Selection for food types is difficult to measure. Preferences for 

 aquatic organisms may vary seasonally as larvae grow in size or emerge as 

 adults. Unfortunately, fish were not collected during all seasons. During 

 the period of capture, Traverella nymphs were mature while other species such 

 as Hydropsyahe were relatively small. Food habits of the insect-feeding fish 

 species may be different during the winter and spring months when caddis 

 larvae are approaching mature size and Traverella nymphs and terrestrial 

 insects are rarely available. 



Ivlev's index and other forage ratios are based on the relative portions 

 of food organisms that occur in the environment, but not all foods are 

 readily available to a predator (Leonard 1941). Baetis , a fast swimmer, is 

 capable of functioning in fast water without the protection of the bottom 

 substrate. Baetis^ therefore, was accessible to sturgeon and goldeye. 

 Heptagenia and Rhithrogena are capable of crawling beneath stones where they 

 are unavailable to predators. Hydropsychid larvae are found in rock crevices 

 or between stones, but may also be exposed on rock surfaces. Chironomid larvae 

 are somewhat immobile and vulnerable to predators (Hess and Swartz 1941, 

 Klarberg and Benson 1975). 



The present study illustrates the importance of aquatic insects in the 

 diets of sturgeon, goldeye, and catfish. The sturgeon was restricted to 

 bottom insects of riffles. The goldeye fed exclusively on insects as well 

 and included a small amount of terrestrial insects in the diet. Insects were 

 significant in the catfish diet, but some fish were also taken. Young 

 sauger and burbot were highly dependent on macroinvertebrates for their food 

 source. Insects were present in the stomachs of carp, white sucker, and 

 freshwater drum. Other conmon fish species of the lower Yellowstone River 

 which are dependent on aquatic macroinvertebrates for a large portion of their 

 diet include longnose sucker [Catostomus aatostorms), shorthead redhorse, 

 river carpsucker (Carpoides aarpio), blue sucker {Cyaleptus elongatus) , 

 smallmouth buffalo (Ictiobus bubalus), bigmouth buffalo (l. aypvinellus) , 

 black bullhead (latalurus melas) , and stonecat. Insects were also important 

 foods for young individuals of crappie, bass, and walleye (Brown 1971). 



Little is known of the habitat requirements and foods of forage fish of 

 the lower river. During the period of study, the flathead chub appears to 

 depend on the backwaters for its food supply of Cladocera and insects. This 

 species is abundant in the river and is a common food item of predator species. 



In the genus Eybognathus , the silvery minnow inhabits backwater areas 

 (Cross 1967) and feeds on bottom ooze and algae (Brown 1971). The plains 

 minnow and brassy minnow are common in small streams and the slow waters of 

 large rivers. Their food habits are similar to those of the silvery minnow. 



Unlike many of the forage species, the longnose dace is adapted to the 

 fast water of riffles. Insect larvae are the main food of this species 

 (Brown 1971). 



FORAGE FISH SURVEY 



Results of the forage fish survey are presented in table 51. The most 

 conmon species, the flathead chub and emerald shiner, frequently occur in both 



132 



