Over the past decade, scientists have documented that many wildlife species in North America are exposed to potentially harmful levels of mercury. Mercury is released into the environment directly or into the atmosphere by both natural (wood burning, volcanoes) and human-caused processes (coal-fired power plants, waste incineration, gold mining, and others). Once deposited in the environment, mercury accumulates in organisms, often placing top predators like Bald Eagles at an increased risk.
Mercury has been linked to a wide variety of negative sublethal effects on birds, and it may be limiting for some populations. Due to west to east wind deposition patterns, wildlife in Northeastern North America are often at highest risk from the toxic effects of mercury. As a result, efforts to evaluate the potential for adverse effects of mercury on wildlife, and to evaluate temporal and geographic mercury trends, are highly warranted.
Sampling Bald Eagle tissues (i.e., blood, feather, eggs) is an optimal way to identify contaminant “hotspots” that place fish-eaters at risk, and to monitor changes in mercury availability to wildlife over time. Several studies documented that Bald Eagles in Maine were exposed to some of the highest levels of mercury in the country (Welch 1994; conducted in 1991-1992).
Since those studies, Maine’s Bald Eagle population has continued to expand, providing new opportunities to evaluate mercury exposure in Bald Eagles from regions of Maine that were previously unavailable for sampling. With support from the Maine Department of Inland Fisheries and Wildlife, the Maine Department of Environmental Protection, FPL Energy Maine Hydro, and the U.S. Fish and Wildlife Service, we initiated a study to evaluate mercury exposure in Maine’s inland Bald Eagle population. Building upon previous efforts we wished to: (1) evaluate the current mercury exposure in Bald Eagles in lake and river habitats, (2) compare mercury exposure among major Maine watersheds, (3) determine if mercury may be slowing the recovery of Maine’s Bald Eagle population by affecting productivity, and (4) evaluate if mercury availability to Bald Eagles has changed since previous measures.
Inland Bald Eagle nestlings in Maine have highly elevated mercury levels compared to the majority of other populations sampled in North America. Blood mercury levels sampled from many Maine lakes – often in remote areas – were similar to populations associated with significant point-source pollution origins (e.g., mercury mines, dredging).
Bald Eagle nestlings sampled at lakes had significantly higher mercury exposure compared to those sampled in river habitats.
Mercury exposure in Maine Bald Eagles varied by major watershed, with the St. Croix River watershed containing the highest levels, and the (inland) Midcoast Maine watershed containing the lowest levels. Eaglet mercury exposure data were often consistent with findings in other biota (i.e., Common Loons, fish) used in other studies to delineate several areas of significant mercury concern (i.e., biological mercury hotspots) in the state.
Temporal comparisons between 1991-1992 and 2004-2006 periods do not suggest notable declines in the availability of mercury to Bald Eagles in Maine. This finding is consistent with previous indications that mercury can be relatively persistent in the environment. Maine’s eagle population is one of the few in the continental U.S. that has showed little evidence of declining mercury levels.
Preliminary comparisons between mercury and eagle productivity suggested that mercury may be slowing the recovery of Maine’s Bald Eagle population. This preliminary relationship is currently being further investigated with recently improved sample sizes. Adverse effect thresholds remain unknown for this species. Bald Eagle pairs nesting at lakes, particularly those with elevated mercury exposure, are at the highest risk for potential adverse health effects.
Efforts to document contaminant exposure and evaluate risks to wildlife are a prerequisite step in informing the general public, environmental agencies, and decision makers about environmental concerns that have potential to impact wildlife, human, and ecosystem health. This research helped demonstrate a need for evaluating mercury exposure in other wildlife throughout Maine. The resulting accumulation of information about mercury exposure in fish and wildlife has helped promote a need for more comprehensive mercury monitoring in this and other regions where contamination may be of concern to wildlife.
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