MERCURY CONCENTRATIONS IN FISH RELATED TO SIZE OF REMOTE CANADIAN SHIELD LAKES

Mercury concentrations in planktivorous, omnivorous, and piscivorous f ishes were inversely related to lake size in six lakes in northwestern Ontario. The lakes were remote from direct anthropogenic influences a nd ranged in surface area from 89 to 35 000 ha. Fish mercury concentra tions were not related to ratios of drainage basin area to lake size, to ratios of epilimnetic area to lake size, to lake alkalinity, or to the concentration of mercury in lake sediments. Rates of mercury methy lation (M) were positively dependent on water temperature whereas rate s of methyl mercury demethylation (D) were inversely related to temper ature. Thus, M/D was strongly temperature dependent. Mercury concentra tions in four fish species were significantly positively correlated wi th mean epilimnetic water temperatures (r2's ranged from 0.66 to 0.88) . This suggested that higher water temperatures in smaller lakes durin g the open-water season influenced M/D ratios and were the cause of hi gher fish mercury levels. No lake size related variation was observed in the mercury concentrations in benthivorous fishes. Our hypothesis t hat epilimnetic temperature affects mercury concentrations in fish des erves further attention, given the possible effects of climate warming on mercury contamination of fishery resources.