Mercury Concentrations in Fish-eating Birds from the Pinchi Lake Area in Relation to Productivity and Reproductive Success

Bald eagles and red-necked grebes were monitored from 2000 2002 on several lakes along the Pinchi fault, an area with a known source of geologic mercury and previous Hg mining (on Pinchi Lake), in an effort to discern whether increased Hg concentrations were affecting reproductive success and productivity. To determine whether or not Hg levels were elevated, fish tissues (rainbow trout and northern pikeminnow) were collected, eagles breeding on 5 lakes (Pinchi, Tezzeron, Stuart, Great Beaver and Fraser) were sampled for bloodand feather-Hg concentrations, and eggs were collected from rednecked grebes. Concentrations of Hg in all grebe eggs, while highest in eggs collected from Pinchi Lake, were below 0.5 μg/g wet wt., often cited as the lowest observed adverse effect level for Hg developmental toxicity in birds. Reproductive success and average productivity of the bald eagles over the 3-year period were 62% and 0.98 chicks/territory on Pinchi Lake compared to 64% and 1.17 chicks/territory on all other study lakes combined. Significant relationships were found between adult and chick blood Hg levels from the same nests as well as chick blood and feather Hg concentrations. Despite the apparently elevated Hg levels found in adult eagles from Pinchi Lake (mean blood Hg = 6.54 μg/g wet wt.), the birds appeared to be in excellent condition. The adult eagle with the highest level of Hg in blood (9.44 μg/g wet wt.) successfully raised two eaglets in each of the summers of 2001 and 2002, making it one of the most productive birds in the study area. Thus, while dietary Hg exposure is greater in eagles and grebes from Pinchi Lake, it does not appear to adversely affect reproductive success or productivity. This study illustrates how reproductive monitoring can be used to verify or refute conclusions that might be drawn from findings suggesting a possible contaminant effect on local wildlife.