DIFFERENT MERCURY BIOACCUMULATION RATES BETWEEN SYMPATRIC POPULATIONSOF DWARF AND NORMAL LAKE WHITEFISH (COREGONUS-CLUPEAFORMIS) IN THE LAGRANDE COMPLEX WATERSHED, JAMES BAY, QUEBEC

Fish mercury concentrations were monitored before and after the constr uction of the La Grande hydroelectric complex, located in the James Ba y region, Quebec, Canada. Monitoring results revealed that sympatric p opulations of dwarf and normal lake whitefish (Coregonus clupeaformis) occur in reservoirs and natural lakes of the area. Dwarf specimens bi oaccumulate mercury more rapidly than normal individuals. In natural S erigny Lake, 6-7 year old dwarf specimens show mean total mercury leve ls 1.5 times higher than normal individuals of the same age (0.33 vs 0 .21 mg kg(-1) wet weight). Eleven years after the impoundment of the C aniapiscau reservoir, 6-7 year old dwarf specimens show total mercury concentrations 3 times higher than normal specimens of similar ages (0 .74 vs 0.27 mg kg(-1)). Stomach content analysis showed that although large normal individuals feed on different prey than do dwarf individu als, similar methyl mercury levels were found in prey of both forms. W e suggest that dwarf individuals bioaccumulate mercury more rapidly th an normal specimens because they begin to mature at an earlier age (us ually at age 2 to 3 compared to age 6 for normal specimens) and thus p roduce proportionally less flesh to dilute the mercury assimilated. To tal mercury analysis indicating higher concentrations in flesh than in gonads support this hypothesis. Our results suggest that when monitor ing mercury levels in fish in areas where populations of dwarf and nor mal lake whitefish coexist, dwarf and normal specimens must be disting uished because of their different bioaccumulation rates.