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A Biointegrity Index for Coldwater Streams of Western Oregon and Washington

Robert M. Hughes, Dynamac Corp, 200 SW 35th St, Corvallis, OR 97333, 541-754-4516, hughes.bob@epa.gov
Shay Howlin, West Inc., 2003 Central Ave., Cheyenne, WY 82001, 307-634-1756, showlin@west-inc.com
Philip R. Kaufmann, USEPA, 200 SW 35th St., Corvallis, OR 97333, 541-754-4451, kaufmann.phil@epa.gov

We developed, tested, and applied an index of biological integrity (IBI) for fish and amphibian assemblages in coldwater streams of the Oregon and Washington Coast Range. A probability sample of 104 wadeable sites was quantitatively sampled for fish and amphibian assemblages, and physical and chemical habitat from 1994 to 1996. Natural gradients and anthropogenic disturbances were assessed by examining digital data for catchment-scale road density and vegetation cover, along with site-scale physical and chemical habitat data. A set of 109 candidate metrics was evaluated for variance properties, redundancy, and responsiveness to multiple measures of disturbance, resulting in the selection of 8 metrics for the index. The IBI itself was subsequently evaluated for variance and responsiveness to disturbance, then compared against an independently selected set of 101 reference sites that had minimal anthropogenic disturbance. Our IBI was fairly precise, with an among-stream variance/index-period (error) variance ratio of 4.7 (indicating a theoretical maximum correlation of 0.83 between IBI and a predictor variable with a similar ratio). The IBI was significantly correlated with multiple estimates of anthropogenic disturbance, and reference sites had significantly higher IBI scores than the non-reference sites. Applying this IBI, we assessed fish assemblage condition in the Coast Range, inferring our results to all mapped (1:100,000-scale) wadeable streams in this region. Using the fifth percentile of reference sites as a biological criterion, 43% of stream kilometers (10, 173 km) were classified as impaired. High IBI scores clustered near national parks and wilderness areas.

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