A process for developing and evaluating indices of fish assemblage integrity

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Abstract
We describe a general process for developing an index of fish assemblage integrity, using the Willamette Valley of Oregon, U.S.A., as an example. Such an index is useful for assessing the effects of humans on entire fish assemblages, and the general process can be applied to any biological assemblage and any region. First, a reference condition was determined from historical information, and then candidate metrics of ecological importance were listed. The variability of the candidate metrics in time and space was estimated and their responsiveness to independent measures of riparian and stream habitat quality assessed. Metrics were scored continuously from 0 to 10, producing an index of biological integrity (IBI) that was weighted to range from 0 to 100 regardless of the number of metrics. The index, developed from a set of 35 sites, was then tested on an independent set of eight urban sites sampled by the Oregon Department of Fish and Wildlife. Thirteen of the 16 candidate metrics were appropriate and produced an IBI with among-site variance triple that of revisit variance. The method distinguished sites with acceptable fish assemblages from marginally and severely impaired sites.

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