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Mercury in Fish

Mercury is a neurotoxin that affects the brain and nervous system and is particularly dangerous for women who are pregnant as it can have adverse effects on a child’s development. Once released into the environment, inorganic mercury is converted to organic mercury (methylmercury) which is the primary form that accumulates in fish and shellfish.

The predators in the ocean generally contain the most mercury because of the quantity of fish they consume. Because when bigger fish eat smaller ones, the predators also absorb their prey’s contamination in a process known as biomagnification. So when a tuna eats a bunch of anchovies, the tuna is accumulating the mercury of those anchovies into its own body.

Tilefish usually have the highest levels measuring as many as 1.45 parts per million of mercury. Other fish found to have high levels of mercury include grouper, swordfish, Spanish mackerel, Chilean sea bass, yellow fin tuna and canned albacore tuna. Species such as cod, halibut, lobster, snapper and canned tuna contain moderate amounts of mercury. Other fish meeting the threshold of more than 0.5 parts per million of mercury, include king mackerel, shark, marlin, orange roughy and both bigeye and ahi tuna.

Because mercury levels build up in the blood over time, it is best to eat other large fish with high levels of mercury sparingly. Eating three servings or less of these fish per month prevents side effects from mercury.

Fish with the lowest mercury levels include smaller species of both fish and shellfish. These varieties include anchovies, crabs, salmon and sardines catfish, clams, scallops and tilapia have the least amounts of mercury. Freshwater trout, shrimp, oysters and flounder are other safe choices. People can eat these as often as desired unless local advisories warn of higher-than-normal mercury levels.

Although these rules of thumb can be helpful, choosing seafood that is both healthy and sustainable can be tricky. Thankfully, there are tools to help you make smart seafood choices more easily. Monterey Bay Aquarium’s Seafood Watch uses a rigorous, science-based process to grade seafood choices as “best choices,” “good alternatives,” or seafood to “avoid.” Checking this guide or downloading the app before heading to the grocery store or ordering from a restaurant is the smartest way to decide which fishing practices you want your dollars to support.

Least Mercury

Enjoy these fish

  • Anchovies
  • Butterfish
  • Catfish
  • Clam
  • Crab (Domestic)
  • Crawfish/Crayfish
  • Croaker (Atlantic)
  • Flounder
  • Haddock (Atlantic)
  • Hake
  • Herring
  • Jacksmelt (Silverside)
  • Mackerel (N. Atlantic, Chub)
  • Mullet
  • Oyster
  • Plaice
  • Pollock
  • Salmon (Canned)
  • Salmon (Fresh)
  • Sardine
  • Scallop
  • Shrimp
  • Sole (Pacific)
  • Squid (Calamari)
  • Tilapia
  • Trout (Freshwater)
  • Whitefish
  • Whiting

Moderate Mercury

Eat six servings or less per month

  • Bass (Saltwater, Striped, Black)
  • Buffalofish
  • Carp
  • Cod (Alaskan)
  • Lobster
  • Mahi Mahi
  • Monkfish
  • Perch (Freshwater)
  • Sheepshead
  • Skate
  • Snapper
  • Tilefish (Atlantic)
  • Tuna (Canned chunk light, Skipjack)

High Mercury

Eat three servings or less per month

  • Croaker (White Pacific)
  • Halibut (Atlantic, Pacific)
  • Mackerel (Spanish, Gulf)
  • Perch (Ocean)
  • Sablefish
  • Sea Bass (Chilean)
  • Tuna (Albacore, Yellowfin)

Highest Mercury

Avoid Eating

  • Bluefish
  • Grouper
  • Mackerel (King)
  • Marlin
  • Orange Roughy
  • Shark
  • Swordfish
  • Tuna (Bigeye, Ahi)