Which is the bad fish in the world? Unveiling the Health and Environmental Risks

October 5, 2025 •

3 min read

According to the Environmental Protection Agency and Food and Drug Administration, several fish species contain levels of mercury high enough to warrant advisories, especially for pregnant women and young children. The question of 'which is the bad fish in the world?' has no single answer, but depends on a variety of health and environmental concerns.

Quick Summary

Several fish species are considered 'bad' for your diet due to high mercury accumulation, presence of natural toxins, environmental unsustainability, and undesirable nutritional profiles. Predatory fish, some farmed varieties, and certain toxic species should be limited or avoided.

Key Points

High Mercury Accumulation: Apex predators such as shark, swordfish, and certain mackerel accumulate high levels of mercury, posing risks to the nervous system, especially for developing fetuses and young children.
Toxins and Poisons: Some fish, like the pufferfish, are lethally poisonous, while others like the barracuda and moray eel can carry ciguatoxins that cause severe food poisoning.
Unsustainable Fishing and Farming: Overfishing has endangered species like Orange Roughy and Atlantic Cod, while unregulated farming, particularly imported shrimp and salmon, can involve harmful chemicals and lead to environmental damage.
Contaminants in Imported Fish: Imported fish, including some farmed shrimp and catfish (Swai/Basa), may contain antibiotics and other contaminants due to less stringent regulations.
Nutrient Imbalance: Some fish, notably conventionally farmed tilapia, have a less favorable ratio of omega-3 to inflammatory omega-6 fatty acids compared to healthier options like salmon.
Best Alternatives: Low-mercury, sustainable fish such as wild Alaskan salmon, sardines, and canned light tuna offer excellent nutritional benefits with fewer risks.

The concept of a 'bad' fish is subjective and multi-faceted. While many fish are praised for their nutritional benefits, such as high omega-3 fatty acids, certain species pose risks from contamination, toxins, or unsustainable practices. Understanding these factors helps consumers make informed choices for their health and the environment.

The Mercury Menace: Top Predators to Avoid

Methylmercury, a neurotoxin, is a primary concern with fish consumption. It builds up in the food chain through a process known as biomagnification. Larger, predatory fish that live longer accumulate the highest levels of mercury from the smaller fish they eat.

Several species are known for particularly high mercury levels and should be avoided or limited, especially for vulnerable populations such as pregnant women and young children:

Shark: An apex predator with high mercury levels and often subject to overfishing.
Swordfish: Another large predator known for high mercury content.
King Mackerel: Varieties from the Pacific or Gulf of Mexico tend to have high mercury.
Tilefish (Gulf of Mexico): This region's tilefish has particularly high mercury levels.
Bigeye Tuna: Contains more mercury than other tuna varieties like canned light tuna.

The Perils of Unsustainable and Contaminated Seafood

Issues like unsustainable fishing practices and contamination from farming or pollution can also make certain fish poor choices.

Imported Shrimp: Often farmed with antibiotics and chemicals, and contributes to habitat destruction.
Orange Roughy: A long-lived, slow-reproducing fish that accumulates high mercury and is severely overfished.
Atlantic Salmon (Farmed): Concerns exist regarding crowded farming conditions, pesticide exposure, and a less favorable omega-3 to omega-6 ratio compared to wild salmon.
Tilapia (Farmed): While low in mercury, conventionally farmed tilapia can have a less ideal fatty acid profile.

Toxic Troubles: Fish with Natural Poisons

Some fish contain natural toxins that are dangerous to humans.

Pufferfish (Fugu): Contains lethal tetrodotoxin in certain organs. Requires specialized preparation to be safe.
Barracuda and Moray Eel: Can carry ciguatoxins from their diet of algae-eating fish, causing ciguatera poisoning.

A Comparison of Fish Choices

To summarize the key differences, here is a comparison of fish to avoid and some recommended alternatives:


Attribute	Examples of Fish to Limit or Avoid	Examples of Healthier Choices
Mercury Content	Shark, Swordfish, King Mackerel, Bigeye Tuna, Gulf Tilefish	Salmon, Sardines, Canned Light Tuna, Anchovies, Catfish
Sustainability	Orange Roughy, Atlantic Cod, Chilean Sea Bass	U.S. Shrimp, Alaskan Salmon, Pacific Halibut
Farming Practices	Imported Farmed Shrimp, Imported Catfish (Swai, Basa), Farmed Atlantic Salmon	Domestic Farmed Catfish, Responsibly Farmed Tilapia, Wild-Caught Salmon
Contaminants	Farmed fish treated with antibiotics and chemicals	Wild-caught fish from clean waters
Natural Toxins	Pufferfish, Moray Eel, Barracuda	Most common market fish (properly prepared)

Healthier Alternatives to Bad Fish Choices

Choosing safer seafood involves selecting species that are lower in the food chain, have shorter lifespans, and come from sustainable fisheries.

Salmon (Wild-Caught Alaskan): A top choice for omega-3s and sustainability.
Sardines and Anchovies: Small, low-mercury fish rich in omega-3s.
Catfish (U.S. Farmed): A responsibly farmed domestic option.
Trout (Freshwater): A good low-mercury fish.
Canned Light Tuna: Lower in mercury than other tuna types.

For more information on sustainable seafood, reputable sources like the Monterey Bay Aquarium's Seafood Watch program can provide up-to-date guidance on the best choices for your region and specific species.

Conclusion: Making Informed Seafood Choices

The notion of 'bad fish' is complex, encompassing a range of risks from heavy metal contamination and natural toxins to ecological harm. By prioritizing smaller, lower-mercury species and opting for sustainably sourced seafood, consumers can enjoy the extensive health benefits of fish while minimizing their exposure to harmful substances and supporting a healthier planet. Always stay informed by checking advisories from health authorities like the FDA and EPA, and consider the source of your seafood to ensure it aligns with both your nutritional goals and environmental values.

Frequently Asked Questions

Not all farmed salmon is bad, but Atlantic salmon is often farmed in crowded, unsanitary conditions where it can be exposed to pesticides and bacteria, and may have a higher concentration of saturated fats and inflammatory omega-6s compared to wild-caught varieties.

The primary issues with imported farmed shrimp are the potential for contamination from antibiotics and other chemicals banned in the U.S., along with unsustainable farming methods that can destroy coastal habitats.

Orange Roughy, a very long-lived fish, accumulates high levels of mercury over its lifespan. It has also been severely overfished, making it an unsustainable seafood choice.

Yes, improperly prepared pufferfish (fugu) contains a lethal neurotoxin called tetrodotoxin. Consumption can lead to respiratory paralysis and death, which is why its preparation is strictly controlled in many countries.

Mercury can cause brain and nerve damage, and is particularly harmful to the developing nervous systems of unborn babies and young children. Long-term exposure to high levels can lead to serious health issues.

No, mercury is stored within the muscle tissue of the fish and cannot be removed by cooking or other preparation methods.

While it is a low-mercury option, conventionally farmed tilapia is sometimes criticized for having a lower concentration of beneficial omega-3 fatty acids and a higher concentration of inflammatory omega-6s than other fish. However, responsibly farmed tilapia remains a viable, low-mercury protein source.