Development and Validation of an Index of Biotic Integrity for Coldwater Streams in Wisconsin

Abstract

The index of biotic integrity (IBI). developed from information on the structure, composition, and functional organization of fish assemblages, is used to assess the health of aquatic ecosystems. We analyzed two large statewide data sets on stream fish assemblages to develop and test a version of the IBI for application to Wisconsin coldwater streams (maximum daily mean water temperature usually <22°C). This new IBI is needed because fish assemblages in Wisconsin coldwater streams differ significantly from those in warmwater streams (maximum daily mean temperature >24°C), for which an IBI already exists. High-quality coldwater streams have few species, with salmonids and cottids dominating. and lack many of the taxonomic groups that are important in high-quality warmwater streams. In contrast, high-quality warmwater streams have numerous species, and cyprinids, catostomids, centrarchids, and percids typically dominate. Environmental degradation often causes an increase in species richness in coldwater fish assemblages, the opposite of what occurs in warmwater assemblages, as a small number of coldwater species are replaced by a larger number of more tolerant eurythermal and warmwater species. The new coldwater IBI has five metrics: (1) number of intolerant species, (2) percent of all individuals that are tolerant species, (3) percent of all individuals that are top carnivore species, (4) percent of all individuals that are native or exotic stenothermal coldwater or coolwater species, and (5) percent of salmonid individuals that are brook trout Salvelinus fontinalis. No regional or stream-size adjustments in metric scoring criteria are needed. Relative coldwater IBI scores and ratings of stream sites throughout Wisconsin closely match independent rankings of environmental quality on the basis of physical habitat and water quality of the sites. Variation in IBI scores within and among years is gencrally low. The new coldwater IBI is not appropriate for coolwater streams (typical maximum summer daily mean temperature 22–24°C).