Evaluation of the critical body burden concept based on inorganic and organic mercury toxicity to rainbow trout (Oncorhynchus mykiss).

Subadult rainbow trout (Oncorhynchus mykiss) were exposed to four waterborne concentrations each of 64-426 microg/L mercuric chloride (HgCl2) and 4-34 microg/L methylmercury chloride (CH3HgCl) until death to evaluate the critical body burden concept. Mean days to death for fish exposed to the highest and lowest concentrations of HgCl2 were 1 and 58 d, and 2 and > 100 d for fish exposed to CH3HgCl. Time to death was an important factor that influenced Hg tissue concentration, and was most evident among fish that died within a few days of exposure. Critical body burdens for Hg could be difficult to establish at the tissue level because no threshold concentrations were clearly indicated among the liver, kidney, spleen, brain, muscle, and gill that were monitored in this study. A critical burden for Hg was derived on a whole body basis for Hg in its organic form. An evaluation of this and other studies suggests whole body concentrations of 10-20 mg/kg Hg could be lethal to fish. Extrapolation from other studies indicate whole body concentrations of 1-5 mg/kg Hg could have chronic effects on fish and possibly other aquatic organisms. This concept could be used to assess the toxicological significance of chemical concentrations that are monitored in feral aquatic organisms. The tissue-based approach appears to have some advantages over current assessment protocols that focus on waterborne concentrations.