Have mummichog (Fundulus heteroclitus) from the lower Penobscot River, Maine, developed tolerance to the toxic effects of mercury?

Fish populations that are chronically exposed to mercury (Hg) can develop resistance to the toxic effects of this metal, including mummichog (Fundulus heteroclitus; Weis 2002). Such resistance allows them to potentially accumulate very high levels of this contaminant (Stefansson et al. 2013). Mercury is a neurotoxin that affects behavior in fish, including swimming and the ability to capture prey (Samson et al. 2001, Zhou et al. 2001), and behavior, reproduction and immune function in birds (Evers et al. 2008, Hawley et al. 2009). Ingestion of mercury-contaminated prey could have significant, and severe, effects on migratory fish and piscivorous birds, including loons (Evers et al. 2008). We found that mercury concentrations in resident mummichog collected from sites in the lower Penobscot River in 2011 were 9 to 16 times higher than Hg levels in mummichog from a control site in Wells, Maine (Elskus 2012). For part one of this study, reproductively mature male and female mummichog were collected in Fall 2015 from two sites in Maine: Bald Hill Cove, a mercury-contaminated site along the mercury gradient of the Penobscot River, and Drake, a reference site at the Wells National Estuarine Research Reserve. These parental fish were housed at the US EPA laboratory (Narragansett, RI). Beginning in Spring 2016, these fish were fed diets containing either low or high concentrations of mercury for 28 days. Embryos were collected from these parents and divided into subsets. One subset was used to measure the mercury concentrations in the embryos, the second subset was shipped to the University of Maine where they were hatched, and the larval fish evaluated for behavioral effects. For part two of this study, adult mummichog were collected from Hg-contaminated Bald Hill Cove and from a reference site, Wells, in the Wells National Estuarine Reserve and evaluated for mercury body burdens and behavior. In Study 1, mercury was maternally transferred to the progeny demonstrating this is one pathway of generational exposure to mercury. Unfortunately, all larvae from the mercury-contaminated Bald Hill Cove population died before behavioral analysis could be done. Behavioral studies of the reference Drake population, however, demonstrated no significant differences between offspring of parents fed high mercury diets and offspring of parents fed low mercury diets. In Study 2, adult fish from the mercury contaminated BHC population had mercury body burdens that were four times higher than those in the reference Wells population, but while fewer BHC adults performed prey strikes than the Wells adults, there were no other significant behavioral differences between the two populations. In conclusion, the main hypothesis, that fish from the site receiving chronic mercury exposure had developed resistance to mercury toxicity, could not be definitively determined due to the death of embryos from the chronically exposed population. However, the present study did reveal some behavioral effects associated with mercury exposure that warrant further study. Behavioral approaches are important because they address some of the basic and difficult questions surrounding contaminant exposures; namely that sub-lethal effects are important to evaluate because they may have consequences for individual and perhaps population survival.