The science of sustainable seafood, explained

Mercury in seafood impacted by fishery management and climate change

We know that climate change and poor fishery management are the largest threats to sustainable fisheries, but new research out of Harvard University shows they also threaten the safety and nutrition of seafood we eat.

In a paper recently published in Nature, researchers showed a troubling relationship between climate change, fishery management, and mercury in seafood.

Mercury in seafood

Mercury is a natural toxin that bioaccumulates in fish, meaning it concentrates in large species higher in the food chain as they eat smaller fish with small amounts of mercury in them. Billfish and sharks contain particularly high levels.

Consuming lots of seafood with high amounts of mercury is not a health concern for most people, but it can cause developmental problems in young children and fetuses. The FDA recommends that young children and pregnant women (or women wishing to become pregnant) consume seafood (there are several important health benefits e.g. omega-3), but limit the amount of high-mercury fish, like billfish and shark.

Here is a handy chart:

FDA guidelines for pregnant women and young children eating seafood

There has always been a natural amount of mercury in fish, but air pollution over the past century has greatly increased the amount of mercury in the ocean and in seafood. Stricter regulations have reduced emissions, but a subsequent decrease in mercury concentrations in fish has not followed for several key species—researchers claim poor fishery management and climate change is to blame.

Mercury concentrations in fish under overfishing and climate change

Researchers studied fish in the Gulf of Maine to see how mercury concentrations changed over time. They found that poor fishery management and warmer water both had an important relationship to mercury levels in dogfish and cod, two commercially important species.

Spiny dogfish and Atlantic cod were perfect study species as both occupy a similar niche as bottom-dwelling predators that feed on herring. Researchers observed that when the herring population in the Gulf of Maine collapsed due to overfishing, the diets of dogfish and cod diverged. Dogfish began eating more squid (higher mercury levels than herring), while cod began eating more sardines (lower mercury levels). With the dietary switch, dogfish began bioaccumulating more mercury while cod less. Though dogfish (a kind of shark) would already be on the “Choices to Avoid” list above, increasing mercury levels in the fish could be dangerous and highlight the importance of good fishery management. With predictable, sustainable ecosystems, mercury levels in fish are stable, eliminating uncertainty and giving seafood consumers peace of mind when making dietary choices.

Researchers also noted the effect of warm water on mercury levels and built a computer model to predict how further climate change will impact mercury in seafood. In warmer water, fish metabolism is higher, meaning they need to eat more. Since mercury bioaccumulates, a fish that eats more will contain more mercury. With further climate change, it will be imperative to monitor mercury levels in fish, particularly tuna which are greatly affected by temperature and one of the most consumed fish in the world. A study from 2014 showed that nearly 40% of American’s exposure to mercury comes from tuna consumption.

Reducing mercury in seafood

However, there are policies that can counteract higher mercury concentrations. Further reducing mercury emissions will reduce the amount that settles in the sea and enters the oceanic food web. An easy way to make an impact is to shut down coal-fired power plants as mercury is commonly released into the atmosphere as a byproduct of coal burning. Shuttering those power plants and switching to clean, renewable electricity will both reduce the amount of mercury in our seafood and mitigate further ocean warming by reducing the amount of carbon in the atmosphere.

A global agreement meant to reduce worldwide mercury emissions was signed in 2017, though President Trump has proposed cutting regulations on coal-fired power plants which would certainly increase the amount of mercury in the atmosphere, the ocean, and eventually our seafood.


Picture of Max Mossler

Max Mossler

Max is the managing editor at Sustainable Fisheries UW.

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