Regionalization of the Index of Biotic Integrity for Texas Streams

Abstract

Aquatic life use designations dictate the level of protection streams receive in accordance with the surface water quality standards prepared by the Texas Natural Resource Conservation Commission. Streams can be assigned one of four aquatic life use categories (exceptional, high, intermediate, or limited). Although streams in Texas are diverse, a statewide Index of Biotic Integrity (IBI) has been applied historically in conjunction with water quality, benthic macroinvertebrate, and habitat data to set aquatic life uses in streams. This study was conducted to regionalize the IBI for Texas’ wadeable streams. Fish were collected from 62 least disturbed reference streams located within 11 of the 12 aquatic ecoregions described for the state. An array of metrics was screened to determine which ones were most suited for Texas. Scoring criteria were developed for each of the respective metrics. Metrics suited for all regions of the state include: total number of species; number of native cyprinid species; number of sunfish species; percentage of individuals as omnivores; percentage of individuals as invertivores; number of individuals per unit effort; percentage of individuals as nonnative species; and percentage of individuals with disease or other anomaly. Other metrics used in selected ecoregions include: number of benthic invertivore species; number of benthic species; number of intolerant species; percentage of individuals as tolerant species (excluding western mosquitofish Gambusia affinis); and percentage of individuals as piscivores. When applied to the least disturbed streams sampled in this study, the statewide IBI produced lower overall scores and aquatic life uses. Scores from the statewide IBI demonstrated a geographical trend, declining from east to west, and resulted in no exceptional aquatic life use designations even though the streams were selected through a screening process and were among the least disturbed in a region. These lower IBI values (and aquatic life uses) result from using a single index over a large land area comprised of a diversity of land forms, soil types, vegetation, climatic conditions, and zoogeographic factors. Regional criteria consider these natural differences and consequently provide a better representation of the integrity of the fish assemblage.