Mercury in fish
Mercury is an element that occurs naturally throughout the globe, but it is also a component of industrial pollution. Small quantities of mercury are present in many fish and shellfish and the level of mercury in fish generally corresponds with its level on the food chain. Mercury levels are highest in top predator fish because mercury biomagnifies, meaning that the mercury present in the many smaller fishes and shellfish that are consumed by top predators remains in its tissues. Consuming fishes with high levels of mercury can be dangerous for humans, especially pregnant and nursing women and small children. To ensure food safety, fishes are tested for mercury content by regulatory agencies such as the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA).
Mercury in the environment
While mercury is a naturally occurring element, industrial activities have increased the concentration of mercury in the atmosphere. Mercury is released by coal-fired power plants, through the extraction of metal from ore (i.e. smelting), and by the incineration of waste1. Mercury that is in the atmosphere can be moved around the globe and deposited back into the environment when it rains or snows. Mercury that ends up in aquatic environments like lakes, rivers, or oceans is taken up by plants and algae and moves up the food chain. At each step in the chain, the mercury is retained in the tissue of the predator.
Mercury levels in freshwater fish can be affected by many different factors ranging from the size of the lake to the acidity of the water to the length of the food chain. Landscape characteristics can also indirectly influence fish mercury levels since the amount of runoff differs depending on plant cover and topography. Local weather and global climate factors can also impact mercury levels in fish since they can alter air circulations patterns, temperature, and precipitation. Understanding how these factors work together to impact mercury levels in fishes is important to ensure food safety.
Miranda Chen and colleagues recently conducted an analysis to determine how mercury levels in four freshwater fishes in Ontario watersheds had changed over time and what factors were driving those changes. They collected publicly available data on fish mercury levels, local weather conditions, and global climate data to investigate these questions. Their analysis focused on four fish species: walleye and northern pike, which inhabit cold water, and smallmouth and largemouth bass, which inhabit warmer waters. These fishes all serve as top predators in their environments and tend to have higher mercury levels than other species.
Changes in mercury levels from 1972-1992
Miranda Chen and colleagues found that in the majority of Ontario watersheds, fish mercury levels have not changed. They did, however, identify several watersheds where there have been changes in fish mercury levels. Examining historical records, Chen and colleagues reported that mercury levels of walleye and northern pike decreased from 1972-1992. During this same time period mercury levels in smallmouth bass remained unchanged or decreased, while levels in largemouth bass increased, but not significantly. The mercury levels decreased more in watersheds where there had been heavy industrial mercury pollution, likely as a result of more stringent environmental regulations.
Changes in mercury levels from 1993-2014
When Chen and colleagues examined more recent records from 1993 to 2014 they determined that many watersheds have reported increasing mercury levels in all four fishes, although these trends were not significant. In fact, there were several watersheds where mercury levels were decreasing during this same time period.
What is causing changes in mercury levels?
After documenting the changes in mercury levels, Chen and colleagues examined what factors may be responsible. They were able to explain 88% of the variability in fish mercury levels with local weather, large-scale climate drivers, and human-caused mercury emissions. It is important to note that each fish species had different factors that led to the patterns Chen and colleagues documented. For example, air temperature explained 41% of the variation in mercury levels of largemouth bass from historical records, while precipitation accounted for 10% of the variation from recent data (1993-2014). Warmer temperatures can impact fish mercury levels because they increase metabolism, resulting in a higher feeding rate and thus increasing heavy metal accumulations. Precipitation, such as rain and snow, can increase mercury levels by bringing mercury trapped in the atmosphere into the aquatic environment and enhancing land run-off.
Overall, Chen and colleagues reported that the mercury levels in all four of the predatory fishes included in the study mostly declined in Ontario from 1972-1993. However, since 1993 the mercury levels in these fishes have been mostly increasing. While local weather, global climate factors, and mercury emissions were able to explain most of these patterns, there are still other factors that warrant further investigation. For example, future research efforts should focus on determining the processes involved in global mercury transport and cycling. Since these fish species are important components of many people’s diets, it is essential to determine how mercury levels will change in the future to ensure food security.
Source: Chen, M.M., Lopez, L., Bhavsar, S.P., Sharma, S., 2018. What’s hot about mercury? Examining the influence of climate on mercury levels in Ontario top predator fishes. Environmental Research 162: 63-73. https://doi.org/10.1016/j.envres.2017.12.018