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FOOD HABITS ANALYSES 



Food habits of fish common to the lower Yellowstone have been investigated 

 in other waters by a number of workers. Unfortunately, many of the studies were 

 conducted for lake (lentic) populations, where species composition differs 

 from that of lotic waters. Also, few investigations addressed food avail- 

 ability, making it impossible to determine selection for specific prey 

 organisms. For these reasons, and because each aquatic ecosystem is unique, 

 many of the conclusions of other researchers cannot be applied to this study. 



Most fish collected for this study were captured near Miles City or below 

 the Intake diversion dam. Some channel catfish were taken above the mouth of 

 the Tongue River. A few sauger and burbot were caught at Forsyth. Most fish 

 were taken from July through October during 1975 and 1976, using several 

 methods. The majority were captured by electrofishing at Miles City and Intake, 

 although fish were taken in seine hauls at the same locations. Catfish from 

 the Tongue River were trapped. Some burbot were caught by fishermen near 

 Forsyth in January. Sampling was limited primarily to backwaters and to 

 slower currents along the shore. 



Digestive tracts were removed from the fish, preserved in 10 percent 

 formaldehyde, and taken to the laboratory for analysis. Food items from the 

 esophagus and intestines were not analyzed. Stomach contents were identified 

 to the species level when possible, using descriptions and keys by Cross (1967) 

 and Brown (1971). Invertebrates found in fish stomachs are listed taxono- 

 mically in appendix D. Many of the fish in the stomachs were digested 

 beyond recognition. Food items were counted and measured volumetrically by 

 water displacement in graduated cylinders (appendix E). Methods of analysis 

 are discussed by Lagler (1964). 



Calculations were made to determine: (1) the percentage of stomachs in 

 which each food type was present (frequency of occurrence), (2) the average 

 number of organisms of each type in the stomachs which contained that food 

 type, (3) the percentage of total number of food organisms, and (4) the 

 percentage of the total food volume. 



Ivlev's (1961) electivity index was used to determine selection for 

 specific food types. The index is a forage ratio of the relative portion of 

 a food type in the diet to the portion that exists in the environment. It 

 takes the form: 



r. - p. 

 E = 1 1 



r. +P. 



where r. = the relative portion of food type i in the ratio, expressed 

 as the percentage of the total number of food items, and 



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