Environmental factors influencing fish assemblage structure in a naturally saline river system

Abstract

This study documents fish assemblages and their relationships to instream, riparian, and land use/cover variables that could be affected by a proposed reduction of chlorides in the Wichita River in Texas. The North, Middle, and South Wichita merge to form the main Wichita River, which then enters the Red River that creates the border between Texas and Oklahoma. These prairie streams provide harsh environments for fishes due to high temperatures, high natural salinity, and low flows. Sites were sampled on the Wichita River, its tributaries, and on the Red River tributaries from the confluence with the Wichita River to Lake Texoma. Forty-five species of fish were collected across the fifty-seven sites that contained fish. Twenty-one species that made up > 1 % of the total abundance of fishes sampled were included in the analyses. Multivariate analyses indicated that fifteen environmental variables explained 62.7 % of the variation in overall fish assemblage. Land use/cover alone explained 11.6 % of the variation in species, instream variables alone explained another 9 %, and riparian habitat alone another 6%. Land use/cover was a good indicator of habitat variability over longer temporal scales than could be measured in this study. Extreme conditions and instability of physicochemical environments separated assemblages of species that were tolerant of such conditions, from assemblages in more stable and benign habitats that had higher numbers of intolerant species. Stochastic and deterministic processes both seem to be involved in determining fish assemblage structure in this harsh prairie stream system. Stochastic processes associated with abiotic variability are most likely to structure assemblages in harsh environments by favoring species that can tolerate reduction in habitat size and increase in salt concentration, then rapidly increase population size as habitats become available. In contrast, biotic factors (competition/predation interactions) are most likely to structure assemblages in stable more mesic habitats by eliminating salt-tolerant species that are less competitive in benign environments, although they otherwise could exist there. Therefore, if chlorides are reduced, extirpation of salt-tolerant species likely will depend on preservation of refuge sites that maintain naturally harsh environmental conditions.