Watch Out for High Mercury Levels in Fish

While eating fish has health benefits, it can also be harmful — especially for high-risk groups — due to elevated mercury levels.
November 23, 2021

Mercury is a heavy metal that is toxic to humans. While people can come into contact with mercury through the environment, exposure happens more frequently through the food they eat, especially some fish.

Often a by-product of industrial processes, at room temperature, mercury is a liquid that can easily vaporize into the air and make its way into soil, plants and the water supply, where it can be absorbed by fish that we catch and eat.

Experts agree that benefits outweigh the risks when it comes to eating fish. Omega-3 fatty acids, vitamins D and B12, calcium and phosphorus, minerals like iron, zinc, magnesium, potassium — these are just some of the important nutrients in fish that are part of a healthy, balanced diet.

While the metal is naturally occurring, at high levels it can cause mercury poisoning, a sickness with severe and long-lasting health complications.

Fish that contain mercury

Fish with elevated levels of mercury should be avoided, or only consumed about once a month to be safe. They are:

  • Orange Roughy
  • Tilefish
  • Marlin
  • Tuna (Albacore, Bigeye, Ahi)
  • Mackerel
  • Bluefish
  • Sea Bass
  • Grouper
  • Swordfish
  • Shark

Other fish that are safer to consume on a regular basis (but still contain some mercury) are:

  • Bass
  • Lobster
  • Halibut
  • Cod
  • Carp
  • Snapper
  • Perch
  • Tuna (Chunk Light, Skipjack)
  • Anchovies
  • Clam
  • Flounder
  • Herring
  • Mullet
  • Salmon
  • Shad
  • Whitefish
  • Whiting

Symptoms of mercury poisoning

Mercury can affect the nervous system. It can be difficult to tell if someone has mercury poisoning, because its symptoms vary widely and are similar to those of many other illnesses.

Some initial symptoms include:

  • nervousness, anxiety
  • memory problems
  • depression
  • numbness or tremors

As mercury levels rise in the body, more symptoms may appear, such as:

  • metallic taste in mouth
  • nausea, vomiting
  • loss of motor skills and coordination
  • changes in vision, hearing, speech
  • muscle weakness

The symptoms develop and worsen over time through increased or prolonged exposure to mercury. It’s crucial to identify signs of mercury poisoning as soon as possible, as severe and irreversible neurological problems can result.

Diagnosis and treatment of mercury poisoning

Mercury poisoning tends to come on slowly over time with prolonged consumption of mercury, but it can also develop quickly. Either way, if you suspect you have been exposed to high levels of mercury, you should consult a physician immediately to avoid long-term health problems.

Doctors diagnose mercury poisoning through a physical exam and blood tests. A blood or urine test can determine the levels of mercury in the body. A physician will likely ask the patient questions about their environment and diet.

There’s no real cure for mercury poisoning. Treatment involves complete elimination of the metal from the diet; the person must stop eating fish that contains mercury. If levels are especially high, a doctor may need to do what’s called chelation therapy, which is a chemical process in which a solution is injected into the bloodstream to remove heavy metals from the body.

Who is most at risk of mercury poisoning?

Mercury poisoning is especially dangerous for vulnerable or high-risk groups, including children under the age of five, the elderly, sick people or those with compromised immune systems.

When it comes to this particular kind of poisoning, pregnant women are the most vulnerable. Mercury can harm the reproductive system, even causing problems with the fetus, including deformity and a decreased survival rate. Women who are trying to become pregnant or are pregnant or breastfeeding should be especially diligent in ensuring they do not consume too much food that could contain mercury in their diet.

What should food businesses do?

Food businesses need to ensure they only source fish from trusted, reputable suppliers and always practice the safe handling and preparation of seafood. Food Handlers should be aware of which fish contain higher levels of mercury and the health risks they pose, especially for vulnerable groups.

The Canadian Institute of Food Safety’s (CIFS) Guide to Food Service to Vulnerable Persons provides comprehensive information to help food businesses protect vulnerable customers from food poisoning and the importance of adhering to modified diet requirements.