Research Spotlight: Wetland Habitats and Aquatic Prey May Drive High Hair Mercury Concentrations in Suisun Marsh Raccoons

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Methylmercury is the most toxic form of mercury for animals, and it biomagnifies through food webs: animals near the bottom of aquatic food webs, such as insects and fish, take in contaminants like mercury from the environment, and then those contaminants become concentrated in predators when they eat many contaminated prey items. In Suisun Marsh, California, a large brackish marsh with a history of mercury contamination, raccoons and skunks are vulnerable to methylmercury toxicity because they are predators that depend, at least partially, on aquatic food webs.

USGS researchers assessed total mercury (an index for toxic methylmercury; hereafter mercury) concentrations in the blood and hair of raccoons and the hair of skunks captured, collared, and released in Suisun Marsh from 2016-2019 to better understand how methylmercury passes through food webs in the marsh ecosystem. Mean mercury concentrations in raccoon hair was strikingly high (28.50 μg/g dry weight) compared to skunk hair (4.85 μg/g dry weight) and to raccoons sampled in other locations, higher even than raccoons sampled close to a mercury mine near Clear Lake, CA. Mercury concentrations in the Suisun Marsh raccoons were also among the highest observed among wild mammals more broadly. Estimates of liver mercury concentrations derived from hair concentrations indicated that 19% of adult raccoons in the study may have been close to or above the proposed liver toxicity benchmark of 30 mg/kg wet weight for impaired reproduction in mammals.

The high concentrations of mercury observed in the Suisun Marsh raccoons relative to other mammals is likely attributable to their diet, which is most likely comprised largely of aquatic or semi-aquatic animals, like crustaceans and duck eggs. In comparison, skunks likely rely less on aquatic prey and more on terrestrial insects, seeds, eggs, and small mammals, making them less vulnerable to mercury contamination.

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The study also examined how mercury levels in raccoon and skunk hair varied in relation to year, season, and availability of nearby wetlands and channels. For both raccoons and skunks, mean mercury concentrations for hair grown during 2016 to 2018 were related to the size of wetted habitat (wetlands and channels) on Grizzly Island the spring prior to hair growth. Interannual differences for both species suggest that mercury exposure varied among years, possibly due to differences in bioavailability to food webs, prey switching, or a combination of these factors.

Methylation of mercury and assimilation of methylmercury into food webs can be influenced by water management and fluctuations in water extent.

The results of this study indicate that water abundance in Suisun Marsh may influence methylmercury exposure to animals throughout the food web. By monitoring contaminants like mercury in mammalian predators like raccoons, researchers and managers get a clearer picture of the marsh’s contaminant status, helping them to manage water and wildlife and work towards maintaining healthy ecosystems in California.

This research spotlight refers to:

Peterson, S. H., Ackerman, J. T., Hartman, C. A., Casazza, M. L., Feldheim, C. L., & Herzog, M. P. (2021). Mercury exposure in mammalian mesopredators inhabiting a brackish marsh. Environmental Pollution, 273, 115808. https://doi.org/10.1016/j.envpol.2020.115808

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