Influence Of Fluvial Geomorphology On Fish Assemblage Structure Within An Agriculturally Impacted Watershed

Abstract

Warmwater stream fish assemblages are often considered limited by instream habitat availability. Thus, much current research has focused on determining habitat features that limit the abundance and persistence of stream fishes. These studies provide valuable information to managers who are concerned with increasing species abundance and persistence, but they may not provide information on the processes structuring the stream fish community. To address this issue, we hypothesized that stream geomorphology differed among stream sizes and types (i.e., channelized vs. unchannelized streams) and that the fish assemblages associated with each geomorphic type would be unique. Fish assemblages were sampled and geomorphic features were delineated in 25 streams within the Embarras River watershed, Illinois, USA. Sampling sites were stratified by upstream watershed areas of 5, 25, and 100 km2, and 8, 7, and 10 streams were sampled per respective size group. Principal Components Analysis was used to elucidate gradients in geomorphology among the sites and Multivariate Analysis of Variance was used to identify significant differences in geomorphology among the size and type groups. The 5 km2 streams were unique in geomorphic structure when compared to the larger streams, while the 25 and 100 km2 streams did differ in geomorphology. The 5 km2 streams differed from larger streams primarily in substrate composition, channel depth and width, sinuosity, and slope. Fish assemblages in the 5 km2 streams were dominated by species that are tolerant of intermittency and by those that require gravel substrates for either reproduction or feeding. Fish assemblages characteristic of the larger streams were quite diverse and contained a mix of habitat specialists and generalists. There was also a significant difference in geomorphic structure between channelized and unchannelized streams. Channelized streams, were less sinuous, had finer substrates, lower gradients, and deeper channels than unchannelized streams. Fish assemblage structure was affected by stream channelization, which led to an assemblage comprised of habitat generalists, while unchannelized streams had an assemblage comprised of habitat specialists and generalists. We found that longitudinal trends in stream geomorphology influence fish assemblage composition in a predictable manner. However, site specific disturbances, in this case channelization, have the most profound effects on fish community structure and must be considered when managing stream systems in agriculturally impacted watersheds.

Date of Award	2008
Original language	American English
Awarding Institution	Eastern Illinois University
Supervisor	Robert Fischer (Supervisor)