AN INDEX OF BIOTIC INTEGRITY TO ASSESS BIOLOGICAL IMPACTS OF SALMONID FARM EFFLUENTS ON RECEIVING WATERS

A major issue for water resource management is the assessment of environmental degradation of lotic ecosystems. In order to quantify the extent of resource degradation resulting from anthropogenic disturbances, a fish-based index, the Index of Biotic Integrity (IBI), has been used throughout North America and recently in Europe. The IBI is based on the assumption that fish assemblage attributes change in a characteristic fashion with stream degradation. In this study, using data from 1979 to 1991, we analysed the impact of 9 salmonid farm effluents on fish assemblages in some Brittany streams. Annual production of these farms varied between 2 and 300 tons. The data were first computed using a principal components analysis. Ordination of the points indicated longitudinal changes in fish assemblages from upstream to downstream and separated sites sampled upstream from sites sampled downstream from the fish farms. Depending on stream size, we found, downstream from trout farms, an increase in both density and biomass of most species, extirpation of sensitive ones (Cottus gobio L. ), and appearance of pollution-tolerant (Rutilus rutilus L.) and exotic forms (Oncorhynchus mykiss W.). Finally, we modified the IBI and compared it with chemical variables to illustrate how this index can be applied to quantify disturbances of the quality of stream water induced by fish farms. This IBI, based on 10 fish assemblage attributes, showed close agreement with these independent chemical measurements.