You’ve probably heard by now that tuna contains mercury. When we eat tuna, we ingest the mercury along with it. Eating too much mercury has bad health consequences for humans. Mercury poisoning can cause vision impairment, loss of emotional stability, anxiety, memory problems, depression, and irritability. None of us want that! However, a small amount of mercury won’t affect your health. So, how much tuna is safe to eat on a regular basis?
Surprisingly, the answer varies based on the species of tuna you eat. There are about a dozen common tuna species that people eat. Some kinds of tuna contain six times as much mercury as others! If we want to understand why some tuna has more mercury than others, we must understand the concept of bioaccumulation.
What is Bioaccumulation?
Bioaccumulation is the gradual concentration of a chemical in an organism. In our case, the chemical is mercury and the organism is tuna. Mercury exists in the oceans, where tuna live. Some of this mercury arrived in oceans via volcanic eruptions and natural events. Human activities, such as mining and industry have also added mercury to the oceans. In the ocean, algae and seaweed absorb mercury during photosynthesis.
Next, herbivores, such as small fish and krill eat these plants. These herbivores need to eat a lot of plants to live and grow. These animals digest the plants and turn most of the material into energy and waste. However, these animals can’t digest mercury at the same rate as the rest of the plant. This means that every time the animals eat more plants, they add more mercury to their bodies.
(Imagine the plus signs as mercury. Notice how the concentration of plus signs increases the higher up the food chain.)
As we move up the food pyramid, mid-level carnivores eat the herbivores. Again, these carnivores must eat many herbivores to stay alive. Each of these herbivores have all that mercury built up in their bodies from the plants they ate. As the carnivore eats the herbivore, it ingests a good portion of the mercury that the herbivore ate over its entire life.
Tuna are near the top of the food chain. They eat the mid-level carnivores. The result? They eat all the mercury from these mid-level carnivores and store it in their bodies. And when humans eat tuna? Yup, we store the mercury in our bodies.
How Do Some Species of Tuna Have More Mercury Than Others?
Not all tuna are at the same place on the food chain. Species such as bluefin, yellowfin, and bigeye tuna are enormous fish that can weigh 400-1000 pounds. These kinds of tuna eat tons and tons of smaller fish throughout their lives. They also live relatively long lives, which means they need to eat more fish compared to a shorter-lived fish over the course of their life. Their large size and longer lifespan result in high levels of mercury. Swordfish and shark are other examples of high-level predators with a lot of mercury in their bodies.
Tuna species such as skipjack, a commonly canned tuna, only get up to 75 pounds and live just over a decade. Since these fish are lower in the food chain and don’t live as long, they don’t accumulate as much mercury as other tuna species. Shrimp have the lowest levels of mercury of any seafood. This makes sense because they are very low on the food chain. Other kinds of seafood low in mercury include oysters, scallops, sardines, and anchovies. These are all small species that live low on the food chain.
How Much Mercury is Safe to Eat?
The EPA suggests the daily maximum mercury an adult should consume is 0.1 microgram per kilogram of body weight. This translates to 55 micrograms per week for an adult who weighs 175 pounds.
Bigeye tuna have the most mercury, at nearly 60 micrograms per 3 ounce serving. Yellowfin and albacore tuna have about half as much mercury, containing 30 micrograms for the same portion. Skipjack, on the other hand, only has 12 micrograms of mercury.
This means that a 3 ounce serving of bigeye tuna exceeds the weekly maximum amount of mercury for a typical adult. The same person could safely eat almost 14 ounces of skipjack tuna per week. For reference, most tuna cans contain 5-8 ounces of fish. This means that most people can safely eat two cans of low-mercury tuna per week. Tuna is a healthy food when eaten in moderation since it is high in omega-3 fatty acids and nutritious proteins.
Beware! Small children, infants, and pregnant women should avoid tuna entirely. Mercury affects young people much more than adults.
Low Mercury Tuna is Also Better for Our Oceans
Overfishing is a huge issue in the tuna industry. In 2010, Greenpeace added albacore, bluefin, yellowfin, and bigeye tuna to their Seafood Redlist. The species on this Redlist are types of seafood commonly sold in grocery stores that are severely overfished. Bluefin tuna are actually endangered with extinction. Albacore tuna is considered threatened with extinction.
Skipjack, on the other hand, is currently considered a sustainable fish. By choosing a lower mercury tuna, such as skipjack, you are also choosing a more sustainable fish. Some cans say ‘chunk light tuna’, which typically means skipjack tuna. However, yellowfin and bigeye tuna can be mixed into these cans as well.
The safest bet for your body and our oceans is to buy fish clearly labeled as ‘skipjack tuna’. Rest assured knowing you can eat a few cans of this tuna each week without having health consequences associated with mercury poisoning.
Disclaimer: this is an opinion piece, please consult your doctor before significantly changing your diet.
Written by Evan Levy on November 21, 2019.