Research finds climate change contributes to rising mercury levels in fish

Underwater photo of a big Northern Pike (Esox Lucius) in a lake.

Existing research suggests that higher levels of mercury in fish consumed by people may harm an unborn baby or young child’s developing nervous system, and concentrated forms of mercury are toxic. Under the supervision of biology Professor Sapna Sharma, York Research Chair in Global Change Biology, MSc student Miranda Chen worked with BSc student Lianna Lopez and the sportfish contaminant group at the Ontario Ministry of the Environment & Climate Change (MOECC) to add to this knowledge. They undertook some pressing research on increasing mercury levels in southern Ontario’s top predator fishes.

“We wanted to evaluate fish mercury trends under multiple stressors and to improve our understanding of how intricate climatic processes can impact fish mercury levels,” Chen explained.

The researchers discovered that a combination of weather, climate and mercury emissions were responsible for rising mercury levels in predatory fishes.

New research looks at the increasing levels of mercury in Ontario fishes, and tries to identify the role of climate. Pictured: northern pike

“Identifying the role of climate on increasing fish mercury levels is crucial, particularly in light of decreasing mercury emissions,” Sharma emphasizes. “It provides a way for us to assess how changing multiple environmental stressors may impact health of wildlife and humans consuming fish.”

Left to right: Miranda Chen and Sapna Sharma
Left to right: Miranda Chen and Sapna Sharma

This research was funded by the Natural Science & Engineering Research Council. The findings were published in Environmental Research (2018).

Why is mercury on the rise after decades of decline?

The idea that there is mercury in the fishes that Ontarians consume is not new. By the 1970s, we knew that this heavy metal was in fishes, and that this was likely caused by elevated industrial emissions such as coal-fired power plants.

Between the 1970s and 2011, as a result of stricter government regulations, mercury emissions declined in North America. But since 2011, they have once again begun to rise. Why is this happening? Sharma and her team believed that this suggests that other factors might be at play. After all, mercury levels in fish can vacillate based on the size or acidity of the lake, the temperature of the water etc. They sought to untangle the potential causes to find the primary cause. More specifically, the study had two primary objectives:

  • to examine how mercury levels in Ontario’s top predator fish have changed between historical and recent time periods; and
  • to investigate how local weather, large-scale climate drivers, and local/global mercury emissions are potentially driving fish mercury trends over time.

“This is one of few studies exploring the changes in fish mercury levels across a large landscape and interacting multiple environmental stressors, including climate change and atmospheric pollution,” Chen said.

To do this, the team examined the relationship between mercury trends and three things:

  • local weather;
  • large-scale climate drivers; and
  • mercury emissions in cool water (for walleye and northern pike) and in warm water (for smallmouth bass and largemouth bass) in both historical (1970-92) and recent (1993-2014) time periods.

The team hypothesized that increases in fish mercury levels over time may more recently be attributed to changes in climate, specifically increases in precipitation events and temperature.

Researchers gather data from variety of sources and records

Total fish mercury measurements were obtained from the MOECC’s Fish Contaminant Monitoring Program, which has collected data from the 1970s. Fish samples were collected in partnership with the Ontario Ministry of Natural Resources & Forestry (MNRF) during late summer or early fall using a variety of methods, including gill netting, trap netting, electrofishing and angling. Climate and emissions variables were obtained from multiple open-access data sources.

This was one of few studies examining the changes in fish mercury levels across a large landscape with multiple environmental stressors

Next, the researchers analyzed the trends in the two time frames – historical (1970-92) and recent (1993-2014). Altogether, the team went through extensive research processes: “Over 1,400 correlation analyses were conducted between predictor variables and fish mercury levels for each species per time period and lag,” Chen explained.

Results corroborate existing research but push beyond

The findings corroborated previous reports that mercury levels in Ontario’s predatory fishes decreased from 1970 to 1992 – again, likely due to regulations – and then increased in the past two decades.

Digging deeper, however, the team unearthed three new findings:

  1. Mercury emission from industry were not the only contributor of higher fish mercury: The recent increase in mercury levels has been attributed to increases in global mercury emissions, but the researchers found this was not the case. “Surprisingly, our results suggest that mercury emissions did not provide any significant explanatory power within the fish mercury,” Chen explained.
  2. Weather and climate were responsible for mercury increase: The researchers’ findings suggest that climate factors have become more influential on fish mercury levels in recent years. In addition to temperature, wetter months translated to increased mercury levels, due to greater runoff and soil erosion. “With a greater amount of mercury entering ecosystems due to increases in precipitation, mercury concentrations in the food web and, ultimately, in fish are also expected to increase,” Chen said.
  3. Climate drivers are key: In this study, mercury levels were positively related with an El Niño Southern Oscillation (ENSO) event. ENSO is one of the greatest drivers of variation in Canadian climate. ENSO is known to influence the winter temperatures and total precipitation in Ontario, particularly the Great Lakes regions. “Changes in climate may be an important driver in Ontario freshwater ecosystems,” Sharma concludes.

Looking to the future, these researchers hope that acquiring improved data on mercury distribution and concentrations will assist modelling capabilities and the ability to predict and manage future changes.

To read the article, visit the website. To learn more about Sharma’s work, visit her lab’s website.

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By Megan Mueller, senior manager, research communications, Office of the Vice-President Research & Innovation, York University,