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

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A new study by scientists from USGS in collaboration with the California Department of Water Resources has found that mercury concentrations detected in hair of raccoons living in Suisun Marsh, California are some of the highest levels observed in raccoons and for wild mammals overall.

Raccoon with a transmitter sits by the edge of the water in Suisun Marsh

Raccoon with a transmitter sits by the edge of the water in Suisun Marsh.

(Credit: Sarah Peterson, USGS Western Ecological Research Center. Public domain.)

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.

Mercury levels in raccoons and skunks track with wetted habitat, with higher levels of both in 2017 than in 2016 or 2018

Annual mean hair total mercury (THg) concentrations for raccoon and skunk from Grizzly Island (Suisun Marsh, California) were highest in hair grown in 2017. Annual mean hair THg concentrations were correlated with the area (km2) covered by seasonal wetlands and canals (wetted habitat) on Grizzly Island (shown using gray bars) during the spring prior to hair growth (during mid-summer and fall) for raccoons and skunks (p = 0.02). 

(From Peterson et al. 2021)

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

Click here to download a one-page PDF version of this research spotlight.

 

 

Chart showing mercury levels for many different types of mammals, with Suisun Marsh raccoons showing some of the highest values

Total mercury (THg) concentrations quantified in hair of aquatic and terrestrial mammals (excluding bats and humans) from the literature, color coded by taxonomic similarity. The dashed horizontal lines are the mean values for raccoon (red) and striped skunk (navy blue) from Peterson et al. 2021 to provide easy comparison with other studies. Circles are means from each species and study and bars indicate the range of the data from a given species and study. When more than one mean hair THg concentration was provided in a study (for multiple sites or separately for males and females), all mean values are shown.

From Peterson et al. 2021

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