Comparison of fish and macroinvertebrate bioassessments from South Carolina coastal plain streams

Stream bioassessments are usually based on a single taxonomic assemblage, often fishes or macroinvertebrates, with the assumption that this assemblage is representative of ecological conditions. However, ecological and physiological differences between taxonomic groups may result in different assessment results. In this study, concurrent fish and macroinvertebrate bioassessments from South Carolina coastal plain streams were compared on the basis of precision, sensitivity, accuracy, and agreement. Multimetric indices used in this comparison were a modified Index of Biotic Integrity based on electrofishing data and a benthic macroinvertebrate multimetric index based on data collected with Hester-Dendy artificial substrates. Benthic macroinvertebrates were also collected from natural substrates for comparative purposes. The Index of Biotic Integrity was more precise than the Hester- Dendy multimetric index but the average difference between disturbed and reference sites was greater for the latter resulting in equal sensitivity (i.e., ability to measure disturbance in relation to index variability). The two indices were significantly correlated (r=0.62, p<0.001), but agreement between them was weak for slightly and moderately disturbed sites. Analysis of species richness and abundance data indicated that fish and macroinvertebrates responded differently to some disturbances regardless of whether macroinvertebates were collected from Hester- Dendy samplers or natural substrates. Disagreement between macroinvertebrate and fish assessments at moderately disturbed sites indicated that biological condition could not always be adequately evaluated from a single taxonomic group. Identification of disturbed sites was most accurate when based on both indices suggesting that future research should emphasize cost-effective sampling and integration of information from multiple taxonomic groups.