^«Unad^ciCtio4/i 



The characteristics of insect and fish populations in streams are deter- 

 mined to a large degree by discharge and by the environmental conditions 

 created by discharge. The composition of the insect community, the food base 

 of most fish populations, is related to water velocity, a function of discharge. 

 To assess the impact of dewatering on fish foods and habitat, it is necessary 

 to determine the importance of the benthic insects to the fishes. In addition, 

 the maintenance of the piscivorous sport and nonsport fish populations in the 

 lower Yellowstone (walleye, sauger, northern pike, burbot, and channel catfish 

 all commonly utilize other fish for all or part of their diet) depends, in part, 

 on an adequate forage fish base. 



PURPOSE 



Fish populations, relative abundances, and food chains vary widely among 

 aquatic ecosystems. Each fish species is adapted to an omnivorous, herbivorous, 

 or carnivorous diet, but the proportions of specific foods eaten differ in each 

 habitat. Prey organisms that are important in one habitat may not exist in 

 other areas which contain the same predator species. Seasonal changes occur in 

 food habits of fish as relative abundances and composition of insect species 

 are altered during the life cycles of insects. Food habits of resident popula- 

 tions must be determined for individual water bodies in order to assess the 

 potential for impact. Impacts on fish populations are dependent on the amount 

 of stress placed on the food supply in a particular habitat. It is important 

 to establish which food species are most critical in fish diets and how 

 environmental change will affect the food supply. 



The objectives of this investigation were: (1) to determine the foods of 

 selected fish species during the period of study, (2) to establish those 

 species' selection for preferred food items relative to their availability, 

 and (3) to describe the possible changes in the fish community based on 

 alterations in the food supply caused by potential flow reductions. 



SCOPE 



The efforts of this phase of research are concentrated on the food habits 

 of some of the major fish species of the lower Yellowstone River, primarily 

 shovelnose sturgeon ( Saaphirhynchus platorynohus) , goldeye (Hiodon alosoides), 

 channel catfish (lotalurus punatatus), burbot (Lota lota), and sauger 

 (Stizostedion aanadense) . Small numbers of northern pike (Esox lucius), 

 carp (Cyprinus cavpio) , white sucker (Catostomus commersoni) , smallmouth 

 bass (Miaropterus dolomieui) , white crappie (Pomoxis annularis), walleye 

 (Stizostedion vitreum) , freshwater drum {Aplodinotus grio^niens), and flathead 

 chub (Hybopsis gracilis) were collected for food analysis. 



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