Abstract-Analyses of biomarkers in fish were used to evaluate exposures among locations and across time. Two types of references were used for comparison, an upstream reference sample remote from known point sources and regional exposure criteria derived from a baseline of fish from reference sites throughout Ohio, USA. Liver, bile, and blood were sampled from white suckers (Catostomus commersoni) and common carp (Cyprinus carpio) collected during 1993 and 1996 in the Ottawa River near Lima, Ohio. Levels of exposure were measured for petroleum by naphthalene-type metabolites, combustion by-products by benzo[a]pyrenetype metabolites, coplanar organic compounds by ethoxyresorufin-O-deethylase (EROD) activity, and urea by blood urea nitrogen (BUN) levels. The four biomarkers analyzed proved effective in determining differences between reference and polluted sampling sites, between geographically close (Ͻ0.5 km) sites, and between sampling years at sites common in both years. Calculated exposure criteria levels of the polycyclic aromatic hydrocarbon bile metabolites were found to be a conservative approximation of levels from a designated reference site and could thereby permit comparison of biomarker levels of fish from the Ottawa River to a regional reference level. Polycyclic aromatic hydrocarbon bile metabolite and EROD activity levels were more reflective of spatial patterns of contamination than BUN, although all biomarkers indicated differences over time. Biomarkers from white suckers seemed to be more responsive in detecting changes in contaminant levels than the same biomarkers from common carp. Lower levels in 1996 of all biomarkers at many sites suggested lower exposures than in 1993 and could be indicative of some improvement over the period.

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Changes in ethoxyresorufin-O-deethylase (EROD) activity were monitored through an extended 6-month dietary exposure to determine the relationship between EROD activity and uptake of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in brook trout, Salvelinus fontinalis. Brook trout were fed labeled TCDD during a 4-week loading phase and an 11-week maintenance phase to achieve whole-body concentrations of 0, 75, 150, 300, 600, and 1,200 pg TCDD/g fish. A spawning phase followed during which no TCDD was introduced. The TCDD had an extended half-life, with maximal levels detected in the late loading-early maintenance phases and 81 d after TCDD had been removed from the diet. Accumulation in liver increased as whole-body target concentration increased but was generally less than half of anticipated whole-body target concentrations. The EROD activity demonstrated a dose-dependent increase. Positive correlations were observed between EROD activity and TCDD body burdens for both males and females. For males, maximal induction was attained early in the maintenance phase and maintained during latter phases. For females, induction was characterized by a biphasic pattern. Maximal induction was attained during late loading-early maintenance, with an attenuated response observed just before spawning. In addition, the induction response was modulated by sex, as induction was lower in females when compared with males. If sexual biases are considered, increased EROD activity may serve as an indicator of level of TCDD exposure and a sublethal predictor of effects of exposure. Keywords-Ethoxyresorufin-O-deethylaseInduction 2,3,7,8-Tetrachlorodibenzo-p-dioxin Brook trout Salvelinus fontinalis EROD response of TCDD-fed brook trout