Choosing fish for a healthy diet requires understanding methylmercury, an environmental contaminant. This organic form of mercury is a potent neurotoxin, posing a particular health concern for the developing nervous system of a fetus or young child. When consumed, methylmercury accumulates in the body, potentially leading to neurological deficits. Seafood provides valuable omega-3 fatty acids and other nutrients, so the goal is to make informed choices that minimize mercury exposure, not avoid fish entirely. Selecting fish naturally low in this compound allows consumers to gain nutritional benefits while mitigating risks.
Understanding Mercury Bioaccumulation in Seafood
Mercury enters aquatic environments primarily from industrial sources and the atmosphere. Bacteria convert inorganic mercury into the more toxic form, methylmercury, which is then efficiently absorbed by small organisms at the base of the food chain.
The concentration of methylmercury increases exponentially as it moves up the food web, a process known as biomagnification. When a smaller fish is eaten by a larger predator, the predator retains the mercury, leading to higher levels in its own tissues because methylmercury binds strongly to muscle proteins and is not easily excreted.
The amount of mercury in a fish relates directly to its diet, position on the food chain, size, and lifespan. Large, long-lived fish that feed on others (high trophic level) accumulate the highest concentrations. Conversely, smaller, short-lived fish that feed on plankton or plants tend to have the lowest levels, making them safer choices.
Recommended Low-Mercury Choices
The best choices for regular consumption are small, short-lived species low on the aquatic food chain. These fish and shellfish consistently have the lowest levels of methylmercury, allowing for frequent inclusion in the diet.
Shellfish and small, oily fish are excellent low-mercury options, providing high amounts of beneficial omega-3 fatty acids with minimal risk. These include:
- Shrimp
- Scallops
- Clams
- Oysters
- Crab
- Anchovies
- Sardines
- Atlantic mackerel
Other commonly available fish in this category are salmon, tilapia, cod, pollock, and catfish. These “Best Choices” can be enjoyed multiple times per week. Canned light tuna (usually skipjack) is also a low-mercury option. Canned white or albacore tuna contains higher mercury levels and should be consumed less frequently.
Fish to Limit or Avoid
Fish that should be limited or avoided entirely are those at the top of the food chain. Their predatory nature and extended lifespan result in mercury concentrations significantly higher than those found in smaller fish.
The fish with the highest mercury levels, which should be avoided entirely, include shark, swordfish, king mackerel, and Gulf of Mexico tilefish. These large predators accumulate mercury far exceeding safety thresholds, making them unsuitable for regular consumption. Orange roughy, a slow-growing, long-lived deep-sea species, should also be avoided.
Other fish categorized as having higher mercury content warrant consumption limits. These “Good Choices” include bigeye tuna, marlin, and Chilean sea bass, and should be eaten less often than “Best Choices” to manage overall mercury intake. Albacore tuna, for instance, has higher mercury content than canned light tuna and is placed in this limited consumption category.
Consumption Guidelines for Vulnerable Populations
The risk associated with methylmercury is greatest for pregnant individuals, those who may become pregnant, nursing mothers, and young children due to the sensitivity of the developing brain and nervous system. Federal health organizations provide specific recommendations to balance the benefits of fish consumption with the need to minimize mercury exposure for these vulnerable groups.
Pregnant and breastfeeding women are advised to consume between 8 and 12 ounces of low-mercury fish per week. This typically translates to two to three servings from the “Best Choices” list. A standard adult serving size is approximately 4 ounces, roughly the size and thickness of a palm.
Children should also eat two servings per week from the low-mercury list, with serving sizes based on age.
Recommended Child Serving Sizes
- Ages 1 to 3: 1 ounce per serving.
- Ages 4 to 7: 2 ounces per serving.
- Ages 8 to 10: 3 ounces per serving.
Consuming fish from the “Good Choices” list should be limited to a single 4-ounce serving per week for vulnerable adults, with no other fish consumed that week. Adhering to these guidelines ensures the neurological benefits of fish are realized without surpassing safe mercury limits.