Mercury in Fish

Warming temperatures increased mercury levels in Atlantic bluefin tuna (Thunnus thynnus)

NOAA

Mercury, a neurotoxin that can cause cognitive deficits, is known to accumulate at increasing rates in fish as the food chain ascends. Recent regulations have reduced global emissions of mercury, yet mercury levels are not decreasing across fish species at similar rates. A new model shows that other factors, such as overfishing and climate change, also affect mercury levels in fish.

To predict different fish species’ levels of methylmercury, a form of mercury easily absorbed and retained by organisms, a team of researchers, led by biogeochemist Amina Schartup at Harvard University, created a “virtual ecosystem” reflecting real-life food webs in the Gulf of Maine. The virtual ecosystem incorporated over three decades of data about environmental parameters, such as seawater temperature and methylmercury emissions, as well as factors such as each fish species’ weight, trophic level, and preferred prey species.

This model allowed the researchers to adjust different parameters to explain how similar species reacted differently to environmental changes. For example, Atlantic cod (Gadus morhua) and spiny dogfish (Squalus aranthias) both prefer to feed on herring (Clupea harengus). In the 1970s, when overfishing reduced herring populations, cod started eating more lower-mercury fish, and spiny dogfish started eating more higher-mercury fish. When herring populations recovered, both predators started eating more herring. According to the virtual ecosystem model, this resulted in mercury levels in cod increasing by 6 to 25 percent and decreasing in spiny dogfish by 25 to 38 percent from the 1970s to the 2000s.

The model also showed that changing temperatures affect methylmercury levels in fish. The researchers hypothesized that as water temperatures rise, fish change their eating habits by moving and hunting more. The more they eat, the more methylmercury they consume. Atlantic bluefin tuna, for example, showed a 3.5 percent increase in methylmercury levels between 2012 and 2017, despite a decrease in global anthropogenic mercury emissions over that time. Changing water temperatures can also cause fish to alter their ranges, which disrupts the food web if predators lose their typical prey and begin eating other species.

The researchers’ model allows them not only to explain historical changes in fishes’ methylmercury concentrations but also to predict how various environmental changes would affect methylmercury concentrations in the future. As senior author Elsie Sunderland notes, “To protect ecosystems and human health, we need to regulate both mercury emissions and greenhouse gases.” (Nature)