TABLE 20. Percentage composition of invertebrate orders derived from kick 

 samples taken at Glendive (17) and Intake(lS) in 1975. 



Mayfl ies 



Mayfly (Ephemeroptera) species diversity (d) was great, with as many 

 as 15 species present in some current-depth samples. Because Ephemeroptera 

 nymphs are much easier to identify to the species level, current preferences 

 were obtained for several abundant species. These data provide some insight 

 into niche separation in the mayfly community and how separation and current 

 preference change throughout the life cycle of several species. 



Densities of Tvaverella albertana and Tricorythodes minutus are 

 presented in figure 49. In this figure and in figures 50-54, the exact 

 nature of the invertebrate/current relationships is not clear from the daU; 

 the following conclusions record only how the data were interpreted by the 

 author. Peak densities in August at Intake for Tvaverella albertana 

 occurred at about 2.25 ft/sec. Nymphs of T. albertana were more abundant 

 in August than in any other month. This species emerges in September and 

 October, and nymphs do not reappear in any number until November. 



At the Intake station during the October samples, peak population 

 densities were determined for several species (figure 50). Heptagenia 

 elegantula were more abundant in slower currents and most abundant at 0.5 

 ft/sec. Traverella albertana was abundant near 2.5 ft/sec as in the August 

 samples. Baetis insignificans was also most abundant at 2.5-3.0 ft/sec, but 

 there was no way to determine at what velocity this population would reach 

 its peak. A similar situation exists with Rhithrogena undulata, although the 

 population seems to reach its greatest density at about 2.75 ft/sec. In 

 November, H. elegantula and B. insignificans exhibited low densities at 

 Intake, but peak densities appear to have occurred at 1.5 ft/sec and 2.5 

 ft/sec, respectively (figure 51). 



Some current preferences were apparent for mayflies at the Glendive 

 station (figure 52). A population extreme was evident for H. elegantula 

 (0.5 ft/sec). In the November samples (figure 53), the highest density of 

 H, elegantula occurred at about 1.5 ft/sec. 



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