Dark fish meat is the reddish-brown strip of muscle you see running along the side of a fillet, sometimes called the bloodline. It gets its color from myoglobin, an oxygen-storing protein packed into the muscle fibers that fish use for sustained, continuous swimming. In tuna, dark muscle contains 5 to 24 mg of myoglobin per gram of tissue, compared to just 0.4 to 1.3 mg in the lighter flesh beside it. That concentration difference is what creates the visible contrast on your cutting board.
Why Fish Have Two Types of Muscle
Fish bodies contain two distinct muscle systems that serve completely different purposes. Red (dark) muscle powers slow, steady movement like cruising and migrating. White (light) muscle fires during explosive bursts of speed, the kind a fish needs to escape a predator or strike at prey. Think of it like the difference between a marathon runner’s legs and a sprinter’s: different fuel systems, different structures, different jobs.
Red muscle fibers are small in diameter, rich in blood vessels, and loaded with mitochondria, the tiny engines inside cells that burn fat for energy. They also store lipid droplets directly in the tissue, particularly palmitic acid and oleic acid, which serve as ready fuel for long-distance swimming. White muscle fibers, by contrast, are thicker, contain far fewer mitochondria, and rely on quick-burning sugars rather than fat. In most fish, white muscle makes up the vast majority of body mass, while red muscle accounts for only a few percent.
In typical fish, the dark muscle sits in a thin strip just beneath the skin. Tunas and certain sharks are unusual: their red muscle is buried deep inside the body, forming dense loins near the spine. This internal placement helps these species retain heat in the muscle, keeping it warm and efficient even in cold water. It’s one reason tuna can sustain high-speed swimming across entire oceans.
Which Fish Have the Most Dark Meat
The species with the most prominent dark muscle tend to be fast, long-distance swimmers. Tuna is the classic example, with large internal loins of red muscle that give the flesh its characteristic deep red color. Salmon, mackerel, sardines, anchovies, herring, and bluefish all have noticeable dark strips along their fillets. Mahi-mahi (dolphinfish) also carries a visible bloodline.
Among sharks, makos and salmon sharks have internalized red muscle similar to tuna, which helps explain why mako is one of the few shark species sold as a steak fish. Slower-moving bottom dwellers like cod, halibut, and flounder have minimal dark muscle, which is why their flesh appears almost entirely white.
Why It Tastes Stronger
Dark fish meat has a noticeably more intense, “fishier” flavor than the white portion of the same fillet. Several things drive this. First, the higher fat content. Dark muscle stores more lipids, and those fats are rich in polyunsaturated fatty acids, including omega-3s. While nutritionally valuable, polyunsaturated fats oxidize easily. When they break down, they produce aldehydes and other compounds that create strong, sometimes sharp flavors and aromas. The more time the fish spends out of the water, the more oxidation progresses, and the fishier the dark meat becomes.
The myoglobin itself also plays a role. Heme iron in myoglobin can accelerate fat oxidation, creating a feedback loop: more myoglobin means faster breakdown of the surrounding fats, which means a more pungent taste. This is why the bloodline on a piece of swordfish or tuna that’s been sitting in a display case for a day or two can taste metallic or overly strong, even when the surrounding white meat still tastes clean.
Nutritional Differences
Because dark muscle is fattier and more metabolically active, it carries a different nutritional profile than white fish meat. It contains higher levels of omega-3 fatty acids, more iron from its concentrated myoglobin, and more B vitamins. In one comparison, dark fish muscle contained roughly 2.8% fat and 10.4% protein, while lighter fish muscle had about 1.2% fat and 6.9% protein. The omega-3 content in the fattier fish tissue made up nearly 16% of its total fat.
For people specifically seeking omega-3s or iron, the dark portions of fish are actually the most nutrient-dense part of the fillet. The trade-off is the stronger flavor.
Histamine and Food Safety
Dark-muscled fish species deserve a bit more attention when it comes to food safety. The same muscle fibers that store myoglobin also contain high concentrations of free histidine, an amino acid. When fish isn’t kept cold enough after being caught, bacteria convert histidine into histamine, the compound responsible for scombroid fish poisoning. Symptoms mimic an allergic reaction: flushing, headache, stomach cramps, and sometimes hives, usually within an hour of eating.
Tuna, mackerel, sardines, anchovies, herring, and bluefish are the species most commonly involved. Their dark muscle tissue is especially rich in histidine, so poor temperature control creates histamine buildup faster than it would in a white-fleshed fish like cod. Cooking doesn’t destroy histamine once it has formed, so freshness and proper refrigeration from the moment the fish is caught are the real safeguards.
How Cooks Handle the Bloodline
Many cooks and fishmongers trim the dark strip from fillets before cooking, especially on salmon, tuna, and swordfish. Removing the bloodline is standard practice in sushi preparation and is common in home kitchens too. The goal is straightforward: it reduces the fishy taste that the dark meat contributes, particularly in milder preparations where you want the clean flavor of the white muscle to come through.
On a salmon fillet, the bloodline appears as a brownish-gray band between the skin and the pink flesh. You can slice it away with a sharp, flexible knife. On tuna steaks, the dark meat often shows up as a wedge-shaped section near the center. Some people leave it in when grilling with bold marinades or soy-based glazes that complement the stronger flavor rather than fighting it.
If you choose to eat the dark portion, cooking it quickly at high heat (searing or grilling) tends to produce better results than slow, gentle methods, which give the fats more time to oxidize and intensify the fishy taste. Acidic ingredients like citrus juice or vinegar can also help temper the flavor. The dark meat is perfectly safe to eat and nutritionally richer than the white, so whether you trim it or keep it is really a matter of personal taste.