Fatty fish are fish species that store significant amounts of oil in their flesh, typically containing 4% or more fat by weight. The most common examples include salmon, mackerel, herring, sardines, anchovies, and trout. These fish are prized nutritionally because their fat is rich in omega-3 fatty acids, the type linked to heart health and reduced inflammation.
What Makes a Fish “Fatty”
The distinction between fatty and lean fish comes down to how much oil is stored in the edible flesh. Fatty fish carry 4 grams or more of fat per 100 grams of tissue. Lean fish, like cod, flounder, sole, and haddock, fall below that threshold and store most of their oil in the liver rather than the muscle.
This isn’t just a technical classification. The fat in these fish is primarily made up of two omega-3 fatty acids called EPA and DHA. Your body can’t produce much of these on its own, so diet is the main source. A single serving of Atlantic mackerel delivers about 2.5 grams of combined EPA and DHA per 100 grams of fish. Cod, by contrast, provides just 0.3 grams. That tenfold difference is why nutrition guidelines specifically call out fatty fish rather than fish in general.
The Best Fatty Fish Species
Not all fatty fish deliver the same amount of omega-3s. Here’s how the most common species compare, ranked by combined EPA and DHA content per 100 grams of raw fish:
- Atlantic mackerel: 2.5 g (0.9 EPA, 1.6 DHA)
- Chinook salmon: 1.4 g (0.8 EPA, 0.6 DHA)
- Atlantic herring: 1.6 g (0.7 EPA, 0.9 DHA)
- Farmed Atlantic salmon: 1.8 g (0.6 EPA, 1.2 DHA)
- European anchovy: 1.4 g (0.5 EPA, 0.9 DHA)
- Sockeye salmon: 1.2 g (0.5 EPA, 0.7 DHA)
- Sardines (canned): 1.0 g (0.4 EPA, 0.6 DHA)
- Bluefin tuna: 1.6 g (0.4 EPA, 1.2 DHA)
- Trout: 1.6 g (0.5 EPA, 1.1 DHA)
- Bluefish: 1.2 g (0.4 EPA, 0.8 DHA)
Sablefish, sprat, and capelin also qualify as fatty fish, though they’re less commonly found in grocery stores. Albacore tuna lands in the middle range with about 1.3 grams combined, making it a solid option from a can.
Nutrients Beyond Omega-3s
Fatty fish are unusually dense in several nutrients that many people don’t get enough of. Vitamin D is the standout. Herring provides about 17 micrograms per 100-gram serving, and salmon provides 15 micrograms. For context, the recommended daily intake for most adults is 15 micrograms, so a single serving of either fish covers an entire day’s worth. Few other whole foods come close.
Vitamin B12 is another strength. Herring delivers 10.2 micrograms per 100 grams, and mackerel provides 9 micrograms. The daily requirement is just 2.4 micrograms, so even a small portion supplies several days’ worth. Selenium, a mineral important for thyroid function and immune defense, is also abundant. Mackerel contains 64 micrograms per 100 grams, and anchovies provide 51 micrograms, both well above the 55-microgram daily recommendation.
How Fatty Fish Affect Heart Health
The connection between fatty fish and cardiovascular health is one of the most studied relationships in nutrition. The omega-3 fats in these fish lower triglycerides, a type of blood fat that raises heart disease risk when elevated. They do this through several pathways: increasing the rate at which your body burns fat, reducing the amount of fat your liver packages into the bloodstream, and speeding up the clearance of fat particles after meals. Research published in The Journal of Clinical Investigation also identified a newer mechanism where omega-3s generate compounds in bile that reduce fat absorption in the intestine.
Beyond triglycerides, omega-3s from fish reduce inflammation throughout the body and lower the tendency of blood to clot. The American Heart Association recommends eating two servings of fish per week, with fatty fish specifically preferred. A serving is 3 ounces cooked, roughly three-quarters of a cup of flaked fish.
Wild-Caught vs. Farmed
Both wild and farmed fatty fish provide omega-3s, but the nutritional profiles differ more than most people realize. Farmed Atlantic salmon contains about three times as much total fat as wild salmon (18% vs. roughly 6%). That sounds like it would mean more omega-3s, but the opposite is true for the most valuable ones. Wild salmon has a higher concentration of EPA and DHA in its fat, about one-third more than farmed on an absolute basis per serving.
The bigger difference is in the ratio of omega-6 to omega-3 fats. Wild salmon has an omega-6 to omega-3 ratio of 0.05, while farmed salmon’s ratio is 0.7, roughly 14 times higher. This happens because farmed fish are fed grain-based diets rich in plant oils like soy and canola, which contain linoleic acid and other omega-6 fats. Wild fish eat smaller marine organisms that are naturally rich in EPA and DHA. Both types still deliver meaningful omega-3s, but wild-caught provides a cleaner omega-3 profile with far less omega-6 along for the ride.
Mercury Levels by Species
Mercury accumulates more in large, long-lived predatory fish than in the smaller species that make up most fatty fish. According to FDA monitoring data, sardines (0.013 ppm), anchovies (0.016 ppm), and salmon (0.022 ppm fresh, 0.014 ppm canned) rank among the lowest-mercury seafood available. Atlantic herring and trout are similarly low.
The exceptions to watch for among fatty fish are king mackerel (0.73 ppm) and bigeye tuna (0.689 ppm), which are both large predators that accumulate mercury over long lifespans. Atlantic mackerel, the smaller species commonly sold in cans and at fish counters, is a different story and carries far less mercury. If you’re choosing fatty fish for regular consumption, salmon, sardines, anchovies, herring, and Atlantic mackerel are the safest picks.
Cooking Methods and Omega-3 Retention
How you cook fatty fish matters for how much omega-3 you actually end up eating. EPA and DHA are heat-sensitive compounds that break down at high temperatures. Research comparing steaming (100°C) to baking in foil (160°C) found that steaming preserved significantly more omega-3 content. In Indian mackerel, steaming retained about 60% of the original EPA, while baking in foil retained only 37%.
The practical takeaway: gentler cooking methods preserve more of what makes fatty fish valuable. Steaming, poaching, and low-temperature baking all keep more omega-3s intact than grilling over high heat or deep frying. If you prefer high-heat methods, shorter cooking times help limit the loss. Canned fatty fish like sardines, salmon, and mackerel are also a reliable option since they’re processed once and then shelf-stable, with omega-3s well preserved in the liquid or oil.