p^ = the relative portion of food type i in the environment, 

 expressed as the percentage of the total number of food 

 items available. 



Values range from -1.0 to 1.0. Negative values indicate that the fish 

 is selecting against a food type, which means that the percentage of a 

 specific food organism in the ratio is lower than the percentage of the 

 food type in the environment. Active selection for a particular food suggests 

 a preference for the food and yields a positive value. A value near indicates 

 that a fish consumes a food type in the same proportion in which it is available. 



The use of the index requires knowledge of the population numbers of prey 

 organisms that occur in the predators' habitat. Relative abundance data for 

 forage fish species were not adequate for purposes of this study; therefore, 

 food selection by sauger and burbot, which feed primarily on forage fish, 

 could not be determined. Ivlev's index was used to determine selection for 

 benthic macroinvertebrates by shovelnose sturgeon, goldeye, and channel 

 catfish. The data from quantitative bottom samples of benthic organisms taken 

 by Robert Newell at Miles City and Intake in August and September 1975 (see 

 Report No. 5 in this study) were used in this study. Newell 's data did not 

 include weight or volumetric measurements of benthos, so electivity was 

 determined on the basis of organism numbers alone. 



Sample sizes of captured fish were too small to divide the individuals 

 into groups by date or location of sampling. The one exception was shovel - 

 nose sturgeon, which were separated into two groups by date of capture prior 

 to stomach analysis. 



A few individuals of northern pike, carp, white sucker, smallmouth bass, 

 white crappie, walleye, freshwater drum, and flathead chub were also taken. 

 For these species, only qualitative results are listed. 



FORAGE FISH SURVEY 



Forage fish samples were collected during 1974 in the vicinities of Myers, 

 Forsyth, Miles City, Sunday Creek, Glendive, Intake, and Sidney. The main 

 channel and backwater areas were sampled with a 50 foot x 6 foot x 1/4 inch 

 bag seine and by electrofishing. In 1975 additional samples were collected 

 at eight locations on the lower river: Hysham, Forsyth, Miles City, the 

 mouth of Sunday Creek, the mouth of the Powder River, Terry, Intake, and 

 Sidney. These samples were also obtained from main channel and backwater 

 areas. In addition to the 50 foot x 6 foot x 1/4 inch bag seine and electro- 

 fishing, a 100 foot x 8 foot x 1/4 inch loose-hung seine was utilized. 



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