What is the unhealthiest seafood to eat? A Deep Dive into Risky Choices

October 16, 2025 •

5 min read

According to the U.S. Food and Drug Administration (FDA), some large predatory fish like shark, swordfish, and king mackerel are on the list of seafood to avoid, particularly for pregnant women and young children, due to high levels of mercury. This often-cited guidance raises the question: what is the unhealthiest seafood to eat and what factors contribute to its risk?

Quick Summary

An examination of seafood health risks reveals that factors like high mercury, toxic pollutants, unsustainable farming, and preparation methods can make some options unhealthier than others. Large predatory fish accumulate high mercury levels, while certain farmed fish may contain unwanted chemicals. The way seafood is cooked, such as deep-frying, also impacts its nutritional value.

Key Points

High Mercury Accumulation in Predators: Large, predatory fish like shark, swordfish, and king mackerel are the unhealthiest seafood choices due to high concentrations of mercury accumulated over their long lifespans.
Risks Associated with Unregulated Farming: Certain farmed fish, such as some imported tilapia, can contain higher levels of inflammatory omega-6s, antibiotics, and other contaminants due to unhealthy farming practices.
Deep-Frying Adds Unhealthy Fats: Preparation methods like deep-frying significantly increase the saturated fat and calorie content of seafood, making even naturally healthy fish less nutritious.
Other Bioaccumulating Pollutants: Seafood can also contain other contaminants like PCBs and dioxins, with fat-soluble pollutants being more concentrated in fattier fish.
Naturally Occurring Marine Toxins: Reef fish and shellfish can sometimes contain natural toxins, such as ciguatoxins from algal blooms, which are not eliminated by cooking and can cause severe illness.
Safer Choices are Smaller Fish: For a healthier diet, prioritize small, short-lived species like sardines, salmon, and shrimp, which have lower mercury levels and are often richer in beneficial omega-3s.

High-Mercury Species: The Apex Predator Problem

The most significant health risk associated with certain seafood is the presence of high mercury levels. Mercury, a neurotoxin, can cause central nervous system damage and is especially harmful to developing fetuses and young children. This accumulation is a direct result of biomagnification, where mercury concentrations increase as they move up the food chain. Larger, long-lived predatory fish at the top of the marine food chain consume smaller fish that have already absorbed mercury from the environment, leading to a dangerous buildup.

Fish with the highest mercury levels

Shark: As a top-tier predator, sharks contain some of the highest mercury concentrations and are also severely overfished.
Swordfish: Another large, predatory species, swordfish is known for its high mercury content, making it a fish to eat sparingly, if at all.
King Mackerel: The King variety of mackerel is consistently listed among the fish with high mercury levels and is a key species to limit or avoid.
Bigeye Tuna: While canned light tuna is generally lower in mercury, bigeye tuna, often used for steaks and sushi, has significantly higher concentrations.
Tilefish (Gulf of Mexico): This species, particularly from the Gulf of Mexico, has dangerously high mercury levels and should be avoided completely.

The Risks of Farmed Fish

Not all farmed fish is created equal, and some practices raise significant health concerns. Certain aquaculture methods, especially those with poor regulation, can expose fish to a range of contaminants. The quality of farmed fish is heavily dependent on the environment and what the fish are fed.

Concerns with poorly regulated farmed seafood

Lower omega-3 to omega-6 ratio: While wild-caught fish feed on a natural diet rich in omega-3s, some farmed fish are fed unnatural diets, like corn and soy, resulting in lower omega-3s and higher, pro-inflammatory omega-6 fatty acids.
Antibiotics and chemicals: In high-density fish farms, antibiotics are often used to prevent the spread of disease. Chemicals and pesticides can also be introduced, which can pollute the surrounding waters and potentially affect the consumer.
Pollutants and contaminants: Some farmed fish, like tilapia from poorly regulated regions, have been associated with concerning reports of unsanitary conditions and higher levels of pollutants.

Unhealthy Preparation Methods

Even a healthy fish can become an unhealthy meal depending on how it's prepared. The addition of excessive calories, fat, and sodium can undermine seafood's nutritional benefits.

How cooking can affect seafood's health profile

Deep-frying: Battering and deep-frying fish turns an otherwise lean protein source into a high-fat, high-calorie meal loaded with saturated fat. Popular dishes like fish and chips fall into this category.
Cream-based sauces and chowders: While ingredients in a dish like New England clam chowder might be healthy individually, the generous use of cream and butter significantly increases its calorie, saturated fat, and sodium content.
Excessive sodium and sugar: Many pre-made sauces, dips (like bottled cocktail sauce), and processed seafood products contain high levels of sodium and sugar, which can negatively impact cardiovascular health.

Comparison Table: Worst vs. Best Seafood Choices


Feature	Unhealthiest Seafood Examples	Healthiest Seafood Examples
Mercury Content	Shark, King Mackerel, Swordfish, Bigeye Tuna	Salmon, Sardines, Herring, Anchovies, Shrimp
Saturated Fat	Battered and deep-fried fish, cream-based chowders	Steamed or grilled fish, lean species
Contaminants	Some farmed Tilapia (depending on source), bottom-dwelling Catfish	Wild-caught species, particularly from clean, monitored waters
Omega-3s	Farmed Tilapia (high omega-6 ratio)	Oily fish like Salmon, Mackerel, Sardines, Herring
Sustainability	Overfished species like Atlantic Salmon (illegal to catch wild) and some Tuna	Sustainably harvested fish with MSC certification

Beyond Mercury: Other Contaminants in Seafood

While mercury is a well-known concern, other toxins and pollutants can also accumulate in seafood, posing additional health risks. Persistent organic pollutants (POPs), like PCBs and dioxins, are fat-soluble and can accumulate in the fatty tissues of fish. Shellfish and reef fish can also be vectors for naturally occurring marine toxins, such as ciguatoxins from algal blooms, which can cause severe food poisoning. These toxins are not destroyed by cooking.

Making Healthier and Safer Choices

To enjoy the nutritional benefits of seafood while minimizing risks, consider these guidelines:

Prioritize low-mercury options: Opt for small, short-lived fish and shellfish such as wild-caught salmon, sardines, anchovies, shrimp, and scallops, which are low in mercury.
Be mindful of farming practices: When choosing farmed fish, look for responsibly sourced options from farms that use sustainable and hygienic practices, such as closed, onshore tank systems.
Use healthy cooking methods: Prepare seafood by grilling, baking, or steaming rather than deep-frying. This preserves nutrients and avoids adding unhealthy fats and calories.
Follow consumption advisories: Pay attention to local and federal seafood advisories regarding mercury and other pollutants. The Environmental Working Group (EWG) and the Monterey Bay Aquarium Seafood Watch offer helpful guides.

Conclusion

While the omega-3 fatty acids and lean protein in seafood offer substantial health benefits, a discerning approach is necessary to avoid the unhealthiest options. High-mercury predators like shark and swordfish, certain poorly-regulated farmed fish like tilapia, and seafood prepared with unhealthy cooking methods like deep-frying are among the least healthy choices. By choosing smaller, lower-mercury species and opting for healthy preparation, you can enjoy the many nutritional upsides of seafood while minimizing potential health risks. Consulting reputable sources like the FDA and EWG can help you navigate these choices confidently.

Key Takeaways

High-Mercury Fish: Large, long-lived predatory species like shark, king mackerel, and swordfish contain dangerous levels of mercury due to biomagnification.
Avoid Poorly Regulated Farmed Fish: Some farmed fish, such as imported tilapia, can have an unfavorable omega-3 to omega-6 ratio and may be exposed to antibiotics and chemicals.
Watch Preparation Methods: Deep-frying and heavy, cream-based sauces can turn a healthy fish into an unhealthy, high-fat, and high-sodium meal.
Beware of Other Toxins: Beyond mercury, seafood can be contaminated with PCBs, dioxins, and natural toxins from algal blooms, especially in reef fish and shellfish.
Choose Lower-Trophic Species: Smaller fish like sardines, anchovies, and shellfish tend to have significantly lower mercury levels.
Opt for Sustainable and Healthy Cooking: Look for sustainably sourced seafood and prepare it with healthy methods like baking, grilling, or steaming.

Frequently Asked Questions

The FDA recommends avoiding fish with the highest mercury levels, including shark, swordfish, king mackerel, marlin, and tilefish from the Gulf of Mexico. Bigeye tuna should also be limited, especially for pregnant women and children.

No, not all farmed fish is unhealthy. The healthiness depends on the farming practices. Sustainably and responsibly farmed fish can be a good source of nutrition, while fish from poorly regulated farms may be exposed to higher levels of antibiotics and pollutants.

Farmed tilapia, particularly from unregulated sources, can be high in inflammatory omega-6 fatty acids compared to healthier omega-3s. Concerns have also been raised about potential contaminants and unsanitary conditions in some international fish farms.

No, cooking does not reduce the mercury content in fish. Mercury binds to the protein in the fish's muscle tissue and cannot be removed through common cooking methods like frying, grilling, or baking.

PCBs are industrial chemicals that are persistent environmental pollutants. They are fat-soluble and bioaccumulate in fish, especially in fatty tissues. These contaminants can pose risks to human health, including affecting blood, liver, and immune functions.

Most shellfish, like scallops, crabs, and shrimp, are generally low in mercury and a healthy option. The main risk comes from marine biotoxins, such as those from harmful algal blooms, which can cause food poisoning if the shellfish are harvested from affected areas. It is important to follow local advisories.

For the healthiest choices, opt for smaller fish lower on the food chain like wild-caught salmon, sardines, and anchovies. Use healthy cooking methods such as grilling, baking, or steaming, and check guides from organizations like the Environmental Working Group (EWG) for current advisories.