A statistical approach for classifying the extent of EROD induction of fish sampled in clean and contaminated waters

Along with conventional chemical analyses, biological assessment is useful for monitoring water quality. Biochemical markers are early warning systems of the exposure of aquatic organisms to pollutants. Among them, the measurement of fish EROD induction is used as an appropriate tool in numerous field studies. Yet, in a large scale biomonitoring perspective, it is necessary to rank results from field sites. Cyprinids were thus sampled in the Rhône river watershed (France) in relatively clean and polluted areas. Reference baseline EROD activities were calculated for two species: chub and gudgeon. The data from the polluted sites were related to contamination and female sexual maturity. With all results, a classification of the EROD levels was proposed through statistical analysis of the characteristics of the EROD distribution. The decision rule was based on type I (false positive) and type II (false negative) error rates. Both measurement and interpretation of the EROD biomarker are then advanced enough to begin a normalisation procedure in biomonitoring and management perspectives.