Mercury exposure in breeding common loons (Gavia immer) in central Ontario, Canada

Total Hg concentrations were determined in blood and feather samples of breeding common loons (Gavia immer) and their chicks from 24 lakes in Ontario with a range of water chemistry and fish Hg concentrations. In paired comparisons, males had significantly higher blood (p < 0.005) and feather (p < 0.05) Hg concentrations than their female mates. Sex differences in Hg concentrations were attributable to size differences between the two sexes and to the possible transfer of Hg to eggs by females during egg-laying. Significantly higher blood (p < 0.001) and feather (p < 0.01) Hg concentrations were found in adult loons compared to their chicks. Adult blood Hg concentrations were significantly correlated with those of their chicks (r = 0.62, p < 0.05). Adult blood concentrations averaged about 13 times those of corresponding chicks. Both adult and chick blood Hg concentrations were positively correlated with fish Hg concentrations (r = 0.71 and 0.51, respectively), indicating that adult loons in central Ontario, like their chicks, probably feed mainly on their breeding lakes and reflect the fish Hg concentrations of those lakes. Three of 24 lakes had 20- to 50-g fish (mainly perch and sunfish) with Hg concentrations at or exceeding the critical concentration (0.3-0.4 mu g/g wet weight or similar to 1-2 mu g/g dry weight) reported to cause reproductive impairment in loons. Monitoring Hg in blood (adult and chick) and feathers (chicks only) is useful for assessing the local bioavailability of methylmercury and the degree of current dietary Hg exposure in loons and other piscivorous birds.