Historical Changes in Large River Fish Assemblages of the Americas

<em>Abstract.</em>—A systematic, standardized approach to monitor fish assemblages has been applied in Ohio’s rivers since 1979. A primary objective is the assessment of changes in response to water pollution abatement and other water quality management programs. All major, nonwadeable rivers were intensively sampled using standardized electrofishing methods and a summer–early fall index period. Most rivers were sampled two or three times, before and after implementation of pollution controls at major point source discharges and best management practices for nonpoint sources. A modified and calibrated index of biotic integrity (IBI) was used to demonstrate and evaluate changes at multiple sampling locations in major river segments. An area of degradation value (ADV) and an area of attainment value (AAV) were also calculated from IBI results to demonstrate the magnitude and extent of changes in fish assemblage condition along segments and between sampling years. Positive responses in the IBI and the ADV/AAV were observed 4 to 5 years after implementing improved municipal wastewater treatment. Positive responses were much less apparent in rivers predominantly influenced by complex industrial sources, agricultural nonpoint sources, and extensive hydrologic modifications. The ADV/AAV showed incremental improvements in river fish assemblages, unlike pass/fail IBI thresholds, and tiered IBI biocriteria provided more appropriate benchmarks than chemical, physical, or qualitative biological criteria. The results show the value of standardized and intensive fish assemblage monitoring and the use of tools that reveal the extent and severity of impairments to determine the effectiveness of water pollution control programs.