Fish assemblage structure along environmental gradients in a coastal plain drainage: influences from a reservoir

Abstract

Because most major river drainages are altered by impoundments, it is necessary to understand how such alterations influence structure and function of fish assemblages. Upstream influences of impoundments have only recently been discussed as potentially altering non-anadromous fish Assemblages. Fishes were collected seasonally for one year in tributaries of the impounded West Fork (12 sites) and unimpounded East Fork (9 sites) of the San Jacinto River, Texas. I evaluated spatial and temporal variation in (1) fish assemblage structure, including species richness, biomass, and trophic groups, (2) environmental factors correlated with fish assemblage structure, and (3) differences in Assemblage structure and correlated environmental factors between the East and West Fork tributaries. Factors related to hydrologic variability, especially minimum discharge, separated East and West Fork sites. Although species richness and biomass were similar in the two forks, macrohabitat generalists here more specious in the West Fork, and fluvial specialists, which require flowing conditions for all life stages, were more specious in the East Fork. Differences in trophic structure between East and West Fork Assemblages included more surface and water-column investitures in West Fork streams, whereas benthic investitures, piscivores, omnivores and herbivores were more abundant in East Fork streams. Differences between East and West Forks were concordant with predicted elects of impoundment and are analogous to faunal changes that have occurred across fragmented terrestrial systems. Because impoundment now is ubiquitous in river systems, these embeds likely go unrecognized and may mask or exacerbate effects of other more recent environmental effects.