Mercury in fish and seafood

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Mercury is a type of heavy metal that is often found in fish and seafood products. It’s almost ubiquitous in seafood, and the best you can do is to minimize your exposure to it.

How mercury gets into fish

Step 1: Contamination

There are several ways that mercury can enter aquatic ecosystems and accumulate in fish:

Industrial emissions: Mercury is released into the air through the burning of coal and other fossil fuels, and it can also be released through the manufacturing of certain products. This mercury can then enter rivers and lakes through rainwater, and it can be absorbed by tiny organisms at the bottom of the food chain.
Agricultural runoff: Mercury can also enter aquatic ecosystems through agricultural runoff from fields where mercury-containing pesticides are used. This runoff can carry the mercury into rivers and lakes, where it can be absorbed by small organisms.
Natural sources: Mercury can also enter aquatic ecosystems through natural sources, such as volcanic eruptions and the weathering of rocks that contain mercury.

Step 2: Conversion

Mercury is then converted into methylmercury. Methylmercury is formed through a process called methylation, which occurs when bacteria in the environment convert inorganic mercury into the more toxic organic form known as methylmercury. This process can occur in the water, soil, or sediments of an aquatic ecosystem.

Step 3: Accumulation

Methylmercury is able to bioaccumulate up the food chain through a process called biomagnification. It starts when small amounts of methylmercury enter an aquatic ecosystem and are absorbed by tiny organisms at the bottom of the food chain, such as plankton and algae.

As these small organisms are consumed by larger ones, such as small fish, the concentration of methylmercury in their bodies increases. This process continues as larger fish consume smaller fish, and the concentration of methylmercury can become much higher in the bodies of the larger predatory fish at the top of the food chain.

Methylmercury is also able to bind to proteins in the bodies of fish and other seafood, making it difficult for the mercury to be eliminated. This means that the mercury can accumulate over time and remain in the bodies of fish and other seafood even when they are no longer exposed to methylmercury.

Regulatory methylmercury limits

The Food and Drug Administration (FDA) has set recommendations for the safe consumption of seafood, including limits on the amount of methylmercury that is considered safe to consume. These recommendations are intended to help protect the public from the potential health risks of consuming too much methylmercury from seafood.

The FDA has established a reference dose for methylmercury, which is the maximum daily intake of methylmercury that is considered to be safe for most people. This reference dose is based on the levels of methylmercury that have been shown to cause no adverse health effects in studies of people exposed to methylmercury. The reference dose for methylmercury is 0.1 micrograms per kilogram of body weight per day. In Canada, Health Canada has set this reference dose to 0.2 micrograms per kilogram of body weight per day.

The FDA has also established a maximum contaminant level (MCL) for methylmercury in seafood, which is the maximum allowable level of methylmercury in seafood products that are sold for human consumption. The MCL for methylmercury in seafood is 1.0 parts per million (ppm). This means that seafood products that contain more than 1.0 ppm of methylmercury are considered to be unsafe for human consumption.

In Canada, Health Canada has established a maximum acceptable concentration (MAC) for methylmercury in seafood, which is the maximum level of methylmercury that is considered to be safe for most people. The MAC for methylmercury in seafood is 0.5 ppm. This means that seafood products that contain more than 0.5 ppm of methylmercury are considered to be unsafe for human consumption by Health Canada.

Health risks of methylmercury

Methylmercury is a toxic substance that can accumulate in the body over time. Consuming too much methylmercury from seafood can be harmful to your health.

The main health risk of consuming too much methylmercury from seafood is that it can cause damage to the nervous system. Symptoms of methylmercury poisoning may include tremors, memory loss, numbness or tingling in the hands and feet, vision problems, and difficulty speaking or walking.

High levels of methylmercury can also affect the development of the brain and nervous system in fetuses, infants, and young children, and can lead to developmental delays and other problems.

In addition to these risks, consuming too much methylmercury from seafood can also increase the risk of cardiovascular disease, including heart attacks and strokes.

Who is at higher risk?

It is generally recommended that pregnant women, nursing mothers, and young children should avoid eating certain types of fish that are known to be high in methylmercury.

This is because methylmercury can accumulate in the body over time, and the developing brains and nervous systems of fetuses, infants, and young children are more vulnerable to the harmful effects of this toxic substance.

Consuming high levels of methylmercury during these critical periods of development can lead to developmental delays and other problems.

Mercury in fish

Generally high

Fish that are high in methylmercury are typically those that are at the top of the food chain and consume smaller fish that have accumulated mercury from their own diet. These types of fish may include:

Shark
Swordfish
King mackerel
Marlin
Orange roughy
Chilean sea bass
Tilefish
Ahi tuna
Bigeye tuna
Southern bluefin tuna

Generally low

Fish that are lower in the food chain and have a diet that consists mainly of plankton and other small organisms are generally lower in mercury. These types of fish may include:

Salmon
Tilapia
Pollock
Catfish
Cod
Haddock
Plaice
Sole
Flounder
Whiting

Mercury in specific fish

Sashimi

Sashimi is a type of Japanese cuisine that consists of thinly sliced raw fish. The levels of methylmercury in sashimi can vary depending on the type of fish that is used. Here are some different types of sashimi and their relative levels of methylmercury:

Salmon sashimi – is generally low in methylmercury.
Tuna sashimi – can vary in its concentration of methylmercury depending on the species and size of the fish. In general, tuna species that are larger and higher in the food chain tend to have higher levels of methylmercury, while smaller and lower-level species tend to have lower levels. As a result, tuna sashimi can vary in its levels of methylmercury.
Yellowtail sashimi – is generally low in methylmercury.
Snapper sashimi – is generally low in methylmercury.
Halibut sashimi – is generally low in methylmercury.
Mackerel sashimi – can have moderate to high levels of methylmercury.
Eel sashimi – can have moderate to high levels of methylmercury.
Sea urchin sashimi – is generally low in methylmercury.
Scallop sashimi – are generally low in methylmercury.

Tuna

Tuna is a type of fish that can vary in its concentration of methylmercury depending on the species and size of the fish. In general, tuna species that are larger and higher in the food chain tend to have higher levels of methylmercury, while smaller and lower-level species tend to have lower levels. Here are some different types of tuna and their relative levels of methylmercury:

Albacore tuna – This type of tuna tends to have relatively high levels of methylmercury, with an average of around 0.34 parts per million (ppm).
Yellowfin tuna – Yellowfin tuna also tends to have relatively high levels of methylmercury, with an average of around 0.30 ppm.
Bigeye tuna – Bigeye tuna tends to have the highest levels of methylmercury among the common species of tuna, with an average of around 0.45 ppm.
Skipjack tuna – Skipjack tuna is the most commonly caught species of tuna, and it tends to have the lowest levels of methylmercury, with an average of around 0.12 ppm.
Bluefin tuna – Bluefin tuna is a highly prized species of tuna and it tends to have relatively high levels of methylmercury, with an average of around 0.35 ppm.
Southern bluefin tuna – Southern bluefin tuna is another highly prized species of tuna and it also tends to have relatively high levels of methylmercury, with an average of around 0.33 ppm.
Longtail tuna – Longtail tuna, also known as northern bluefin tuna, is a species of tuna that is found in the Indian and Pacific Oceans. It tends to have relatively high levels of methylmercury, with an average of around 0.29 ppm.
Black tuna – Black tuna is a species of tuna that is found in the Atlantic Ocean and the Mediterranean Sea. It tends to have relatively low levels of methylmercury, with an average of around 0.16 ppm.
Pacific bluefin tuna – Pacific bluefin tuna is a species of tuna that is found in the Pacific Ocean. It tends to have relatively high levels of methylmercury, with an average of around 0.30 ppm.
Atlantic bluefin tuna – Atlantic bluefin tuna is a species of tuna that is found in the Atlantic Ocean. It tends to have relatively high levels of methylmercury, with an average of around 0.32 ppm.
Frigate tuna – Frigate tuna is a species of tuna that is found in the Pacific and Indian Oceans. It tends to have relatively low levels of methylmercury, with an average of around 0.16 ppm.
Rainbow tuna – Rainbow tuna is a species of tuna that is found in the Pacific Ocean. It tends to have relatively low levels of methylmercury, with an average of around 0.14 ppm.
Bullet tuna – Bullet tuna is a species of tuna that is found in the Pacific Ocean. It tends to have relatively low levels of methylmercury, with an average of around 0.13 ppm.
Little tuna – Little tuna, also known as kawa kawa, is a small species of tuna that is found in the Pacific and Indian Oceans. It tends to have relatively low levels of methylmercury, with an average of around 0.12 ppm.
Black skipjack – Black skipjack is a species of tuna that is found in the Pacific and Indian Oceans. It tends to have relatively low levels of methylmercury, with an average of around 0.12 ppm.
Striped tuna – Striped tuna is a species of tuna that is found in the Pacific and Indian Oceans. It tends to have relatively low levels of methylmercury, with an average of around 0.11 ppm.

Salmon

Salmon is a type of fish that is generally low in methylmercury. However, the levels of methylmercury in salmon can vary depending on the type of water in which the fish live and the level of mercury in their diet. Here are some different types of salmon and their relative levels of methylmercury:

Wild salmon – Wild salmon, which is caught in the wild, tends to have lower levels of methylmercury than farmed salmon.
Farmed salmon – Farmed salmon, which is raised in fish farms, can have higher levels of methylmercury than wild salmon. This is because farmed salmon are often fed a diet that includes fishmeal, which can contain mercury.
Sockeye salmon – Sockeye salmon is a type of wild salmon that is generally low in methylmercury.
Coho salmon – Coho salmon is a type of wild salmon that is also generally low in methylmercury.
Chinook salmon – Chinook salmon, also known as king salmon, is a type of wild salmon that tends to have moderate levels of methylmercury.
Pink salmon – Pink salmon is a type of wild salmon that is generally low in methylmercury.
Chum salmon – Chum salmon is a type of wild salmon that is also generally low in methylmercury.

Mercury in other seafood

Non-fish seafood generally tends to be lower in methylmercury compared to many species of fish. Here is a list of non-fish seafood that tend to be low in methylmercury:

Crustaceans – includes species such as crab, lobster, shrimp, and crayfish.
Mollusks – includes species such as clams, mussels, scallops, and oysters.
Cephalopods – includes species such as squid and octopus.
Echinoderms – includes species such as sea urchins and sea cucumbers.

Mercury in seaweed

Seaweed is a type of marine algae that is commonly used in culinary applications. It generally tends to be low in methylmercury. Here is a list of some common types of seaweed and their typical concentrations of methylmercury:

Nori – Nori is commonly used to make sushi rolls. It tends to have very low levels of methylmercury, with an average of around 0.02 ppm.
Wakame – Wakame is commonly used in soups and salads. It tends to have very low levels of methylmercury, with an average of around 0.02 ppm.
Kombu – Kombu is commonly used to make dashi, a Japanese broth. It tends to have very low levels of methylmercury, with an average of around 0.02 ppm.
Dulse – Dulse is commonly used as a snack or in salads. It tends to have very low levels of methylmercury, with an average of around 0.02 ppm.
Hijiki – Hijiki is commonly used in salads and side dishes. It tends to have very low levels of methylmercury, with an average of around 0.02 ppm.
Arame – Arame is commonly used in salads and side dishes. It tends to have very low levels of methylmercury, with an average of around 0.02 ppm.
Irish moss – Irish moss is commonly used as a thickener or emulsifier in food products. It tends to have very low levels of methylmercury, with an average of around 0.02 ppm.