BIOLOGICAL AND ENVIRONMENTAL-CONTROL OF MERCURY ACCUMULATION BY FISH IN LAKES AND RESERVOIRS OF NORTHERN MANITOBA, CANADA

Enlargement of northern Manitoba lakes to form hydroelectric reservoirs caused a rise in the Hg content of walleye (Stizostedion vitreum), northern pike (Esox lucius), lake whitefish (Coregonus clupeaformis), yellow perch (Perca flavescens), and spottail shiner (Notropis hudsonius) owing to stimulation of Hg methylating microbes by sub-merged terrestrial organic matter. However, an increase in the supply of organic substrates beyond a critical amount mitigated this effect, apparently by promoting Hg demethylation and production of Hg-binding agents such as sulfides. Fish species, even at the same trophic level, differed markedly in their Hg content, Hg - fork length relationships, and regional and temporal variations in Hg content; this is ascribed to differences in habitat preference, metabolic rate, age, growth rate, size, biomass, diet, and excretory pathways, and hence different responses and degrees of exposure to spatially varying conditions influencing Hg speciation and availability. Whitefish had an anomalously weak tendency to accumulate CH3Hg+ and a high ratio of liver CH3Hg+ to muscle CH3Hg+, suggesting a special mechanism of excretion. Only walleye Hg correlated significantly with CH3Hg+ production in offshore mud, indicating the value of walleye for whole-lake Hg bioassays and the validity and utility of the methods used to quantify CH3Hg+ production.