Mercury Exposure May Suppress Baseline Corticosterone Levels in Juvenile Birds

Mercury exposure has been associated with a wide variety of negative reproductive responses in birds, however few studies have examined the potential for chick impairment via the hypothalamic-pituitary-adrenal (HPA) axis. The HPA axis regulates corticosterone levels during periods of stress. We examined the relationship between baseline fecal corticosterone metabolite concentrations and mercury concentrations in down feathers of recently hatched (<3 days) and blood of older (15–37 days) Forster’s tern (<i>Sterna forsteri</i>) chicks in San Francisco Bay, California. Baseline fecal corticosterone metabolite concentrations were negatively correlated with mercury concentrations in blood of older chicks (decreasing by 81% across the range of observed mercury concentrations) while accounting for positive correlations between corticosterone concentrations and number of fledgling chicks within the colony and chick age. In recently hatched chicks, baseline fecal corticosterone metabolite concentrations were weakly negatively correlated with mercury concentrations in down feathers (decreasing by 45% across the range of observed mercury concentrations) while accounting for stronger positive correlations between corticosterone concentrations and colony nest abundance and date. These results indicate that chronic mercury exposure may suppress baseline corticosterone concentrations in tern chicks and suggests that a juvenile bird’s ability to respond to stress may be reduced via the downregulation of the HPA axis

Relative abundance of fish returned to Forster’s tern (<i>Sterna forsteri</i>) colonies.

<p>Prey fish were collected at tern breeding colonies in south San Francisco Bay, California during 2005–2015. The total number of individuals collected within a year are shown above the bars. The other category includes additional species of fish. The bars are presented in the same order as the legend.</p

Sample size, dry standard length (SL; mm), and dry mass (mass; g) of individual fish returned to Forster’s tern (<i>Sterna forsteri</i>) colonies in south San Francisco Bay during 2005–2015 by colony and species.

<p>The other category includes additional species of fish and invertebrates. Refer to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193430#pone.0193430.g001" target="_blank">Fig 1</a> for locations of individual colonies.</p

Relative dry mass by fish species returned to Forster’s tern (<i>Sterna forsteri</i>) colonies.

<p>Prey fish were collected at tern breeding colonies in south San Francisco Bay, California during 2005–2015. The total number of individuals collected within a year is shown above the bars. The other category includes additional species of fish. The bars are presented in the same order as the legend.</p

Fish returned to 17 Forster’s tern (<i>Sterna forsteri</i>) breeding colonies in south San Francisco Bay.

<p>Fish were collected during weekly nest-monitoring visits during 2005–2015 (April through September while the colony was active). Terns did not nest at every colony in every year. Colonies included in statistical analyses are indicated by an * (sampled in ≥ 4 years with ≥ 25 fish collected/year). Colonies in the southern-most portion of the bay are separated into Moffett (A1, A2W, AB1, and AB2) and Alviso regions (A5-A16 and NCM). Imagery Service Layer Credits: Source: Esri, Digital Globe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community.</p

Relative abundance of four species of fish returned to Forster’s tern (<i>Sterna forsteri</i>) colonies.

<p>Forster’s tern breeding colonies were sampled in south San Francisco Bay, California during 2005–2015. Temporal trends for four main prey species of Forster’s terns are color-coded by colony. Silversides (<i>Menidia audens</i> and <i>Atherinops affis</i>) increased in relative abundance over time, whereas longjaw mudsucker (<i>Gillichthys mirabilis</i>) and three-spined stickleback (<i>Gasterosteus aculeatus</i>) decreased over time. Trends of staghorn sculpin (<i>Leptocottus armatus</i>) relative abundance varied among colonies. Not every location had a breeding colony of Forster’s terns each year.</p

Relative abundance of fish returned to Forster’s tern (<i>Sterna forsteri</i>) colonies in south San Francisco Bay during 2005–2015, calculated for each colony and year and all colonies together.

<p>Relative abundance of fish returned to Forster’s tern (<i>Sterna forsteri</i>) colonies in south San Francisco Bay during 2005–2015, calculated for each colony and year and all colonies together.</p

Estimating Mercury Exposure of Piscivorous Birds and Sport Fish Using Prey Fish Monitoring

Methylmercury is a global pollutant of aquatic ecosystems, and monitoring programs need tools to predict mercury exposure of wildlife. We developed equations to estimate methylmercury exposure of piscivorous birds and sport fish using mercury concentrations in prey fish. We collected original data on western grebes (<i>Aechmophorus occidentalis</i>) and Clark’s grebes (<i>Aechmophorus clarkii</i>) and summarized the published literature to generate predictive equations specific to grebes and a general equation for piscivorous birds. We measured mercury concentrations in 354 grebes (blood averaged 1.06 ± 0.08 μg/g ww), 101 grebe eggs, 230 sport fish (predominantly largemouth bass and rainbow trout), and 505 prey fish (14 species) at 25 lakes throughout California. Mercury concentrations in grebe blood, grebe eggs, and sport fish were strongly related to mercury concentrations in prey fish among lakes. Each 1.0 μg/g dw (∼0.24 μg/g ww) increase in prey fish resulted in an increase in mercury concentrations of 103% in grebe blood, 92% in grebe eggs, and 116% in sport fish. We also found strong correlations between mercury concentrations in grebes and sport fish among lakes. Our results indicate that prey fish monitoring can be used to estimate mercury exposure of piscivorous birds and sport fish when wildlife cannot be directly sampled

Experimental Dosing of Wetlands with Coagulants Removes Mercury from Surface Water and Decreases Mercury Bioaccumulation in Fish

Mercury pollution is widespread globally, and strategies for managing mercury contamination in aquatic environments are necessary. We tested whether coagulation with metal-based salts could remove mercury from wetland surface waters and decrease mercury bioaccumulation in fish. In a complete randomized block design, we constructed nine experimental wetlands in California’s Sacramento–San Joaquin Delta, stocked them with mosquitofish (Gambusia affinis), and then continuously applied agricultural drainage water that was either untreated (control), or treated with polyaluminum chloride or ferric sulfate coagulants. Total mercury and methylmercury concentrations in surface waters were decreased by 62% and 63% in polyaluminum chloride treated wetlands and 50% and 76% in ferric sulfate treated wetlands compared to control wetlands. Specifically, following coagulation, mercury was transferred from the filtered fraction of water into the particulate fraction of water which then settled within the wetland. Mosquitofish mercury concentrations were decreased by 35% in ferric sulfate treated wetlands compared to control wetlands. There was no reduction in mosquitofish mercury concentrations within the polyaluminum chloride treated wetlands, which may have been caused by production of bioavailable methylmercury within those wetlands. Coagulation may be an effective management strategy for reducing mercury contamination within wetlands, but further studies should explore potential effects on wetland ecosystems

Appendix A. Survey coverage.

Survey coverage