you're reading...


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.



  1. […] Top predators reveal the extent of mercury pollution in Canadian waters by Samantha DeCuollo […]

  2. […] common form of mercury, is a powerful neurotoxin.  (For more information on mercury, check out this OceanBites article!)  The toxin is incorporated in lower trophic level organisms (e.g., algae, […]

  3. […] content due to body changes associated with pregnancy. The researchers measured concentrations of mercury, polychlorinated biphenyls (PCBs), and flame retardants in the blubber of these whales. They also […]

Post a Comment


  • by oceanbites 3 months ago
    Happy Earth Day! Take some time today to do something for the planet and appreciate the ocean, which covers 71% of the Earth’s surface.  #EarthDay   #OceanAppreciation   #Oceanbites   #CoastalVibes   #CoastalRI 
  • by oceanbites 4 months ago
    Not all outdoor science is fieldwork. Some of the best days in the lab can be setting up experiments, especially when you get to do it outdoors. It’s an exciting mix of problem solving, precision, preparation, and teamwork. Here is
  • by oceanbites 5 months ago
    Being on a research cruise is a unique experience with the open water, 12-hour working shifts, and close quarters, but there are some familiar practices too. Here Diana is filtering seawater to gather chlorophyll for analysis, the same process on
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  on  #oceanbites  we are featuring Hannah Collins  @hannahh_irene  Hannah works with marine suspension feeding bivalves and microplastics, investigating whether ingesting microplastics causes changes to the gut microbial community or gut tissues. She hopes to keep working
  • by oceanbites 6 months ago
    Leveling up - did you know that crabs have a larval phase? These are both porcelain crabs, but the one on the right is the earlier stage. It’s massive spine makes it both difficult to eat and quite conspicuous in
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring Cierra Braga. Cierra works ultraviolet c (UVC) to discover how this light can be used to combat biofouling, or the growth of living things, on the hulls of ships. Here, you
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Elena Gadoutsis  @haysailor  These photos feature her “favorite marine research so far: From surveying tropical coral reefs, photographing dolphins and whales, and growing my own algae to expose it to different
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  on Oceanbites we are featuring Eliza Oldach. According to Ellie, “I study coastal communities, and try to understand the policies and decisions and interactions and adaptations that communities use to navigate an ever-changing world. Most of
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Jiwoon Park with a little photographic help from Ryan Tabata at the University of Hawaii. When asked about her research, Jiwoon wrote “Just like we need vitamins and minerals to stay
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring  @riley_henning  According to Riley, ”I am interested in studying small things that make a big impact in the ocean. Right now for my master's research at the University of San Diego,
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Gabby Stedman. Gabby is interested in interested in understanding how many species of small-bodied animals there are in the deep-sea and where they live so we can better protect them from
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Shawn Wang! Shawn is “an oceanographer that studies ocean conditions of the past. I use everything from microfossils to complex computer models to understand how climate has changed in the past
  • by oceanbites 9 months ago
    Today we are highlighting some of our awesome new authors for  #WriterWednesday  Today we have Daniel Speer! He says, “I am driven to investigate the interface of biology, chemistry, and physics, asking questions about how organisms or biological systems respond
  • by oceanbites 9 months ago
    Here at Oceanbites we love long-term datasets. So much happens in the ocean that sometimes it can be hard to tell if a trend is a part of a natural cycle or actually an anomaly, but as we gather more
  • by oceanbites 10 months ago
    Have you ever seen a lobster molt? Because lobsters have exoskeletons, every time they grow they have to climb out of their old shell, leaving them soft and vulnerable for a few days until their new shell hardens. Young, small
  • by oceanbites 10 months ago
    A lot of zooplankton are translucent, making it much easier to hide from predators. This juvenile mantis shrimp was almost impossible to spot floating in the water, but under a dissecting scope it’s features really come into view. See the
  • by oceanbites 11 months ago
    This is a clump of Dead Man’s Fingers, scientific name Codium fragile. It’s native to the Pacific Ocean and is invasive where I found it on the east coast of the US. It’s a bit velvety, and the coolest thing
  • by oceanbites 11 months ago
    You’ve probably heard of jellyfish, but have you heard of salps? These gelatinous sea creatures band together to form long chains, but they can also fall apart and will wash up onshore like tiny gemstones that squish. Have you seen
  • by oceanbites 12 months ago
    Check out what’s happening on a cool summer research cruise! On the  #neslter  summer transect cruise, we deployed a tow sled called the In Situ Icthyoplankton Imaging System. This can take pictures of gelatinous zooplankton (like jellyfish) that would be
  • by oceanbites 1 year ago
    Did you know horseshoe crabs have more than just two eyes? In these juveniles you can see another set in the middle of the shell. Check out our website to learn about some awesome horseshoe crab research.  #oceanbites   #plankton   #horseshoecrabs 
WP2Social Auto Publish Powered By : XYZScripts.com