Mercury Levels in Fish: Which Fishes Have the Lowest and Highest Mercury Levels?

As they move through their life cycles, fish obtain mercury from their surroundings and respective diets. Though fish and shellfish are an essential component of a healthy diet, it’s best to avoid species high in mercury.

Mercury in fish is associated with the decrease of fine motor skills, memory, and attention over time. Fortunately, a balanced diet can mitigate the risks associated with mercury consumption.

This guide will tell you how fish accumulate mercury, the risks of consuming mercury in fish, and what fish species deserve a place in your diet.

What is Mercury, and How Does It Get Into Fish?

Mercury is a naturally occurring heavy metal that occasionally results from human activities, such as burning fossil fuels, industrial processes, and mining.

In aquatic environments, bacteria transform elemental mercury into methylmercury, a highly toxic form that can accumulate in the tissues of fish.

As smaller fish consume plankton and other organisms, they can take in small amounts of methylmercury.

When larger predatory fish consume these smaller fish, the mercury accumulates and becomes more concentrated as it moves up the food chain.

This biomagnification process results in higher mercury levels in larger fish, making them a potential source of exposure to humans who consume them.

Why is Mercury in Fish a Concern?

Mercury in fish is a concern due to its potential adverse health effects on humans.

Methylmercury, the toxic form of mercury that accumulates in fish, is a potent neurotoxin that can harm the nervous system, particularly in developing fetuses, infants, and young children.

Prolonged or excessive consumption of fish with high mercury levels can lead to cognitive and developmental issues, impacting learning, memory, and overall neurological function. Pregnant women, nursing mothers, and young children are especially vulnerable to these effects.

As fish is a valuable source of nutrition, striking a balance between its benefits and the risks posed by mercury is crucial to ensuring both optimal health and minimal exposure to this harmful element.

Fish Low in Mercury

Fish with low mercury levels are safer options for consumption, especially for pregnant women, nursing mothers, and young children. Here are some examples of fish with relatively low mercury levels:

1. Salmon

Salmon is low in mercury because it occupies a relatively low position in the marine food chain. This species feeds on plankton and smaller organisms that contain lower levels of mercury.. Additionally, salmon's relatively short lifespan and migratory behavior contribute to its lower mercury levels compared to larger, longer-lived predatory fish.

2. Sardines

Sardines are low in mercury because they consume phytoplankton and small zooplankton, which are lower in mercury content compared to larger fish. This diet limits their exposure to higher levels of mercury. Additionally, sardines are a shorter-lived species, further reducing the time for mercury accumulation in their bodies, much like salmon.

3. Anchovies

Like sardines and salmon, anchovies are low in mercury due to their dietary preference for plankton and small organisms that are lower in the food chain and thus contain less mercury (they only have 0.09 ppm, among the lowest mercury levels in all fish).

4. Trout

Feeding habits aren’t the only reasons trout is low in mercury (though they play a significant role, as this fish feeds primarily on insects, smaller fish, and aquatic plants). They also live in freshwater environments less exposed to mercury and are lower down the food chain.

5. Herring

This low-mercury fish dwells in the Atlantic Ocean, North Pacific, and Baltic Sea, in environments with limited mercury exposure. In addition, they consume plankton and small marine organisms far lower on the food chain. Instead of being high in mercury, herring is stuffed with omega-3 fatty acids, vitamin D, calcium, zinc, and iron.

6. Cod

While they feed on larger organisms like fish and crustaceans, cod is not a top predator in the marine food chain, making them lower in mercury. In addition, they dwell in colder, deeper waters where mercury levels are generally low.

Cod isn’t just a low-mercury fish – it’s a low-fat source of protein, ideal for individuals limiting fat intake and monitoring heart health. It also contains high iodine levels, good for balancing thyroid function. It has a mild flavor and dense, flaky meat.

7. Flounder

Flounder is a saltwater white fish texturally similar to tilapia and high in vitamin B12. They have very low amounts of mercury and are a delicious option for children and pregnant women.

Because they are bottom-dwellers, flounder are less exposed to predatory fish that accumulate higher levels of mercury. In addition, they dwell in coastal and estuarine environments with lower mercury contamination levels.

8. Tilapia

Tilapia is a freshwater fish found in lakes, ponds, and streams, physically identifiable by protruding mouths and swollen lips. Because they are primarily herbivores and don’t prey on other fish, tilapia boast lower mercury levels and are safer to consume.

They also have faster reproductive cycles, which limits the time available for mercury accumulation.

9. Pollock

This greenish-black fish is rich in proteins and omega-3 fatty acids while maintaining low mercury content.

Like cod, pollock is a bottom-dweller and are less exposed to mercury-infested environments.

Fish High in Mercury

Fish with high mercury levels include larger predatory fish, as mercury tends to accumulate as you move up the food chain. High-mercury fish can be harmful to human health if consumed in excessive amounts, as mercury is a toxic heavy metal. Some examples of fish with high mercury levels include:

1. Shark

Sharks high levels of mercury because they are at the top of the oceanic food chain. As apex predators, sharks consume a variety of smaller fish and marine organisms that may already contain traces of mercury. Some body concentrations of mercury in sharks are ten times higher thant he species they consume.

2. Swordfish

Many advise against eating more than one portion of swordfish a week, as they contain mercury loads nearly identical to those found in sharks. As an apex predator, swordfish often consume organisms with high mercury accumulation.

In addition, swordfish live a relatively long time and can accumulate more mercury over the years. When consumed responsibly, however, swordfish is a good source of selenium, which provides heart health benefits.

3. King Mackerel

While it doesn’t contain as much mercury as shark or swordfish, king mackerel has a mercury concentration of 0.730 ppm. Like sharks and swordfish, king mackerel rest at the top of the food chain, preying on fish with high mercury levels.

They also have longer lifespans, allowing for more time to accumulate mercury. In addition, they tend to be oily and are listed by the FDA as a “once a week choice.”

4. Tilefish

Tilefish are a predatory species that dwell in the bottom of the ocean with an average ppm of 1. According to the FDA, they are among the species that warrant greatest concern because of mercury levels.

Like most apex predators, tilefish are large, feed on other species that contain mercury, and have a longer lifespan, allowing for mercury accumulation. However, when consumed in moderation, tilefish are generally low in fat and sodium and are a good source of niacin and phosphorus.

5. Bigeye Tuna

While bigeye tuna is very nutritious, most dieticians recommend limiting intake because of its high mercury levels. On addition, they are often fished with a moderate-to-high bycatch of seabirds, turtles, and other marine mammals that put this species at risk for mercury contamination.

As their name suggests, they are large animals and feed on smaller species that contain a lot of mercury.

Related: Tuna vs. Salmon: What’s the Difference?

6. Marlin

Their size, coupled with their predatory behavior, increases their risk of mercury contamination, making them a high-mercury seafood choice.Though still high in mercury, marlin is a healthier alternative to bigeye tuna, as it’s low in saturated fat and rich in vitamins B6 and B12. In addition, it’s full of hearty protein. Still, be mindful to consume it only in moderation.

Summary

While mercury in fish poses a legitimate concern, balancing your meal is the key to healthy outcomes. Always consume fish with lower mercury levels. If you can’t help the delectable taste species like shark, bigeye tuna, and marlin leave in your mouth, consume them in moderation to avoid contamination and health complications.