Mercury Levels in Freshwater Fish Pose Threat to Consumers

Photo: Eric Engbretson/U.S. Fish and Wildlife Service (CC0)

Invasive species in the Great Lakes are draining the food supply of native fish, forcing fish to eat the less nutritious mercury-containing goby, which poses health risks for fish consumers, a study finds.

Mercury levels in the lakes have dropped since the 1980s after regulations halted waste burning from local hospitals. Though pollution is lower, researchers found that large game fish are consistently contaminated with mercury, according to a journal published by the Proceedings of the National Academy of Sciences.

By tracing the food consumption of fish over time, scientists discovered that zebra mussels and other invasive species compete for the fish’s food sources.

“Concentration doesn’t equate to failure,” said Ryan Lepak, the lead researcher and immunologist from the Environmental Protection Agency. “The rules were changed when the zebra mussels came in.”

The issue of mercury contamination goes beyond the Great Lakes. Forms of energy production like coal and natural gas emit mercury globally. When mercury is released into the air, there are threats to human and environmental health.

The United States has implemented regulations since the early 1980s to combat these emissions. Including The U.S. Clean Air Act and the Mercury Export Ban of 2008. In 2011, the Environmental Protection Agency issued the Mercury and Air Toxics Standard which eliminated coal use by 30% to 40%.

While mercury levels remain stagnant in fish, scientists found that the regulations of emissions are increasingly beneficial for the Great Lakes.

“Despite not decreasing like we expected in concentration, those emission regulations are a wild success,” Lepak said.

Mercury is naturally occurring in aquatic ecosystems. Fish have small amounts of a mercury type, called methylmercury, in their proteins which can accumulate when eaten by humans.

Excessive consumption of fish containing methylmercury can damage the nervous system in humans and animals. Women of childbearing age and children are most at risk of impairment caused by mercury consumption.

Methylmercury is harmful to brain development and can hurt the female reproductive cycle. In the worst cases, excessive mercury intake can deteriorate IQ or cause loss of physical abilities.

Since being introduced in the 1990s and early 2000s, invasive species like quagga and zebra mussels eat phytoplankton which is the base of Lake Michigan’s food chain. These mussel species have exploded in population. Researchers estimate there are trillions of mussels in the lakes.

With increased competition for phytoplankton, Lake Michigan’s food web is changing.

Game fish, like trout, used to eat alewives which ate phytoplankton. Alewives’ populations are now smaller because of the competition for phytoplankton. Trouts have turned to alternative food sources which are less nutritious but contain the same amount of mercury.

Trout and other big fish are a commodity for fishermen in the Great Lakes, but consumers should avoid eating them often. People fishing recreationally should exercise caution when catching and eating large fish regularly.

Lepak and a team of researchers assessed sediment samples that also demonstrate changes in environmental mercury levels over time. The sediment samples proved that atmospheric mercury contents decreased since the reduction of emissions and that the mercury levels in fish are coming from another source.

The researchers traced mercury through the ecosystem by analyzing samples of fish in Lake Michigan. In these samples, the scientists were looking for isotopes of mercury, which is a modified form of the element.

These isotopes show how fish consumed mercury over time in the ever-changing food web. The scientists noticed differences in the isotope ratios when the invasive species arrived in the lakes.

“It allowed us to track a changing mercury source without being able to visually see a change,” Lepak said.

The method is called ‘fingerprinting’ and helps the researchers determine where the mercury in the samples comes from.

The isotopes and trout sampled from 1978 to 2012 showed the complete food web transformation.

The study found that after 2000, lake trout began to feed on goby. Though gobies and alewives have similar levels of mercury, fish must consume more goby to be nourished. The more goby the trout eat, the more mercury they consume.