What's the unhealthiest fish to eat? A Guide to Contaminants and Health Risks

October 25, 2025 •

3 min read

According to the FDA, certain fish species contain dangerously high levels of mercury, a neurotoxin that can cause serious health problems, particularly for developing fetuses and young children. Answering the question, "what's the unhealthiest fish to eat?" often involves examining species high in contaminants like mercury and PCBs, as well as those with less favorable nutritional profiles. This guide delves into the specifics of which fish to approach with caution and why.

Quick Summary

This article discusses factors that determine a fish's healthiness, including mercury levels, polychlorinated biphenyls (PCBs), and omega-3 content. It identifies specific species like shark, swordfish, and certain farmed fish as less healthy options due to high contaminant levels. The guide also provides safer, healthier fish alternatives to consider for your diet.

Key Points

High-Mercury Fish: Large, predatory fish like shark, swordfish, and king mackerel are considered the unhealthiest due to high levels of neurotoxic mercury.
Contaminants in Farmed Fish: Some farmed fish, especially Atlantic salmon, can contain higher levels of PCBs and dioxins from their feed and environment.
Omega-6 Ratio Issues: Farmed tilapia has a less favorable omega-3 to omega-6 ratio compared to wild fish, which can contribute to inflammation.
Smarter Seafood Choices: To reduce risks, prioritize smaller, short-lived fish like sardines, anchovies, and wild-caught salmon, which are typically low in mercury and other contaminants.
Check Your Source: The origin of your fish (wild-caught vs. farmed) and sourcing practices can significantly impact its nutritional and contaminant profile.
Vary Your Intake: Eating a variety of fish, and checking local advisories for recreationally caught fish, helps minimize exposure to specific contaminants.

The Primary Health Risks in Fish

While fish is often celebrated for its omega-3 fatty acids and other nutritional benefits, not all fish are created equal. Several factors can make a particular species a less healthy choice.

Mercury Contamination: The Top Concern

Mercury is a neurotoxin that accumulates in the food chain. Larger, longer-living predatory fish tend to have the highest concentrations. High mercury intake can damage the nervous system, posing a severe risk to pregnant women and developing babies.

Common high-mercury fish to limit or avoid include:

Shark
Swordfish
King Mackerel
Tilefish (Gulf of Mexico)
Orange Roughy
Marlin and Bigeye Tuna

PCB and Dioxin Contaminants in Farmed Fish

PCBs are persistent pollutants banned in the US in the 1970s but still present in the environment. They accumulate in fatty tissues. Some farmed fish, like farmed salmon, can have higher levels of PCBs and dioxins than wild fish due to feed and living conditions. PCBs are classified as probable human carcinogens.

Poor Nutritional Profile: The Case of Tilapia

Farmed tilapia is popular but contains low beneficial omega-3s and a high amount of omega-6 fatty acids. While omega-6s are essential, an imbalanced ratio can promote inflammation. Farmed tilapia can also be susceptible to disease and may require antibiotic use.

The Impact of Farming Practices on Nutritional Value

Farmed fish often consume processed diets, affecting their nutritional makeup. Farmed salmon, for example, may have more fat and saturated fat than wild salmon, and potential benefits can be offset by higher contaminant levels and environmental impact.

Comparison of Fish Options


Feature	Least Healthy Fish (Examples)	Healthiest Fish (Examples)
Mercury Levels	High: Shark, Swordfish, King Mackerel, Gulf Tilefish, Bigeye Tuna, Orange Roughy	Low: Sardines, Anchovies, Wild Salmon, Catfish, Shrimp, Scallops
PCB/Dioxin Risk	Higher: Farmed Salmon, some Fatty Fish	Lower: Wild-caught fish, especially smaller ones
Omega-3:Omega-6 Ratio	Poor Ratio: Tilapia	Excellent Ratio: Salmon, Sardines, Mackerel
Sourcing (Sustainability)	Often linked to overfishing or damaging aquaculture practices	Often sustainably sourced (look for MSC certified)
Fat Content	Farmed varieties can be higher in overall fat, including saturated fat	Can be low (e.g., white fish) or rich in healthy omega-3 fats (e.g., oily fish)

Making Healthier and More Sustainable Seafood Choices

Choosing healthier fish means prioritizing species lower on the food chain or sustainably farmed. The FDA and EPA provide guidance, advising sensitive populations to avoid high-mercury fish.

To make better choices:

Prioritize smaller fish: Anchovies, sardines, and Atlantic mackerel are low in mercury and high in omega-3s.
Choose canned light tuna over albacore: Canned light tuna has less mercury than albacore or fresh tuna steaks.
Opt for sustainable options: Look for certifications like MSC or consult guides like Seafood Watch.
Vary your fish intake: Eating different species minimizes exposure to any single contaminant.
Consider wild-caught: Wild-caught options like Alaskan salmon often have better nutritional profiles and fewer contaminants than farmed counterparts.

Conclusion: Balance Your Diet and Your Risks

Making informed choices about fish consumption is key. The unhealthiest fish to eat are typically large predatory fish high in mercury, species with poor omega ratios like tilapia, and some farmed fish with higher PCB concentrations. By choosing smaller, lower-mercury, and sustainably sourced alternatives, you can enjoy the nutritional benefits of seafood while minimizing potential health risks. For official guidance, refer to the FDA's advice on eating fish.

Frequently Asked Questions

Larger, older predatory fish at the top of the aquatic food chain tend to have the highest levels of mercury because they eat many smaller fish, causing the neurotoxin to accumulate in their bodies over time, a process called bioaccumulation.

Wild-caught salmon is generally considered healthier. While some farmed salmon can have comparable omega-3 levels, they often contain higher concentrations of PCBs and other contaminants due to their diet and crowded farming conditions.

No, cooking fish does not remove mercury or PCBs. These contaminants are bound to the fish's flesh and are not affected by heat. The best way to reduce exposure is by choosing low-contaminant fish species.

Farmed Tilapia is often a less healthy choice due to its low levels of beneficial omega-3 fatty acids and a less favorable ratio of omega-3 to omega-6 fatty acids, which can be pro-inflammatory.

The FDA recommends that pregnant women and young children consume 8 to 12 ounces per week of fish from the 'Best Choices' list, which includes low-mercury options like shrimp, salmon, canned light tuna, and catfish. High-mercury fish should be avoided entirely by these groups.

As a general rule, yes. Smaller fish like sardines and anchovies are lower on the food chain and have shorter lifespans, resulting in less time to accumulate mercury and other contaminants.

You can look for certifications from organizations like the Marine Stewardship Council (MSC) or consult resources such as the Monterey Bay Aquarium's Seafood Watch guide to find fish that are responsibly caught or farmed.