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FAUNAL ZONATION 



Streams vary greatly in size, velocity, gradient, nature of the bed, 

 temperature, and other features. Generally, streams change from steep torrents 

 to sluggish meandering waterways as they proceed from source to mouth (Allen 

 1969). Usually there are stages between the two extremes which are char- 

 acterized by specific environmental features and a particular assemblage of 

 fish species. 



Many attempts have been made to associate particular fish faunas with 

 these defined zones. Huet (1959) devised a scheme for European streams using 

 four categories, naming each zone from the characteristic fish found in them. 

 In descending order of current velocity they are: 



1 ) the trout zone 



2) the grayling zone 



3) the barbel zone 



4) the bream zone 



North American streams have been classified by Lagler et al . (1962) as 

 follows: 



1) grayling 



2) stream char 



3) flowing water minnows and pike 



4) bass 



5) catfish, suckers, and quiet water minnows 



Each of these zones is characterized by a particular set of combinations of 

 stream gradient and stream width. As the gradient diminishes, the headwater 

 fishes disappear and are replaced successively by others better adapted to the 

 changing environment. This was evident in the change of species diversity with 

 progression downstream in the Tongue River. 



Fish population in the Tongue River exhibit a succession from torrent-zone 

 fishes (trout) to the quiet-zone fishes (catfish and suckers). While the upper 

 Tongue River in Montana cannot be considered a trout stream, the presence of 

 brown trout and other species associated with fast-water habitats suggests 

 zone 2 according to the classification given above. 



RESIDENT FISH POPULATIONS 



SPECIES DISTRIBUTION 



Thirty-one species of fish representing 11 families were collected on the 

 Tongue River during 1974 and 1975. Of these species, four (burbot, paddlefish, 



19 



