Spatial patterns of fish assemblage structure in a tributary system of the upper Colorado River basin

This study was conducted to describe the distributions of both native and normative fishes and to identify spatial patterns in fish assemblage structure of Muddy Creek in the upper Colorado River basin of Wyoming using data collected from 77 reaches during 1999-2004. Fish assemblages in high-elevation reaches were characterized by brook trout (Salvelinus fontinalis) and represented a coldwater faunal zone. Reaches at lower elevations were characterized by warmwater fish species. The upper segment of the warmwater faunal zone contained four native (i.e., bluehead sucker [Catostomus discobolus], flannelmouth sucker [C. latipinnis], speckled dace [Rhinichthys osculus], roundtail chub [Gila robusta]) and two normative (i.e., white sucker [C. commersoni] and creek chub [Semotilus atromaculatus]) species. The lower segment of the warmwater faunal zone included species present in upstream segments and three additional notmative species (i.e., common carp [Cyprinus carpio], redside shiner [Richardsonius baleatus], and fathead minnow [Pimephales promelas]). Differences in fish assemblage structure between coldwater and warmwater faunal zones were likely due to physiological constraints of species adapted for either coldwater or warmwater habitats. Changes in fish assemblages over the warmwater faunal zone are due to introduction and naturalization of normative fishes and anthropogenic barriers preventing upstream movements and colonization by some of these species.