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We live in an era—the Anthropocene—where humans and societies are reshaping and changing ecosystems. Pollution, human-made climate change and overfishing have all altered marine life and ocean food webs.

Increasing ocean temperatures are . This especially affects top predators including marine mammals such as fish-eating killer whales that strongly rely on large fish as seafood for energy.

Now the combination of mercury pollution, climate change and overfishing are conspiring together to . This has . The risk of consuming mercury-contaminated fish and seafood is growing with climate change.

Mercury rising

but mercury is still present in the marine environment.

Methylmercury builds up in the muscle tissue of fish across the food web, "bioaccumulating" in larger and high trophic level predators. This is why larger pelagic fish (for example, tuna, marlins, billfishes and sharks)—those that eat a lot of fish—are in general considered riskier to eat than smaller ones.

In humans, . Children who are exposed to mercury during fetal development and childhood have a .

Climate change can amplify the accumulation of methylmercury in fish and marine mammals at the top of their food webs due to . A warmer and more acidic ocean may increase the amount of methylmercury that enters the food web.

Overfishing can also exacerbate the mercury levels in some fish species. , as well as Atlantic bluefin tuna and Atlantic cod and other fish species are susceptible to increases in methylmercury due to rising ocean temperatures.

Our modelling research work shows that Chinook salmon, the largest Pacific salmon species and main prey of endangered southern resident killer whales, is projected to be exposed to high methylmercury accumulation due to changes in its prey that are driven by climate change.

Under a worst-case climate-change scenario, where greenhouse gas emissions continue to increase and global temperatures , Chinook salmon will see a 10 percent increase in methylmercury. But under a best-case scenario, where emissions are low and global temperature rise is in the order of 0.3C to 1.7C at the end of the century, mercury levels would increase by only one percent.

For forage fish, such as Pacific sardine, anchovy and Pacific herring, which are key ecological and commercial species in the Pacific Rim ecosystem, the methylmercury increase is projected to be 14 percent under the influence of high emissions and three percent under low emissions. Here again, this increase is driven by dietary shifts and changes in the food web composition due to warmer oceans.

Fishing down the food web

. Chinook salmon stocks from the northeastern Pacific Ocean are also dwindling because of natural factors and environmental stressors, including predation, habitat loss, warming oceans and fishing. The combination of these pressures can make Pacific salmon more susceptible to methylmercury bioaccumulation.

When one species is overfished, fishing fleets expand and adjust their targets, often . The cascading effects lead to changes in prey and foodweb composition for the remaining species, likely altering the transfer of organic contaminants such as persistent organic pollutants and methylmercury in top predators.

When fish are removed from the food web, larger fish and top predators may be forced to consume more or different prey, or smaller fish than they usually do. These fish can be highly contaminated with mercury.

The combination of climate change and overfishing are further shifting the composition of fish in the ocean and where they are found. They are also altering the way these species are exposed to pollutants, —fish that are often eaten by humans.

Protecting health and the planet

Based on this evidence, the public health community should revisit and revise fish consumption guidelines for those who are most likely to be exposed to mercury (coastal communities) or experience negative effects (pregnant women, infants and children).

Our simulations show that the projected methylmercury concentrations in forage fish and Chinook salmon will surpass this century, as well as the consumption advisory level issued by the World Health Organization.

In our human-dominated world, it is imperative that we consume fish and shellfish that come from sustainable fisheries and make efforts to reduce ocean pollution. International and national environmental policies, such as the UN and the , can conserve marine species and protect our blue planet for generations to come.

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