1. The Dark Stripe Is a Muscle Built for Endless Slow Swimming
A fish's body is mostly muscle, and it comes in two main kinds. A review of fish muscle puts the difference this way: the red fibers are "slow but virtually inexhaustible" and run on oxygen, while the white fibers are "fast, exhaust quickly and use anaerobic metabolic pathways," meaning they work without much oxygen [1].
The dark stripe is generally made up of red muscle. Its color comes from a rich supply of blood vessels and from myoglobin, a red protein that stores oxygen inside muscle [2]. Japanese encyclopedias add that the dark muscle is also richer in iron, vitamin B, and glycogen (stored fuel) than the ordinary white muscle [2].
So the stripe is not a leftover, a bruise, or a streak of spilled blood. It is working muscle, built to keep going.
2. A Fish Uses Slow Muscle to Cruise and Fast Muscle to Sprint
The two muscles split the work. For steady, everyday swimming a fish relies on its red muscle. The white muscle in the same body is mainly called on for bursts of fast swimming and sudden escapes [1]. A Japanese encyclopedia entry says the same about tuna and bonito, and notes that recordings of the muscles' electrical activity show the dark muscle working in slow swimming and the ordinary muscle in sudden, fast swimming [2].
A simple way to picture it is a person who walks all day without tiring but can only sprint for a few seconds. A fish has both abilities built into one body. The comparison is only a picture: fish and human muscles are built differently.
That also explains why the stripe is bigger in some fish. The review says the share of red muscle is highest in pelagic fish, the ones that swim constantly in open water, and gives mackerel, at about 15% of total muscle, as an example [1]. Fish that mostly rest on the bottom tend to have little red muscle.
3. Where the Dark Muscle Runs Through a Fish, and Why It Differs by Species
In most fish the red fibers lie just under the skin [1]. Japanese encyclopedias call this the "superficial" dark muscle, strongest along the side of the body near the lateral line, the faint line that runs from head to tail. Every fish has some. Fish that cruise the open ocean have more, and bottom or deep-sea fish have only a little [2].
Tuna and bonito also have a "true" dark muscle deeper in the body, around the backbone, which is found only in these large, ocean-crossing fish [2].
How much of a fish is dark muscle? The share varies a lot. One encyclopedia says dark muscle is usually 10 to 15% of a fish's total meat, can exceed 20% in some species, and can be only a few percent in others [2]. So in the fish it describes, dark muscle is a minority of the meat. The stripe looks bigger than it is because it sits in the middle of a pale fillet.
4. How a Three-Piece Cut Follows the Backbone and the Small Bones
In Japan the standard way to prepare a whole fish is sanmai oroshi, literally "three-piece lowering." The fish ends up as three pieces: two fillets and the central skeleton. Many cooks around the world fillet fish in a similar way.
The Chiba Prefecture fisheries office, describing how to fillet a Pacific saury, tells cooks to put the knife in from the head end on top of the central bone and draw it along that bone all the way to the tail. It adds that the whole blade should be used, because small sawing motions make the flesh fall apart. Afterward the knife is laid flat to shave off the thin belly bones [4]. The central bone helps guide the knife.
There are also tiny bones inside the fillet itself, often called pin bones. Research on teleost fish, the large group that includes most food fish, describes them as intermuscular bones. They form in the thin sheets of tough tissue that divide the body's muscle into segments, and they develop from tendons that harden into bone [3]. So they sit inside the flesh, not on the backbone, and a cut along the backbone alone does not take them out.
How the dark muscle lines up with these bones depends on the species. In a fish with only superficial dark muscle, the stripe lies along the side just under the skin. In a tuna, the deep dark muscle lies near the backbone [2].
5. Why Raw Fish Is Often Trimmed, and What Scientists Found in Tuna's Dark Muscle
For raw dishes like sashimi, cooks often trim the dark muscle away. One Japanese encyclopedia explains that its blood and iron lower the taste for many people, even though it has plenty of vitamins and good nutritional value [2]. That is a matter of taste, not safety, and many cooks keep it for cooked dishes.
Tuna's dark muscle has drawn scientific attention since 2010. That year, researchers Yamashita and Yamashita reported selenoneine, a compound that contains the element selenium, as the main form of organic selenium in bluefin tuna blood. They also measured it in red muscle at 190 nanomoles of selenium per gram [5].
Kanagawa Prefecture's fisheries technology center then compared the two cuts. In bigeye tuna, average selenoneine was 1.04 nanomoles per gram in the lean red flesh and 95.9 in the dark muscle, about 92 times as much. That is one fish species in one study, not a rule for every fish [6].
The same center ran a trial with 100 healthy adults in their 20s to 60s, who ate tuna three times a week for three weeks. Selenoneine in their blood rose a lot after dark muscle, and only slightly after lean flesh. The report says it is "expected" to help with staying healthy, which is a hope, not a result [6]. The trial shows that eating the dark muscle raises a certain substance in blood. It does not show that eating it prevents illness.
6. How to Spot the Dark Muscle Yourself at the Table
Next time fish is on the menu, look at a piece from the side. Where is the dark band, close to the skin or deep in the middle? If you can compare a tuna or mackerel with a white-fleshed fish such as cod, does the dark band look thicker in one than the other? If fish is not on the menu, look at photos of fish fillets and see if the pattern repeats.
For a cooked piece, gently open it with a fork and look for the fine bones inside. Go slowly and leave any small bones on the side of the plate. Leave all cutting with a sharp knife to an adult, and just watch how the blade follows the backbone.
Question to explore further: does the dark muscle of other fish, not just bigeye tuna, hold more of that selenium compound? The sources below are a place to start.