Fish still sits near the top of the healthy food list. It is rich in protein and omega 3 fats that support the heart, brain and even healthy aging. For the most part, that reputation is well deserved.
But not every fish on ice at the supermarket is a smart choice for weekly meals. Health agencies now flag certain species and products for higher mercury, harmful chemicals or handling issues, while encouraging people to favor smaller, fresher options instead.
So which fish are worth thinking twice about before they land in your pan?
Big predatory fish that concentrate mercury
Large hunters near the top of the food chain slowly build up mercury as they eat smaller fish over many years. That process, called bioaccumulation, leaves species such as marlin, shark, king mackerel, bigeye tuna, orange roughy and swordfish with some of the highest mercury levels seen in routine testing.
The Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) group these fish in a top risk category and advise people who are pregnant, planning a pregnancy or feeding young children to avoid them and choose low-mercury species instead.
Their technical guidance treats fish with average mercury at or below about 0.15 micrograms per gram as the safest routine choices.
For healthy adults, an occasional tuna steak or piece of swordfish is unlikely to be a crisis. Making high-mercury fish a weekly habit is a different story and can gradually push exposure above recommended limits.
Fish that is not truly fresh
Some of the riskiest fish are not a particular species, but fish that simply have not been kept cold enough. When dark meat fish such as tuna, mackerel, sardines or mahi mahi sit too long at warm temperatures, bacteria convert natural histidine in the flesh into histamine and related toxins.
The Centers for Disease Control and Prevention (CDC) describes the result as scombroid or histamine fish poisoning, an allergic type reaction that can cause flushing, headache, a racing heartbeat and sometimes a trip to the emergency room within hours of a meal.
For shoppers, the basics still matter. Fresh fish should have clear, bright eyes, firm flesh that springs back when pressed and a clean, ocean-like smell rather than a strong fishy odor. If the fillet looks dull, mushy or has an ammonia scent, it is safer to leave it in the case.
Freshwater fish from poorly managed farms
Aquaculture can provide affordable protein with a smaller climate footprint. At the same time, research in parts of Southeast Asia has documented crowded freshwater ponds that rely heavily on antibiotics to control disease in species such as striped catfish and tilapia.
In the Mekong Delta for example, scientists have measured drugs like enrofloxacin and ciprofloxacin in pangasius farm waters and effluents, raising concerns about antibiotic resistance and residues in the wider environment.
Regulators set strict limits for residues in fish flesh and export products are generally tested before reaching major markets. Even so, many experts encourage buyers to favor fish from certified farms.
Programs such as the Aquaculture Stewardship Council (ASC) audit antibiotic practices, water quality and stocking density, allowing consumers to pick farmed fish that meets higher environmental and health standards.
If a freshwater fish looks unusually bloated, with very soft flesh and a swollen belly, that can be a red flag that invites questions about how it was raised. Choosing trusted brands and certification labels helps take some of the guesswork out of that decision.
Heavily salted or long-stored fish
Traditional dried or heavily salted fish is part of many cuisines. When these products sit for long periods, however, nitrogen compounds can form nitrosamines, a family of chemicals that includes known carcinogens.
The International Agency for Research on Cancer (IARC) has classified Chinese-style salted fish as carcinogenic to humans and points to nitrosamine formation as one likely mechanism. Not every dried fish product carries the same level of risk and recent studies suggest average nitrosamine exposure from dried aquatic foods in some populations remains relatively low.
In everyday terms, that means salty preserved fish is better treated as an occasional flavor booster rather than a daily staple, especially for people who already struggle with high blood pressure or who consume other processed meats.
If you enjoy tuna sashimi, there is another nuance. Studies find that mercury tends to collect in lean red muscle, while some fat soluble pollutants, such as certain pesticides and dioxins, are more concentrated in fattier belly cuts.
Keeping portions moderate and not relying on large tuna species as your main fish source is a sensible compromise.
Small oily fish that still deliver the benefits
The good news is that you do not have to give up seafood to stay safe. Nutrition experts often highlight small species like sardines, anchovies and smaller mackerel as affordable, nutrient dense choices. They sit low on the food chain, tend to carry much less mercury and still pack plenty of omega 3 fats.
Eating the tiny bones in canned sardines or anchovies also provides a handy calcium boost, which matters for kids, older adults and anyone who does not drink much milk. A simple tray of sardines baked with lemon, or anchovies melted into a pasta sauce, can check the seafood box without blowing the budget.
Whenever you shop, the basic checklist stays the same. Look for clear eyes, firm flesh and a mild smell, avoid very large predatory fish for routine meals and pick farmed products with clear certification whenever possible.
The official advice was published on the FDA website.
