The Relation of Fish Assemblages to Channel Units in an Ozark Stream

Abstract

To evaluate a hierarchical channel-unit-based habitat classification system for warmwater streams, we sampled fishes in 11 channel units in both headwater and downstream reaches of an Ozark stream during summer and winter. There was significantly greater species richness (total number of species) at the downstream reach and greater biomass at the upstream reach, as revealed by multifactor analyses of variance. There were significant differences in both species richness and biomass among channel units and seasons and significant channel unit × season interactions, which together suggest that fish changed habitat use between summer and winter. Cross-validation procedures indicated that the fish assemblages in channel units at the headwater site, which had an average channel unit classification accuracy of 47%, were less predictable than those of downstream reaches, which averaged 61% accuracy. These differences were attributed to differences in habitat specialization between fishes of the headwater and downstream reaches. A cross-classification procedure also indicated that channel unit fish assemblages differed substantially between the headwater and downstream reaches. Canonical correspondence analysis indicated that fish distribution at both sites was highly related to mean depth and current velocity. The magnitude of the association of individual habitat characteristics and fish distribution also differed between seasons and suggested that the availability of bluff pools and backwater habitats during winter may influence the population size of some fish species. Channel units may be useful for defining warmwater stream habitats, but the utility of the classification is influenced by longitudinal factors. We propose a hierarchical model for the utility of a channel-unit-based habitat system, with upper-level constraints such as longitudinal position having the greatest influence on stream fish community structure and the utility of the classification.