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Fisheries

Ocean to Table Mercury: a Rising Risk

 

Source: Lavoie, R.; Bouffard, A.; Maranger, R.; Amyot, M. Mercury transport and human exposure from global marine fisheries, Nature Scientific Reports 2018, 8 (6705). DOI: 10.1038/s41598-018-24938-3

 

Fish come in all shapes and sizes, but nearly all carry mercury in their tissues, regardless of species. They weren’t born with this mercury in their bodies. Instead, the metal enters the ocean through mining activity by humans. This mercury can be passively absorbed into the tissues of sea creatures from the surrounding water, or by eating other mercury-containing creatures. Mercury is passed up the aquatic food chain as bigger fish eat smaller fish and collect mercury in their own bodies in a process called bioaccumulation (Figure 1). Fish high on the food chain, like tuna and bonito, have an average whole-body mercury concentration of 0.38 micrograms mercury/gram body mass, while creatures low on the food chain (oysters, mussels, clams) have an average mercury concentration of only 0.015 micrograms/gram of body mass (that is less than 4% than that of the larger fish!).

 

Figure 1. Cartoon explaining the process of methylmercury bioaccumulation. A helpful video explaining mercury bioaccumulation can be found by following this link: (https://youtu.be/Q1ZA8ZrK3U4 ). Image source: https://www.canada.ca/en/environment-climate-change/services/pollutants/mercury-environment/health-concerns/food-chain.html

 

While excess mercury is also harmful to fish, much more attention is given to human exposure to methylmercury, a form of mercury that can cause damage to the nervous, digestive and immune systems and can interfere with the development of fetuses in pregnant women. In their paper, Lavoie and the researchers provide an estimate of how much methylmercury and total mercury are leaving the ocean via fish caught for commercial purposes, how much of that mercury is actually making its way to our plates, and finally how this mercury export has varied over the past 60 years and in different geographic regions.

To follow how much mercury is being fished out of the ocean, the researchers paired data on fish catches from the Food and Agriculture Organization of the United Nations (FAO) with published values for mercury concentrations in a variety of fish species. Based on this information, they were able to measure how much mercury was exiting the ocean in different regions around the world, as well as how this varied in each region over time (Figure 2). They found that the total amount of mercury leaving the ocean through fishing has been increasing steadily since the 1950’s, and that the majority of this fishing has occurred in coastal waters. They estimate that in 2014, nearly 29,000 pounds of total mercury was exported from the global ocean via fishing.

 

Figure 2. Marine fish catches measured in megatons (1 megaton roughly equaling 2.2 billion pounds) on the right-hand vertical axis, and amount of total mercury (Hg) being exported from the ocean through fishing each year measured in kilomoles (1 kmol roughtly equaling 442 pounds) on the left-hand vertical axis. Source: Lavoie et al. (2018) Open Access under the Creative Commons Attribution 4.0 International License. https://doi.org/10.1038/s41598-018-24938-3.

 

Additionally, the researchers found that the proportion of fish coming from coastal regions has been decreasing. This phenomenon is a result of coastal waters being overfished, forcing fishermen to move further from shore and deeper into the sea. This move has an important implication: the open ocean is home to much larger fish that contain higher concentrations of accumulated mercury. The researchers found that between 1950 and 2014, large fish high on the food chain accounted for 63% of the total catch by weight, but contributed to 88% of the mercury exiting the ocean through fishing. The researchers showed that this mercury flow from the ocean to the food economy isn’t just a function of total weight of catch but of the types of fish being caught as well.

The researchers also showed that the amount of mercury being exported from the ocean by fishing varies by location as well. The Northwest Pacific region was the leading exporter of mercury over the entire study period. The Northeast Atlantic followed, with the Southeast Pacific and Western Central Pacific in the successive spots (Figure 3). Again, while the amount of mercury exported in different regions tended to be a function of total mass of caught fish, there were some locations that defied this trend because of the types of fish being caught, specifically large fish with high bioaccumulation of mercury.

 

Figure 3. Regional total mercury export from the ocean via fishing for different time periods (the bottom panel being the most recent). The Northwest Pacific region can clearly be seen as the leading exporter of mercury. Source: Lavoie et al. (2018) Open Access under the Creative Commons Attribution 4.0 International License. https://doi.org/10.1038/s41598-018-24938-3.

 

So what does this mean for human health? Lavoie and his colleagues calculated estimates of consumption of the toxic methylmercury via fish for the populations of 173 different countries and compared these values to the recommended weekly limit of methylmercury consumption provided by the FAO and World Health Organization (WHO). They recommend consuming no more than 1.6 micrograms of methylmercury/kg of body weight. The researchers found that the global weekly mercury intake has been increasing over time, and that the mean intake by people from a whopping 38% of countries assessed exceeded this recommended intake (Table 1). The risk for mercury consumption was particularly high in populations in coastal regions of Southeastern Asia, the Western Pacific, and the Mediterranean, all of which are amongst the top exporters of mercury through fishing.

 

Country

1961–1970 1971–1980 1981–1990 1991–2000 2001–2011

Maldives

9.5 ± 6.6 14.1 ± 2.5 20.1 ± 4.4 24.0 ± 4.3 23.1 ± 5.3

Kiribati

4.7 ± 0.4 5.0 ± 0.2 6.4 ± 1.5 7.5 ± 0.3 8.0 ± 0.8

Iceland

4.7 ± 1.6 7.6 ± 2.3 7.8 ± 2.0 6.2 ± 1.7 7.5 ± 1.6
Malaysia 2.6 ± 0.3 3.9 ± 1.0 5.0 ± 0.3 6.0 ± 0.7

6.4 ± 0.4

Samoa 4.1 ± 0.3 3.5 ± 0.8 4.4 ± 0.5 5.2 ± 1.6

6.4 ± 0.8

French Polynesia

5.2 ± 0.7 3.9 ± 0.6 3.8 ± 0.4 5.0 ± 0.4 5.0 ± 0.1

Lithuania

NA NA NA 2.5 ± 0.9

4.8 ± 0.4

Japan 5.3 ± 0.2 5.9 ± 0.2 6.0 ± 0.2 5.4 ± 0.4

4.8 ± 0.5

Barbados

2.9 ± 0.5 3.3 ± 0.6 4.0 ± 0.7 4.0 ± 0.4 4.8 ± 0.2
Republic of Korea 1.4 ± 0.3 2.5 ± 0.5 3.0 ± 0.4 3.6 ± 0.8

4.7 ± 0.1

Grenada

3.2 ± 0.6 4.4 ± 0.5 3.6 ± 0.8 3.1 ± 0.7 4.6 ± 0.4
Vanuatu 4.6 ± 0.4 5.6 ± 1.0 3.6 ± 0.4 3.5 ± 0.4

4.3 ± 0.3

Fiji

2.2 ± 0.3 2.9 ± 0.6 3.3 ± 0.8 2.2 ± 0.5

4.1 ± 0.7

Sao Tome and Principe 1.7 ± 0.7 1.3 ± 0.6 3.9 ± 1.0 4.1 ± 0.6

3.9 ± 0.2

Philippines

3.4 ± 0.5 4.2 ± 0.4 3.6 ± 0.2 3.7 ± 0.3 3.9 ± 0.2
China (Hong Kong) 2.4 ± 0.5 2.9 ± 0.4 2.3 ± 0.3 3.3 ± 0.3

3.9 ± 0.2

World average 1.3 ± 0.1 1.5 ± 0.0 1.6 ± 0.0 1.6 ± 0.0

1.7 ± 0.0

Table 1. Per capita mean methylmercury consumption as measured by Lavoie and his colleagues. The countries are ordered based on decreasing methylmercury exposure for the 2001-2011 period. Values exceeding the recommended limit provided by the FAO and WHO are bolded. It is important to note that these values are the potential mercury consumption (based on how much fish was available for consumption), and not necessary the actual amounts consumed. Source: Lavoie et al. (2018) Open Access under the Creative Commons Attribution 4.0 International License. https://doi.org/10.1038/s41598-018-24938-3.

The researchers note that in their analysis they calculated the amount of methylmercury in fish available for human consumption as a proxy for human consumption of methylmercury. However, because they did not directly measure how much of this fish was actually being consumed, they may be overestimating the amount of methylmercury consumed. That being said, they excluded the consumption of freshwater and market fish in their calculations, and that the global marine fisheries (the focus of their paper) account for only 49% of the total global seafood production, leaving open the possibility that they instead are underestimating the average methylmercury consumption. Finally, nuances arise when considering whether cooking methods or co-ingestion of other foods, including tea, may influence how much of the mercury actually remains in its harmful methylmercury form when it enters our body. They recommend further research on both country-specific mercury limits (which could vary depending on eating habits and genetics), as well as research focusing on fish consumption to estimate methylmercury exposure rather than the amount of fish available to be consumed.

This paper from Lavoie et al. gives an updated estimate of how much mercury is moving from the ocean to our seafood economy, how this value varies by region, and how it has changed over time. Much of the attention given to the dangers of human activity is centered around greenhouse gases and global warming, but mercury is an example of a compound released into our environment through our pursuit for fuel that can have detrimental impacts on human populations in ways other than climate change. Lavoie, Bouffard, Maranger and Amyot sound the alarm for countries whose habits put them above the recommended intake of methylmercury and draw attention to the need for further research to better understand how much mercury we’re consuming and the effects it may have on our bodies.

Julia Dohner

Julia is a second-year PhD student at Scripps Institution of Oceanography in La Jolla, California. Her focus is on chemical oceanography, which often manifests as the intersection of the biology, chemistry, and physics of the ocean. She joined Dr. Ralph Keeling’s group and is modeling large-scale air-sea gas exchange to better understand how much carbon dioxide the ocean is absorbing from the atmosphere. When not at her computer or reading papers, Julia is usually in the ocean on her surfboard and/or thinking about food.

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