Different mercury bioaccumulation rates between sympatric populations of dwarf and normal lake whitefish (Coregonus clupeaformis) in the La Grande complex watershed, James Bay, Québec

Abstract

Fish mercury concentrations were monitored before and after the construction of the La Grande hydroelectric complex, located in the James Bay region, Quebec, Canada. Monitoring results revealed that sympatric populations of dwarf and normal lake whitefish (Coregonus clupeaformis) occur in reservoirs and natural lakes of the area. Dwarf specimens bioaccumulate mercury more rapidly than normal individuals. In natural Serigny Lake, 6–7 year old dwarf specimens show mean total mercury levels 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 Caniapiscau 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 individuals, similar methyl mercury levels were found in prey of both forms. We suggest that dwarf individuals bioaccumulate mercury more rapidly than normal specimens because they begin to mature at an earlier age (usually at age 2 to 3 compared to age 6 for normal specimens) and thus produce proportionally less flesh to dilute the mercury assimilated. Total mercury analysis indicating higher concentrations in flesh than in gonads support this hypothesis. Our results suggest that when monitoring mercury levels in fish in areas where populations of dwarf and normal lake whitefish coexist, dwarf and normal specimens must be distinguished because of their different bioaccumulation rates.