The worst fish to eat are large, long-lived predators that accumulate dangerous levels of mercury and other contaminants. Tilefish from the Gulf of Mexico tops the list at 1.123 parts per million (ppm) of mercury, followed closely by swordfish (0.995 ppm) and shark (0.979 ppm). But mercury isn’t the only concern. Some fish carry natural toxins, others are loaded with antibiotics from farming operations, and a few can cause bizarre digestive reactions that no amount of cooking can prevent. Here’s a breakdown of the fish you’re better off skipping entirely and why.
High-Mercury Fish: The Biggest Health Risk
Mercury builds up in fish through a process called bioaccumulation. Small organisms absorb mercury from the water, small fish eat those organisms, and bigger fish eat the smaller ones. Each step up the food chain concentrates mercury further, which is why the largest, longest-lived predatory fish carry the highest levels.
According to FDA monitoring data, these species consistently test highest for mercury:
- Tilefish (Gulf of Mexico): 1.123 ppm
- Swordfish: 0.995 ppm
- Shark: 0.979 ppm
- King mackerel: 0.73 ppm
- Bigeye tuna (fresh/frozen): 0.689 ppm
- Orange roughy: 0.571 ppm
- Marlin: 0.485 ppm
The EPA and FDA recommend completely avoiding tilefish from the Gulf of Mexico, swordfish, shark, and king mackerel, especially for pregnant women and young children. Orange roughy is a particularly sneaky offender because it can live over 100 years, giving it decades to accumulate mercury in its flesh. For everyone else, the general guidance is to eat two to three servings per week from lower-mercury “best choices” like salmon, sardines, shrimp, and pollock, or one serving per week from the moderate-mercury “good choices” category.
Escolar: The Fish That Causes Orange Diarrhea
Escolar is sometimes sold as “white tuna,” “super white tuna,” or “snowfish,” particularly in sushi restaurants. It is none of those things. Escolar’s flesh is roughly 20% oil by weight, and more than 90% of that oil consists of wax esters that the human body simply cannot digest. Eating it causes a condition called keriorrhea: the uncontrollable excretion of oily, orange-to-brown liquid that can come on suddenly and without warning.
Beyond the oily diarrhea, escolar can trigger abdominal cramps, nausea, headaches, and vomiting. The FDA classifies the wax esters in escolar as a natural toxin called gempylotoxin. Several countries, including Japan and Italy, have banned its sale outright. In the United States it’s still legal but frequently mislabeled, so if you see “white tuna” at a sushi bar priced suspiciously low, there’s a real chance it’s escolar.
Reef Fish and Ciguatera Poisoning
Ciguatera is a type of food poisoning caused by toxins that originate in tiny algae living on coral reefs. Small fish graze on the algae, and predators accumulate the toxin as they eat their way up the food chain. You can’t taste, smell, or cook away ciguatera toxin, and it doesn’t change the appearance of the fish.
The species most commonly linked to ciguatera include barracuda, grouper, amberjack, moray eel, red snapper, sea bass, and Spanish mackerel. Barracuda is widely considered the riskiest because of its position at the top of the reef food chain. Symptoms are distinctive and can be unsettling: tingling or numbness in the mouth, fingers, and toes, along with a bizarre reversal of temperature sensation where hot things feel cold and cold things feel hot. In serious cases, people experience blurred vision, hallucinations, and even temporary paralysis. Symptoms can linger for weeks or months.
Ciguatera is most common in tropical and subtropical waters, particularly the Caribbean, South Pacific, and parts of the Indian Ocean. If you’re eating reef fish from these regions, smaller specimens are generally safer because they’ve had less time to accumulate the toxin.
Imported Farmed Shrimp and Swai
Farmed shrimp from parts of Southeast Asia has repeatedly been flagged for containing unapproved antibiotic residues, including chloramphenicol, a drug that’s been shown to be toxic to some people and is banned for use in food-producing animals in the United States. The problem stems from crowded farming conditions in countries with less stringent regulations, where antibiotics are used heavily to prevent disease outbreaks in the ponds.
Swai (also sold as basa or pangasius) is an inexpensive imported catfish relative that lands on most environmental “avoid” lists. The Monterey Bay Aquarium’s Seafood Watch program red-lists imported swai along with imported farmed shrimp, tilapia from China, and farmed salmon from several countries due to pollution, habitat damage, and weak management of the farming operations. These fish aren’t necessarily high in mercury, but the farming practices introduce other contaminants and contribute to environmental harm that makes them poor choices overall.
Bottom Feeders and Heavy Metals
Fish that feed along the bottom of rivers and lakes are exposed to metals that settle into sediment. Research comparing fish from different water zones has found that bottom feeders accumulate significantly higher levels of lead, cadmium, nickel, and chromium than fish feeding at the surface or mid-water. In some studies, lead and cadmium levels in bottom-feeding species exceeded the limits set by the World Health Organization in every season tested.
This is particularly relevant for freshwater fish caught recreationally. Larger catfish, carp, and perch from local waterways are among the species most likely to be subject to state fish consumption advisories. If you catch your own fish, checking local advisories is worth the two minutes it takes. The EPA recommends that when no advisory exists for a given body of water, you eat only one serving of locally caught fish per week and skip other fish entirely during that week.
Tilapia’s Fatty Acid Problem
Tilapia is one of the most popular and affordable fish in the United States, but its nutritional profile is surprisingly poor compared to other seafood. The reason people eat fish in the first place is largely for omega-3 fatty acids, which reduce inflammation and support heart health. Tilapia contains very little omega-3. What it does contain is a high amount of omega-6 fatty acids, which in excess promote inflammation.
Researchers at Wake Forest University found that tilapia’s ratio of inflammatory omega-6 to anti-inflammatory omega-3 fatty acids averaged about 11 to 1. For comparison, salmon and trout have ratios below 1 to 1, meaning they contain more beneficial omega-3s than omega-6s. This doesn’t make tilapia dangerous in the way that high-mercury fish are, but it does mean that choosing tilapia over salmon, sardines, or mackerel (Atlantic, not king) is a missed opportunity. If you’re eating fish for the health benefits, tilapia delivers very few of them.
Orange Roughy: A Triple Threat
Orange roughy deserves special mention because it fails on nearly every measure. Its mercury level of 0.571 ppm puts it in the high-mercury category. It’s severely overfished, with populations that recover extremely slowly because the fish doesn’t reach reproductive maturity until around age 20 and can live past 100. And Seafood Watch lists it as a species to avoid due to the environmental damage caused by deep-sea trawling used to catch it, which destroys ancient coral habitats on the ocean floor.
Orange roughy was heavily marketed in the 1980s and 1990s under its original, less appetizing name: slimehead. The rebranding worked so well that global populations crashed before fisheries managers could react. If you see it on a menu today, it’s one of the clearest “skip it” choices you can make.
The Quick Reference List
Pulling all of this together, the fish you’re best off avoiding entirely:
- Tilefish (Gulf of Mexico): highest mercury of any commercial fish
- Swordfish: extremely high mercury
- Shark: extremely high mercury, environmentally devastating
- King mackerel: high mercury (Atlantic mackerel is a safe alternative)
- Orange roughy: high mercury, severely overfished, habitat destruction
- Escolar: causes uncontrollable oily diarrhea, often mislabeled
- Barracuda: high ciguatera risk
- Imported farmed shrimp: antibiotic residues, environmental concerns
- Swai/pangasius: poorly regulated farming, environmental red flag
- Bluefin tuna (farmed/ranched): high mercury, critically overfished
Safe, nutritious alternatives that are low in mercury and high in omega-3s include salmon (wild-caught), sardines, anchovies, Atlantic mackerel, herring, and trout. These give you the health benefits that make fish worth eating in the first place, without the risks that put the worst offenders on this list.