POTENCY OF A COMPLEX MIXTURE OF POLYCHLORINATED DIBENZO-P-DIOXIN, DIBENZOFURAN, AND BIPHENYL CONGENERS COMPARED TO 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN IN CAUSING FISH EARLY-LIFE STAGE MORTALITY

Use of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity equivalents concentration (TEC) assumes that polychlorinated dibenzo-p-dioxins (P CDDs), dibenzofurans (PCDFs), and biphenyls (PCBs) act additively and via a common mechanism to cause toxicity. To test these assumptions, 1 1 TCDD-like congeners and three non-TCDD-like congeners were combined at ratios typically found in Lake Michigan lake trout. The potency of the mixture, expressed as TEC based on fish-specific toxic equivalency factors, was compared to TCDD for producing lake trout and rainbow tr out early life stage mortality. Signs of toxicity following exposure o f newly fertilized eggs to the mixture or to TCDD were indistinguishab le; sac fry mortality associated with blue-sac disease, and slopes of the dose-response curves for percentage sac fry mortality versus egg T EC or versus egg TCDD were parallel. However, the mixture dose-respons e curves were significantly shifted to the right of the TCDD dose-resp onse curves by 1.3- to 1.8-fold as illustrated by LD(50) values. Follo wing exposure to the mixture or TCDD, LD(50) for lake trout early life stage mortality were 97 (89-110) pg TE/g egg and 74 (70-80) pg TCDD/g (LD(50), 95% fiducial limits) and for rainbow trout were 362 (312-406 ) pg TE/g egg and 200 (148-237) pg TCDD/g egg. These data suggest that TCDD-like congeners act via a common mechanism to cause toxicity duri ng trout early development, but may not act strictly additively when c ombined in a mixture of TCDD- and non-TCDD-like congeners at ratios fo und in Great Lakes fish. The deviation from additivity, however, is le ss than current safety factors of 10-fold commonly applied in ecologic al risk assessments, providing support for the continued use of a TE a dditivity model for assessing risk posed by complex mixtures of PCDDs, PCDFs, and PCBs to fish. (C) 1996 society of Toxicology