Top predators reveal the extent of mercury pollution in Canadian waters


Article: Depew, D.; Burgess, N.M.; Campbell, L. Spatial patterns of methylmercury risks to common loons and piscivorous fish in Canada. 2013. Environ. Sci. Technol. DOI: 10.1021/es403534q

It’s that time of year again.  The beautiful colors of peak foliage have come and gone and, as the autumn season progresses, some of us cannot help but dread the cold winter season ahead. It might sound like a bit of a disappointment, but don’t fret – nature’s beauty abounds in other ways, especially near our coastlines!

During dawn hours throughout the winter and spring months, you may be surprised by an eerily nocturnal cry that echoes across the water. This spooky, beautiful tremolo arises from the common loon revealing its arrival on the water. Luckily for us, populations of this bird distribute themselves along the entirety of the Eastern and Western coastlines of the United States throughout the winter and into the spring months. It is a top predator and therefore its presence on rivers, lakes, estuaries, and coasts continually reminds us of the healthy ecosystem in which it resides.


Gavia immer is the scientific name for the common loon, a top predator and a prominent symbol of the northern wilderness. Picture by Bob Weaver.
Gavia immer is the scientific name for the common loon, a top predator and a prominent symbol of the northern wilderness. Picture by Bob Weaver.


During the summer, populations of the common loon travel to Canada to breed. Once they breed, they rely heavily on the presence of fish to feed their hungry young. Yellow perch are considered to be an especially delicious and desirable catch for the loon and so it is favored over other species of fish. In fact, walleye and northern pike, two fish that are piscivorous (hunt for fish), also favor yellow perch and share the position of top predator in Canadian waters. Unfortunately, these top predators of the food web have a greater risk of building-up toxic levels of chemical in their tissues. Dangerous levels of mercury have been found in prey such as yellow perch. Across the Great Lakes region and North America, strong correlations have been made between the mercury levels in adult and juvenile common loons and the mercury levels found in perch. Other investigations found similar correlations pertaining to walleye and northern pike. We know that high levels of mercury enter top predators primarily via the diet, but how does mercury enter the ecosystem and how does it affect loons and piscivorous fish?

Sources of mercury pollution

Mercury pollution is attributed to a variety of anthropogenic activities that release the chemical into our atmosphere. Burning waste, combusting fossil fuels, and smelting metal ores are some ways that mercury is introduced into the atmosphere, where it is eventually deposited onto bodies of water. The metal, if left in its inorganic form, is not very harmful. However, a problem arises once mercury interacts with bacteria. It becomes methylated, or readily converted to a more toxic form called methylmercury.  This toxic form of mercury – MeHg – travels through all levels of the ecosystem, but puts the organisms at the top of the food chain at greatest risk due to bioaccumulation, much like the aforementioned pollutants in humpback whales that you read about last month.

A common loon feeding. If food sources are polluted, toxic levels of chemicals can bioaccumulate the tissue of top predators. Picture by Bob Weaver.
A common loon feeding. If food sources are polluted, toxic levels of chemicals can accumulate in the bird, altering behavior and decreasing reproductive success. Picture by Bob Weaver.


Concern over MeHg levels arose after a series of high-profile poisoning events that took place between the 1950’s and the 1970’s, including the event in Minimata, Japan and the English-Wabigoon River off the Northwest Ontario. Although initial efforts were conducted on fish in a targeted group of former industrial mercury pollution sites, expanded testing revealed that mercury pollution is much more widespread throughout the ecosystem. MeHg’s extremely invasive consequences apply to a variety of vertebrate species, altering neurobehavioral, endocrine, reproductive, and immune system processes. MeHg exposure can disrupt reproduction and productivity by influencing mate selection, hatchling success, and embryonic development of many avian species, including loons. Today, the USA and Canada are leaders in the monitoring of the chemical. Canadian scientists have already monitored mercury contamination levels within 1000 species of fish over the past forty years as part of CARA- Canada’s Clean Air Regulatory Agenda Program.

Determining toxic mercury levels in Canada

As an extension of CARA, the Canadian scientist Dr. Depew and his research team set out to determine the MeHg risks associated with common loons and piscivorous fish throughout Canada. The research crew used the top predator’s favorite food, yellow perch, as an indicator of MeHg exposure. They estimated the amount of mercury in one average-sized yellow perch. Next, they estimated mercury concentrations in yellow perch by generating data, which included nearly 2000 locations in Canada over a twenty-year period. They assigned each location a risk quotient, based on mercury concentrations in yellow perch, to indicate the level of risk to adult breeding common loons, sexually mature female walleyes, and northern pikes.

Risks to common loons and piscivorous fish

Depew and his research team found that although piscivorous fish and common loons were exposed to similar concentrations of MeHg, each group indicated varying degrees of susceptibility to behavioral, health, and reproductive impairments. For example, a very low percentage of the sampling locations (<0.5%) indicated a risk of behavioral impairment for piscivorous fish, but almost 40% of the locations indicated a behavioral impairment risk to common loons. This suggests that common loons are at a greater risk of developing behavioral impairments due to MeHg exposure. However, it appears that piscivorous fish are largely at risk for reproductive and health impairments. For common loons, a low percentage (10%) of the sites exhibited a risk of reproductive impairment, whereas a large majority of the sites (>70%) indicated potential risks for reproductive and health impairment for walleye and northern pike.

Increased mercury concentrations in southeastern Canada

In addition, Depew and his team found that locations associated with the most intense risks were located in southeastern Canada, an area known to have extensive forest cover, acidic soils, and high levels of mercury deposition. This most eastern region of Canada includes the provinces of Ontario, Quebec, and Atlantic. It is estimated that 60% of the total common loon pairs come to these regions to breed, thereby highlighting a potential threat to the population of this species. A similar west to east gradient of increasing mercury levels matched previous models of mercury cycling, which were developed using mercury content in loon feathers, blood, and eggs from the northwestern USA to northeastern North America. The finding suggests the likely presence of a very broad, prominent pattern that exists across the entirety of the North American continent!

Map of Canadian freshwater locations indicating those areas associated with potential risks to common loons due to mercury poisoning (Fig. 1; Depew et al. 2013).


In the past, comprehensive programs in the USA (i.e. MercNet) and in Canada (i.e. CARA) have prompted awareness of mercury pollution. Depew’s work is evidence that top predators are still exposed to unsafe mercury levels. Although similar findings were already established for the Great Lakes region and Northeastern North America, Depew’s research extends the findings to a broader scale. Scientists are continuing to assess the uncertainties pertaining to mercury pollution’s effects. Eventually, data from this project can provide informative results to guide mercury emission regulations in order to protect the common loon and other important keystone predators and to direct future environmental policy decisions.

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