POTENTIAL RISK TO COMMON LOONS (GAVIA-IMMER) FROM METHYLMERCURY EXPOSURE IN ACIDIFIED LAKES

Piscivorous birds and mammals in areas remote from point sources of Hg contamination may be exposed to dietary methylmercury concentrations that are sufficiently high to cause reproductive impairment. Common lo ons (Gavia immer) were observed to show aberrant nesting behavior and low overall reproductive success when Hg concentrations in prey (small fish and crayfish) averaged > 0.3 mug g-1 wet weight (Barr, 1986), le vels known to occur in fish from many lakes in central Ontario. We use d data on Hg in Ontario fish to estimate the proportion of lakes where fish small enough for loons to eat (< 250 g) had Hg concentrations th at exceeded estimated thresholds for reproductive impairment. Up to 30 % of lakes exceeded thresholds for reproductive impairment, depending on the species of fish and the threshold Hg concentrations chosen. The re was a significant negative correlation between fish-Hg concentratio n and lake pH in most fish species examined. For these species, reduct ions in sulfate deposition rates are predicted to result in a correspo nding reduction of lakes in Ontario having fish with potentially toxic concentrations of Hg.