designed to survey the 

 bottom fauna and to test equipment. The data (available in Newell 1976 or 

 in the files of the Montana Department of Natural Resources and Conservation, 

 Helena) are, therefore, semiquantitative and difficult to compare with later 

 sampl ing. 



Quantitative bottom fauna sampling began in the summer of 1975. No 

 sampling is possible in the lower river during the winter because of ice cover. 

 Shortly after the ice is removed, spring runoff begins; bottom samples from 

 this period would be of little value. The data gathered by Schwehr (see 

 Report No. 8 in this series) were added here to compare the density of 

 invertebrates of the midriver (stations 5-11) to that of the lower river 

 (stations 12-20). Field data from samples taken at stations 15, 17, and 

 18 are presented in Newell 1976 and are on file at the Montana DNRC. 



In August, bottom fauna population estimates ranged from about 50/m2 at 

 station 9 to about 2,000/m at station 5 (figure 33). Station 19 exhibited 

 the lowest mean, 250/m2. Generally, there was a gradual downstream decrease 

 in mean population size. 



September population estimates (figure 34) exhibited a greater range, 

 from 20/m2 at station 19 to 8,500/m2 (station 5). Estimates from the lower 

 river were much lower than those from upper river stations. 



In October, less variation in range was observed (figure 35). The 

 minimum population estimate was 250/m2 at station 18 and the maximum was 

 400/m2 (station 11). The trend again was a gradual downstream decrease in 

 the density of organisms. 



In November samples, data from stations 1 and 3 were also available 

 (figure 36). Population estimates at stations 1 and 3 were similar and 

 were much higher than for the remaining sampling stations (range 4,500-12,000/m2) 

 The trend was a decrease in population downstream. 



The percentage composition of all invertebrate orders collected in 1975 

 is presented in tables 4-7. The mean percentage composition of each order is 

 found in table 8. Ephemeroptera dominate the fauna in August, and 

 Trichoptera begin to dominate in September and October; the Diptera became 

 dominant in November. Plecoptera and others are a minor portion of the 

 fauna. Figure 37 graphically illustrates the longitudinal changes in 

 percentage composition of invertebrate orders. 



52 



