Environmental Biology of Fishes 68: 129–141, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. Differences in species composition and feeding ecology of catostomid fishes in two distinct segments of the Missouri River, North Dakota, U.S.A. Tim L. Welker a & Dennis L. Scarnecchia University of Idaho, Department of Fish and Wildlife Resources, Moscow, ID 83844-1136, U.S.A. a Current address: BIO-WEST, Inc., 1063 W. 1400 N., Logan, UT 84321, U.S.A. (e-mail: twelker@bio-west.com) Received 4 January 2002 Accepted 23 June 2003 Key words: sucker, resource partitioning, niche overlap, niche breadth, food habits Synopsis In 1997 and 1998, we sampled the Missouri River, North Dakota to determine if anthropogenic disturbances had influenced catostomid species composition and feeding ecology. We compared two distinct river segments, the Missouri River between the mouth of the Yellowstone River and Lake Sakakawea (the Yellowstone–Sakakawea segment (YSS)), a moderately altered segment and the Missouri River between Garrison Dam and Lake Oahe (the Garrison–Oahe segment (GOS)), a highly altered segment. The segments exhibited greatly different sucker communities. Bigmouth buffalo, Ictiobus cyprinellus, smallmouth buffalo, Ictiobus bubalus, and river carpsucker, Carpiodes carpio, represented 94% of the sucker catch in the YSS, whereas in the GOS, white sucker, Catostomus commersoni, and longnose sucker, Catostomus catostomus, constituted 98% of the sucker catch. In the YSS, high zooplankton densities led to greater sucker zooplanktivory and food niche overlap than in the GOS. Intense anthro- pogenic disturbances to the GOS are associated with the differences in sucker species composition, prey density and composition, and sucker feeding ecology between the two segments. Introduction Catostomid fishes include about 70 species of relatively large, chiefly North American fishes commonly known as suckers (Helfman et al. 1997). These cypriniform fishes are predominately bottom dwelling and possess specialized lips and pre-maxillaries which allow them to feed effectively on a variety of invertebrate organ- isms. Suckers generally inhabit unpolluted waters and can be found living under a variety of riverine and lake conditions, but most species generally cannot tolerate extensive modification of their habitats (Smith 1979). Impoundment of rivers through damming has frequently led to changes in native fish ecology and abundance in rivers below dams (Miller 1959). Declines in the abundance and changes in the ecol- ogy of native species following river regulation have been documented in many large, turbid rivers, such as the Missouri (Hesse et al. 1989). In the Missouri River, the decline of many native species and the concomitant increase in non-native species coincided with mainstem dam construction that began in the 1930s and ended in the 1960s (Hesse et al. 1989). The sucker commu- nity in the Missouri River was once very diverse with many of its species very abundant throughout much of the river. It appears, however, that numbers of most native suckers have been reduced substantially in river reaches below mainstem Missouri River dams (Funk & Robinson 1974). Despite the historical widespread distribution of sucker species in the Missouri River, the feeding ecol- ogy of these fishes has received little attention. Detailed knowledge of food habits for a fish assemblage can give valuable insight into inter-specific interactions, niche dimensions, food resource partitioning, and the trophic status of individual species (Litvak & Hansell 1990, Gray et al. 1997). Discovering and understand- ing these components of feeding ecology is key to proper management and conservation of suckers in the Missouri River. Our objectives were to: (1) contrast