Written by Nereus Research Associate Ryan Swanson,
Nereus research fellow Colin Thackray (Harvard University) and Principal Investigator Elsie Sunderland (Harvard University) are co-authors with others on a new publication in the journal Nature. For their study, they investigated how warming seawater temperatures and overfishing are contributing to the bioaccumulation of the potent neurotoxin methylmercury (MeHg) in top ocean predators, looking at Atlantic cod (Gadus morhua), spiny dogfish (Squalus acanthias), and Atlantic bluefin tuna (Thunnus thynnus) in the Gulf of Maine in the Northwest Atlantic ocean.
With 80% of the mercury (Hg) from human activity and emissions ultimately settling in the ocean, large predatory fish can end up with “[Hg-derived] MeHg concentrations amplified a million times or more”. Globally, over 3 billion people rely on seafood for their nutritional needs, and consuming oceanic fish is the main source of human exposure MeHg. The authors write that, in the U.S. alone, 82% of the population’s exposure to MeHg is from oceanic seafood. This leaves large population segments at risk for toxic MeHg exposure, with children particularly susceptible to the associated long-term neurological deficits.
First, the authors investigated how overfishing has affected MeHg levels in marine predators. They synthesized three decades (1970-2000s) worth of ecosystem data from the Gulf of Maine, looking at environmental MeHg concentrations and performing stomach content analysis. From this, they developed a simulation model based on bioenergetics and predator-prey interactions to show how shifts in marine food web structures influence MeHg concentrations in Atlantic cod and spiny dogfish. Their results showed that overharvesting their prey (e.g. herring) impacted each species differently, with Atlantic cod having 6-20% lower MeHg concentrations and spiny dogfish having 33-61% higher MeHg concentrations in the 1970s compared to 2000s.
In each time period when either species had lower MeHg levels, their diet contained more herring and less prey known to have higher amounts of MeHg (e.g. squids and other cephalopods). The authors therefore show that the situation is more complex than species accumulating environmental mercury in their tissues, and that other potential contributing factors (e.g. overfishing) need to be further investigated.
Next, the authors explored how warming seawater temperatures in the Gulf of Maine affected MeHg levels in fish species. Here, their model predicted that a 1°C increase in seawater temperature (relative to 2000) would lead to a 32% increase in the MeHg concentration in Atlantic cod (>15kg), a 70% increase in spiny dogfish, and a projected 30% increase in Atlantic bluefin tuna in 2015 that would go through 2030 (after previously declining 31% from 1990 to 2012).
Taken together, both overfishing and warming seawater temperatures associated with climate change are driving MeHg concentrations higher in marine predatory fish, some species of which a substantial number of humans rely on for their nutritional needs.
Since 2011, global mercury emissions have been relatively stable, largely due to aggressive regulations in North America and Europe that have been in place since the 1970s. Because of these regulations, the authors state that “seawater temperatures and prey availability will be two factors that strongly influence [MeHg] in tissues of marine predators, such as Atlantic cod and bluefin tuna in the Gulf of Maine.” In order to combat MeHg bioaccumulation in marine species, the authors suggest a “two-pronged regulatory approach” that involves reducing both greenhouse gas (GHG) and mercury emissions, and warn that weakening regulations will likely increase human exposure to MeHg.
Schartup, A.T., Thackray, C.P., Qureshi, A., Dassuncao, C., Gillespie, K., Hanke, A. & Sunderland, E.M. (2019). Climate change and overfishing increase neurotoxicant in marine predators. Nature, 2019. DOI: 10.1038/s41586-019-1468-9