Oily fish are species that store fat throughout their flesh rather than only in the liver. This fat is rich in omega-3 fatty acids, specifically EPA and DHA, which are the reason oily fish gets so much attention in nutrition guidance. Common examples include salmon, mackerel, sardines, herring, anchovies, and trout. The NHS recommends eating at least one portion (about 140g cooked) of oily fish per week as part of a balanced diet.
What Makes Fish “Oily”
The distinction between oily and white fish comes down to where fat is stored. White fish like cod, haddock, and flounder keep most of their fat in the liver, leaving the flesh very lean. Oily fish distribute fat through the muscle tissue itself, which is why their flesh tends to be darker, richer in flavor, and visibly more moist.
This fat distribution matters because when you eat a fillet of oily fish, you get omega-3s with every bite. A 100g serving of Atlantic mackerel delivers about 2.5g of combined EPA and DHA. Atlantic herring provides around 1.6g. Sardines and salmon sit in the range of 1.0 to 1.8g. Compare that to cod or flounder, which offer just 0.2 to 0.3g per serving. The gap is enormous.
The Most Common Oily Fish
Not all oily fish are created equal when it comes to omega-3 content. Based on data from Oregon State University, here’s how popular species stack up per 100g serving (combined EPA and DHA):
- Atlantic mackerel: 2.5g
- Farmed Atlantic salmon: 1.8g
- Pacific herring: 1.7g
- Atlantic herring: 1.6g
- Bluefin tuna: 1.6g
- Chinook salmon: 1.4g
- European anchovy: 1.4g
- Albacore tuna: 1.3g
- Sprat: 1.3g
- Sockeye salmon: 1.2g
- Sardines (canned): 1.0g
Canned sardines and canned salmon are worth highlighting because they’re affordable, shelf-stable, and still deliver a meaningful dose of omega-3s. You don’t need to buy fresh fish to get the benefits.
How Omega-3s Affect Your Body
The omega-3 fatty acids in oily fish have well-documented effects on the cardiovascular system. EPA and DHA help lower triglycerides by reducing the liver’s production of fat-carrying particles and speeding up their clearance from the bloodstream. They also compete with pro-inflammatory fats for the same enzymes in your body, which shifts the balance toward compounds that reduce blood clotting and relax blood vessels.
Research published through the American Heart Association has shown that EPA in particular can stabilize arterial plaques, making them less likely to rupture and cause a heart attack or stroke. EPA accumulates in vulnerable, thin-capped plaques and promotes the activity of anti-inflammatory immune cells at those sites. It also reduces arterial stiffness independently of blood pressure or cholesterol changes, likely through its effects on inflammation and oxidative stress.
Beyond heart health, DHA is a structural component of brain tissue and the retina. It plays a role in brain development during pregnancy and infancy, which is one reason fish intake is recommended for pregnant women, with some caveats around mercury.
Mercury: Which Fish to Limit
Nearly all fish contain traces of methylmercury, which accumulates in larger, longer-lived predatory species. The good news is that many of the best oily fish for omega-3s are also among the lowest in mercury. FDA monitoring data shows sardines at just 0.013 ppm mercury, anchovies at 0.016 ppm, and fresh salmon at 0.022 ppm. Atlantic mackerel comes in at 0.05 ppm, and herring at 0.078 ppm. All of these are well within safe levels for frequent consumption.
The oily fish to watch out for are the large, predatory ones. King mackerel averages 0.73 ppm of mercury, swordfish hits 0.995 ppm, and fresh bigeye tuna reaches 0.689 ppm. These sit in the FDA’s “choices to avoid” category for pregnant women and young children. Albacore tuna (canned white tuna) contains about three times more mercury than canned light tuna, so if you’re pregnant or breastfeeding, the FDA suggests limiting albacore to one serving per week with no other fish that week.
Cooking does not reduce mercury levels. It’s found throughout the fish tissue, so the only way to manage exposure is through species selection and portion control.
How Much to Eat
The NHS recommends at least two portions of fish per week, with at least one being oily fish. A portion is about 140g cooked. Women who are pregnant, breastfeeding, or planning a future pregnancy should cap oily fish at two portions per week due to potential pollutant accumulation. The same limit applies to girls. Boys and men can safely eat up to four portions weekly.
The FDA’s guidance for pregnant and breastfeeding women aligns broadly: 8 to 12 ounces of lower-mercury fish per week, spread across two to three servings. Sticking with salmon, sardines, anchovies, and herring makes it easy to hit these targets without worrying about mercury.
Wild vs. Farmed Fish
A common question is whether farmed salmon delivers the same benefits as wild-caught. Research compiled by Harvard Health Publishing found that both types provide similar amounts of omega-3s per serving. Farmed salmon tends to have more total fat, which means more omega-3s in absolute terms, but also more saturated fat. Omega-3 content across five salmon species tested ranged from 717mg to 1,533mg per 100g serving, with farmed varieties generally landing at the higher end.
If budget is a factor, farmed salmon is a perfectly reasonable choice for omega-3 intake. The nutritional tradeoff is modest, and it’s typically half the price of wild-caught.
Best Ways to Cook Oily Fish
How you cook oily fish matters more than you might expect. Steaming preserves omega-3 fatty acids better than any other common method. In one study, steamed fish retained up to 60% of its EPA content, while baking in foil kept only about 37%. The difference comes down to temperature: steaming operates at 100°C, while oven baking typically hits 160°C or higher. Both methods outperform grilling and deep-frying for omega-3 retention.
Deep-frying is the worst option. The high heat degrades EPA and DHA, and the cooking oil introduces additional fats that dilute the nutritional profile. If you prefer pan-frying, keeping the heat moderate and the cooking time short will help preserve more of the beneficial fats. Canned oily fish, which is typically cooked once during processing, retains a solid omega-3 profile and requires no additional cooking at all.
Alternatives for People Who Don’t Eat Fish
If you follow a vegetarian or vegan diet, algal oil supplements are the closest equivalent to eating oily fish. Algae are where fish get their omega-3s in the first place, so going straight to the source cuts out the middleman. A randomized study of 31 adults found that DHA from algal oil capsules was at least as well absorbed as DHA from fish oil. Over two weeks, vegetarians and vegans taking 600mg of algal DHA daily reached the same blood DHA levels as supplemented omnivores, despite starting with significantly lower baseline levels.
Plant foods like flaxseed, chia seeds, and walnuts contain a different omega-3 called ALA. Your body can convert ALA into EPA and DHA, but the conversion rate is low, typically under 10%. These foods are nutritious for other reasons, but they’re not a reliable replacement for the EPA and DHA found in oily fish or algal oil.