Impacts of Mercury Exposure on Free-Ranging Post-Fledged Piscivorous Birds

Mercury is one of the priority pollutants of concern for several EPA programs, other federal agencies, and state governments. The concern is especially focused on methyl mercury because of its high toxicity and its propensity for extremely high bioaccumulation in aquatic food webs. Much of the concern for methyl mercury in the environment is focused on piscivorus (i.e., fish-eating) wildlife. Despite several risk assessments suggesting that several species of avian piscivores are experiencing significant risks from current mercury environmental concentrations, field data on methyl mercury impacts on birds are extremely limited. As a consequence, the positive findings of current risk assessments, which usually rely on laboratory-generated toxicity data from non-piscivorus birds, cannot be corroborated in the field. This gap was noted as a major source of uncertainty by EPA in its 1997 Mercury Study Report to Congress and subsequently became a major research recommendation in its 2000 Mercury Research Strategy.

The Carson River system in western Nevada is believed to have received millions of pounds of mercury from the direct use of elemental mercury and production of waste tailings during gold and silver mining activities initiated in the watershed during the late 1800's. Investigations in the 1970's and 1980's on the nature and extent of the contamination led to the 1990 listing the Carson River Mercury Site as a Federal Superfund Site on EPA's National Priorities List. Mercury concentrations in the Carson River system are some of the highest reported in the Nation. Recent research published on three species of wading birds inhabiting mercury impacted areas of the Carson River system (double crested cormorant, snowy egret, black-crowned night heron) indicate strong associations between mercury exposure and various effects including biochemical, organ weight, histopathological responses.

Despite much higher residues in adult birds, effects were most prominent in young (pre-fledged) birds. This may have been a function of a reduced capacity of young birds to detoxify methyl mercury (via demethylation) compared to adults. Because mercury disposition in growing feathers is known to be a significant excretion route in birds, concern has been raised here (and elsewhere in the scientific literature) regarding risks to post-fledged birds when this excretion route is lost, but at a time when inherent sensitivity appears to remain high. The objective of this project is to further investigate the impacts of mercury (methyl mercury) exposure on post-fledged birds inhabiting the Carson River Mercury Sites. Because of the high exposure to mercury in the Carson River system, studies at this site present a valuable opportunity to evaluate the predictability of laboratory-based forecasts of methyl mercury risks and effects.

Citation: Henny, C. J., H. F. Hill, D. J. Hoffman, R. A. Grove and J. L. Kaiser. 2005. An Evaluation of Mercury Exposure and Effects on Waterbirds along the Lower Carson River in Nevada (1997-2005), with Special Emphasis on Survival Rates of young Snowy Egrets (2002-2004). Forrest and Rangeland Ecosystem Science Center, U.S. Geological Survey. 110 p.


U.S. EPA. Impacts of Mercury Exposure on Free-Ranging Post-Fledged Piscivorous Birds.