Summer fish assemblages in channelized and unchannelized reaches of the South Sulphur River, Texas

Abstract

I used a conceptual model proposed by Schlosser (1987) to compare channelized and unchannelized reaches of the South Sulphur River, Texas. This model suggests that fish assemblage structure can be predicted based on the level of habitat heterogeneity, especially with regard to the level of pool development. Based on Schlosser's model, I hypothesized that habitat heterogeneity would be greater in the unchannelized (as compared to channelized) reach of the South Sulphur River, which would therefore have more stable fish assemblages. Fish assemblages in this reach would have similar total fish density and higher species richness, in addition to lower density and higher biomass of larger bodied fish (primarily piscivores and omnivores), as well as lower density and biomass of juveniles and adults of small-bodied species (primarily invertivores) as compared to the channelized reach. Habitat characteristics conformed to my predictions, but fish assemblage attributes were opposite those hypothesized. Schlosser's study focused on biotic processes more than the abiotic effects of a highly variable, stochastic environment. I propose that abiotic processes, particularly extreme fluctuations in flow regimes, are likely to be the most influential factors affecting fish assemblages in the South Sulphur River. Streams in this region are naturally subject to extreme variations in streamflow, but unchannelized sites may have been more directly influenced by water release or retention from the relatively recent construction of Cooper Dam located just upstream, whereas channelized sites, located much further downstream, were probably less affected. Most fish species present in the South Sulphur River are considered habitat generalists, have evolved to cope with extreme changes in environmental conditions, and are able to populate a variety of available habitats. Therefore, future management of this stream should reflect the needs of the few remaining fluvial specialists in this system, such as the intolerant freckled madtom and mimic shiner.