What is the most unhealthy seafood? A guide to making better nutrition diet choices

October 8, 2025 •

4 min read

Large predatory fish contain higher levels of mercury due to a process called bioaccumulation. Understanding what is the most unhealthy seafood is crucial for making informed choices that protect your health and minimize exposure to potentially harmful contaminants.

Quick Summary

Identify which seafood to limit or avoid due to high mercury content, persistent pollutants, and potential foodborne illness risks from improper farming or preparation.

Key Points

High Mercury Accumulation: Large, long-lived predatory fish like shark, swordfish, and king mackerel contain the highest levels of mercury, a neurotoxin.
Farmed Fish Concerns: Some farmed fish, such as tilapia and certain imported shrimp, are raised in conditions involving antibiotics and can be higher in pollutants like PCBs.
Pollutants in Fatty Tissues: Fat-soluble pollutants like PCBs and dioxins accumulate in the fatty tissues of some fish, posing potential health risks to consumers.
Raw Seafood Risks: Raw seafood, often consumed in sushi or sashimi, carries a risk of parasites and bacteria, requiring careful handling and preparation.
Unhealthy Preparation: Cooking methods such as deep-frying can transform a healthy seafood choice into an unhealthy meal, adding excessive calories and harmful compounds.

Identifying the most unhealthy seafood

While seafood is a valuable source of omega-3 fatty acids, protein, and essential vitamins, certain types carry health risks due to contaminants or unsustainable practices. The designation of "most unhealthy" often depends on factors like mercury levels, pollutant accumulation, and preparation methods. Large, long-lived predatory fish tend to accumulate the highest concentrations of mercury, while some farmed fish may carry other concerns like antibiotics and pollutants.

The risk of high mercury levels

Mercury is a neurotoxin that can be particularly harmful to the developing brains of unborn babies and young children, making it a serious concern for women who are pregnant or planning to be. Larger, older predatory fish bioaccumulate methylmercury from the prey they consume. As the toxin builds up the food chain, the top predators end up with the highest levels.

High-mercury fish to limit or avoid:

Shark: As a top-tier predator, sharks accumulate very high levels of mercury.
Swordfish: Like sharks, swordfish are large predators with significant mercury concentrations.
King Mackerel: The larger species of mackerel, especially king mackerel, are known to have high mercury levels.
Tilefish: The FDA warns against eating tilefish from the Gulf of Mexico due to extremely high mercury content.
Bigeye Tuna (Ahi): This type of tuna, especially when served as steaks or sushi, has higher mercury levels than canned light tuna.
Marlin: Similar to swordfish, marlin is a large predatory fish with high levels of mercury.
Orange Roughy: These fish can live for over 100 years, giving them a long time to accumulate mercury.

The problem with pollutants and farmed seafood

Besides mercury, fish can also absorb other environmental pollutants like polychlorinated biphenyls (PCBs) and dioxins. These fat-soluble chemicals accumulate in the fatty tissues of fish and can cause adverse health effects in humans. Farmed seafood, depending on the practices used, can sometimes pose different risks.

Farmed Salmon: While many modern farmed salmon operations are cleaner, some studies have shown higher levels of pollutants like PCBs in farmed fish compared to wild-caught varieties. Overcrowding can also necessitate the use of antibiotics, contributing to antibiotic resistance.
Tilapia and Swai (Pangasius): Often raised in crowded, polluted conditions in some parts of the world, these fish are associated with poor farming practices. Tilapia is also noted for having a less favorable omega-3 to omega-6 fatty acid ratio compared to wild fish.
Imported Shrimp: Some imported shrimp from countries with lax regulations have been found to contain antibiotics, chemical contaminants, and bacteria like Salmonella and E. coli.

Making smart choices: Unhealthy preparation and raw risks

Even a healthy fish can become unhealthy based on preparation. Deep-frying adds significant calories and saturated fat, while also creating harmful compounds. Furthermore, consuming raw or undercooked seafood carries risks of foodborne illnesses from bacteria, viruses, and parasites.

Sushi and Sashimi: Consuming raw fish requires careful handling to prevent parasites (e.g., Anasakinae) and bacteria (e.g., Vibrio). Safe sushi-grade fish must be properly frozen to kill parasites. High-mercury tuna used in sushi is also a concern for frequent consumers.
Clam Chowder: While the clams themselves are low in mercury, traditional creamy clam chowder is often high in calories, saturated fat, and sodium due to the addition of butter, cream, and salted pork.
Fish and Chips: The deep-frying process used for this popular dish adds excessive calories and unhealthy fats, outweighing the nutritional benefits of the fish.

Comparison table: High vs. low mercury seafood choices


High Mercury (Limit or Avoid)	Low Mercury (Best Choices)
King Mackerel	Salmon (especially Alaskan)
Shark	Sardines
Swordfish	Anchovies
Tilefish (Gulf of Mexico)	Catfish (U.S. farm-raised)
Bigeye Tuna	Shrimp
Marlin	Oysters
Orange Roughy	Canned Light Tuna
Chilean Sea Bass	Tilapia

Making healthier seafood choices

To maximize the health benefits of seafood while minimizing risks, consider the following strategy: opt for smaller, lower-mercury fish more frequently, and limit consumption of larger predators. When choosing farmed fish, look for responsibly sourced options, and always prepare seafood using healthier methods like baking, grilling, or steaming instead of frying. Cooking seafood thoroughly is also essential for killing bacteria and parasites.

Conclusion

While the answer to what is the most unhealthy seafood is complex, focusing on specific risks such as high mercury, pollutants, and food safety is key. Large predatory fish like shark and swordfish, as well as some unethically farmed species, pose the highest health risks due to bioaccumulation of toxins. By choosing smaller, sustainable fish and preparing them healthfully, individuals can enjoy the benefits of seafood as part of a nutritious diet. Consulting resources from agencies like the FDA or Environmental Working Group can provide further guidance on safe seafood consumption for all populations.

Frequently Asked Questions

Fish with the highest mercury levels include shark, swordfish, king mackerel, tilefish from the Gulf of Mexico, bigeye tuna, marlin, and orange roughy.

Not all farmed salmon is unhealthy. Modern practices have improved, and some farmed salmon has omega-3 levels comparable to wild. However, some farmed fish may still contain higher levels of pollutants or be treated with antibiotics, so it's wise to source from reputable suppliers.

Eating raw seafood risks exposure to bacteria (like Vibrio and Salmonella) and parasites. To mitigate this, sushi-grade fish must be frozen to specific temperatures to kill parasites before being served.

Larger, older fish that are high on the food chain accumulate more contaminants like mercury through a process called bioaccumulation. They eat smaller contaminated fish, and the toxins build up in their bodies over their longer lifespans.

Yes, cooking methods matter. Deep-frying adds unhealthy fats and calories, often cancelling out the health benefits of the fish. Healthier methods include baking, grilling, or steaming.

PCBs are fat-soluble environmental pollutants that were banned in the 1970s but persist in the environment. They accumulate in the fatty tissues of fish and are a concern because of potential links to health issues in humans.

Examples of safer, low-mercury seafood include salmon (especially Alaskan), sardines, anchovies, shrimp, oysters, and canned light tuna. These can be consumed more frequently as part of a balanced diet.