Large‐scale assessments of river health using an Index of Biotic Integrity with low‐diversity fish communities

Summary 1. Effective tools are needed to measure the ‘health’ of rivers at scales large enough to be useful for management. Indicators for assessing the complex of variables that constitutes river health need to be ecologically based, efficient, rapid and consistently applicable in different ecological regions. 2. A large‐scale survey of rivers in New South Wales, Australia provided data to test the Index of Biotic Integrity (IBI). The IBI employs the fish‐community attributes, identified using regional and river‐size data, expected for a river reach of excellent environmental quality. It uses metrics based on species richness, abundance, community structure and the health of individual fish. IBI metrics were established to suit a relatively low‐diversity and unspecialized freshwater fish fauna in south‐eastern Australia, totalling 55 species. 3. The IBI was able to discriminate between relative levels of environmental quality within a diverse set of stream systems and four presumptive ecological regions. The index was validated by testing the repeatability of scores, and by comparison of IBI scores at eighty sites with an independent measure of potential catchment condition, the River Disturbance Index. 4. Assessments of metric performance showed that eleven of the twelve metrics contributed satisfactorily. One metric based on trophic guild performed poorly and should be deleted from the index. Six other recommendations are made to enhance the performance of the IBI. 5. Results show that, while all large rivers have been disturbed, rivers in the Murray region and those in many coastal montane areas are particularly degraded. 6. The IBI results presented here demonstrate a validated method for large‐scale monitoring of river health based on a fish fauna of limited diversity, in the absence of suitable reference sites.