All of the data on mayfly current preference were pooled and are 

 presented in figure 54. Several characteristics are evident. Current 

 preference seems to change with different periods in the life cycle of a 

 species. Greatest population densities for Heptagenia elegantula changed 

 from 0.5 ft/sec in October to 1.5 ft/sec in November. Populations of 

 Baetis insignif leans exhibited a similar trend but at higher velocities. 

 The two samples of Traverella albertana, however, were similar (near 2.5 

 ft/sec). 



Figure 54 gives some insight into niche separation of six species of 

 Ephemeroptera. Each of these species had its highest densities at slightly 

 different current velocities, thus reducing interspecific competition for 

 food and resting areas. The remaining mayfly species were present in numbers 

 too small to illustrate current preference and made up an insignificant part 

 of the fauna in the lower Yellowstone River. 



Stonefl ies 



Stonefly (PlecopteraJ nymphs were not common in the lower Yellowstone 

 River, and little information on current preference was obtained. At Intake, 

 however, Plecoptera were found only at the fastest currents. 



Caddisflies 



Caddisfly (TrichopteraJ larvae, Hydropsyehe in particular, exhibited 

 a distinct current preference, with the greatest number of larvae found at 

 the fastest currents sampled. Larvae could not be identified to species, 

 although at least three species of Hydropsyehe have been collected at 

 Glendive and Intake. Samples taken in August and September were not 

 significant (p=.05) when relating numbers of individuals to current. Samples 

 taken in October and November at both stations were highly significant. 

 Regression lines varied little from October to November at Glendive and 

 at Intake (figures 55 and 56). 



There is some evidence that Hydropsyehe reached its greatest densities at 

 about 2.5 ft/sec at Intake in October (figure 55) and November (figure 56). 



91 



