Mackerel is one of the oiliest fish you can eat. With a fat content that ranges from roughly 2% to 11% depending on the season, mackerel falls squarely into the “fatty fish” category, which covers fish with more than 6% fat. It sits alongside salmon, sardines, and herring as a top-tier source of omega-3 fatty acids.
How Fish Are Classified by Fat Content
Nutritional scientists group fish into three categories based on how much fat their flesh contains. Lean fish like cod and sole carry 2 to 5% fat. Medium-fat fish like hake and sea bass sit at 5 to 6%. Fatty (oily) fish range from 6 to 25% fat, and this group includes anchovies, herring, sardines, tuna, salmon, and mackerel.
What makes oily fish different from lean fish isn’t just the total amount of fat. It’s where that fat lives. In lean fish, most fat is stored in the liver. In oily fish, fat is distributed throughout the muscle tissue, which is why mackerel has a richer, more pronounced flavor compared to something like cod. That same fat distribution is also why oily fish deliver far more omega-3s per serving.
Mackerel’s Fat Content Changes With the Season
One interesting detail about mackerel: its oil content fluctuates dramatically throughout the year. Atlantic mackerel caught in November contained about 11% total fat per 100 grams in North Atlantic sampling, while the same species caught in February had just 2.4%. This swing happens because mackerel build up fat reserves in the warmer months when food is abundant, then burn through those reserves over winter.
This seasonal shift directly affects the omega-3 content. November-caught mackerel provided 2.68 grams of EPA and DHA (the two most beneficial omega-3 fats) per 100 grams. February-caught mackerel delivered only 0.23 grams. So while mackerel is always classified as an oily fish, the nutritional payoff can vary tenfold depending on when it was caught. Canned mackerel is typically processed during peak-fat months, making it a reliable year-round option.
What Omega-3s Actually Do in Your Body
The reason oily fish get so much attention from health organizations comes down to those omega-3 fatty acids. These fats lower triglyceride levels by reducing the amount of fatty acids your liver produces and sends to fat cells. They also help lower blood pressure by reducing oxidative stress in blood vessels and influencing how ion channels and receptors function on cell surfaces. On top of that, omega-3s improve insulin sensitivity and have anti-inflammatory effects throughout the body.
Major health advisories recommend eating one to two servings of seafood per week to reduce the risk of heart failure, coronary heart disease, stroke, and sudden cardiac death. These benefits are strongest when fish replaces less healthy protein sources in your diet. For people with existing heart disease, the American Heart Association recommends about 1 gram per day of EPA plus DHA, preferably from oily fish. A single serving of peak-season mackerel easily exceeds that.
Other Nutrients Beyond Omega-3s
Mackerel isn’t just about fat. It’s one of the best natural sources of vitamin B12, delivering over 300% of the daily value per 100 grams. B12 is essential for nerve function, red blood cell production, and DNA synthesis. Many people, especially older adults and those on plant-based diets, don’t get enough of it. A serving of mackerel covers your needs several times over.
Mackerel also provides meaningful amounts of selenium and vitamin D, two nutrients that many people fall short on. Vitamin D is notoriously hard to get from food alone, and oily fish are one of the few dietary sources that deliver it in significant quantities.
Mercury Varies Hugely by Species
Not all mackerel is equal when it comes to mercury, and this is a distinction worth paying attention to. Atlantic mackerel has one of the lowest mercury levels of any fish at just 0.05 parts per million. King mackerel, on the other hand, contains 0.73 ppm, nearly 15 times more. The FDA considers king mackerel a high-mercury fish that pregnant and breastfeeding women should avoid entirely.
If you’re buying mackerel specifically for its health benefits, Atlantic mackerel is the clear winner: high in omega-3s, extremely low in mercury. Spanish mackerel falls somewhere in between. Always check the label, because “mackerel” on its own doesn’t tell you which species you’re getting. Pregnant or breastfeeding women are advised to eat 8 to 12 ounces of low-mercury seafood per week, and Atlantic mackerel fits comfortably within those guidelines.
How Mackerel Compares to Other Oily Fish
- Salmon: Similar omega-3 content to peak-season mackerel, but typically more expensive. Lower in B12, comparable in vitamin D.
- Sardines: Slightly less omega-3 per serving but eaten whole (bones included), making them an excellent calcium source. Very low mercury.
- Herring: Close to mackerel in fat content and omega-3 levels. Often sold pickled or smoked, which adds sodium.
- Tuna: Omega-3 content varies widely by species. Canned light tuna has moderate omega-3s and low mercury. Bigeye and albacore carry more mercury.
Mackerel holds its own against any of these options. Its combination of very high omega-3 content, extremely low mercury (for Atlantic mackerel), and exceptionally high B12 makes it one of the most nutrient-dense fish available, often at a lower price point than salmon.