Climate Change and Overfishing Increase Neurotoxicant in Marine Predators

Abstract

Over three billion people rely on seafood for nutrition but fish are also the predominant source of exposure to the potent neurotoxicant, methylmercury (MeHg). In the United States (U.S.), 82% of the population-wide MeHg exposure is from consumption of marine seafood with almost 40% from fresh and canned tuna alone.1 Inorganic mercury (Hg) is emitted to the atmosphere from natural and human sources. Most of this Hg (~80%) is deposited to the ocean, where some is converted by microbes to MeHg. Fish are the main vector for human MeHg exposure because environmental concentrations are magnified by a million times or more in predatory fish. Elevated MeHg exposure has been associated with long-term neurocognitive deficits in children that persist into adulthood, resulting in societal costs that exceed $20B USD globally. To mitigate these risks, the first global treaty (Minamata Convention) on anthropogenic Hg emissions reductions entered into force in 2017. However, the effects of ongoing and dramatic changes in marine ecosystems on MeHg bioaccumulation in marine predators most frequently consumed by humans (e.g., tuna, cod, swordfish) have not been considered when setting targets for global policy. Based on more than 30 years of data and ecosystem modeling, we show MeHg concentrations in Atlantic cod (Gadus morhua) increased by up to 23% between the 1970s and 2000s due to dietary shifts initiated by overfishing. When fully realized, the effects of unprecedented seawater temperature warming since a low observed in 1969 will contribute to an estimated 56% increase in tissue MeHg concentrations in Atlantic bluefin tuna (ABFT: Thunnus thynnus). This is greater than reductions from declining seawater MeHg concentrations in the late 1990s and 2000s (22% decrease). A recent plateau in global anthropogenic Hg emissions means ocean warming and fisheries management programs will be major drivers of future MeHg concentrations in marine predators.