124 UNIVERSITY OF COLORADO STUDIES 



are various species of minnows whose abundance depends upon local 

 conditions. These last-mentioned species are always found either in 

 the lower courses of the stream where they come in from the plains or 

 in the more quiet parts back of beaver dams and other obstructions. 

 Still other species may be taken near the outlets of high mountain 

 lakes, as Pimephales promelas. 



Above the foothill region and below the region of the high mountain 

 lakes, the fishes of the average mountain stream are, however, the 

 Dace, Suckers, Sculpins (these are abundant only west of the Divide) 

 and the Trout. The first three forms feed upon trout eggs when these 

 are available, but this injury to the trout is offset by the fact that the 

 young suckers and both young and adult Dace are eaten by the adult 

 trout. The Sculpins are a disturbing factor in this balance, since their 

 young are rarely eaten by the trout. 



The major interrelations of the fishes of the mountain and foothill 

 streams are shown in the diagram on p. 125. In this diagram the aver- 

 age conditions are considered and it is to be borne in mind that with 

 local complications these interrelations are not so simple as the diagram 

 suggests. In general there is a progressive elaboration of the food 

 material from the slime and protozoa, through small crustaceans and 

 insect larvae, young fishes and adult suckers to the higher carnivorous 

 forms hke the trout. As has been pointed out by several writers, the 

 trout are normally carnivorous fishes but like any other group of 

 animals they feed upon the available food material when the supply 

 of that regularly taken falls short of the demand. 



Plains Streams 



The plains streams are larger and broader than the mountain 

 streams and with much less fall. Near the foothills, plains streams 

 have beds of gravel or small stones, the results of sorting by the 

 mountain streams, but away from the foothills the stream beds are 

 sand or clay except where changed by local conditions. Flowing 

 through a semi-arid region of low rainfall, the voliune of water carried 

 is subject to periodic variations correlated with fall rains and, in the 

 spring, melting of the snow. In spring and late fall with the increase 



