1. In Japan, Fish Are Sorted Into Red-Flesh and White-Flesh, and Salmon Breaks the Color Rule
Japanese cooking splits fish flesh into akami (red flesh) and shiromi (white flesh). The Japanese Wikipedia article explains that the split is based on how much red pigment (it names hemoglobin and myoglobin) a fish's muscle contains, so the name does not always match the color. It lists salmon as one of the exceptions [1].
English has a different habit. English-speaking cooks tend to sort fish by fat: "white fish," such as pollock or halibut, are lean (pollock has about 1 gram of fat per 100 grams), while "oily fish," such as herring (about 12 grams) and salmon, hold much more [2]. So salmon is on the "oily" side in English and the "white-flesh" side in Japanese. Neither list is wrong. They are answering different questions.
This article follows the Japanese question: why can a pink fillet count as white flesh, and what is the sorting really tracking?
2. Salmon's Pink Comes From Its Food, Not From Its Muscle
In the ocean, salmon eat krill and other small crustaceans that contain a pigment called astaxanthin, which algae make and which passes up the food chain. The pigment collects in the salmon's muscle and gives wild salmon its orange-red color. Some wild salmon carry genetic changes that stop them from storing it, and their flesh is pale or ivory [3].
Farmed salmon eat manufactured pellets instead of wild prey. Without carotenoids in the feed, the flesh would be pale or grayish, so farms add the pigment to get the pink that buyers expect [3]. The Japanese Wikipedia article says the same thing in a sentence: if the pigment is left out of the feed, farmed salmon flesh turns white [1].
That settles the first half of the puzzle. The pink is borrowed color. The red-flesh and white-flesh groups look at a different pigment, one the fish makes for itself.
3. Fish Have Two Kinds of Swimming Muscle, and Only One Is Red
That pigment is myoglobin, a red protein that stores oxygen inside muscle. A 2025 study of young yellowfin tuna describes the two muscle types. Red muscle sits mainly near the lateral line, the sensing line along a fish's side. It is packed with mitochondria (the parts of a cell that burn fuel with oxygen), fat droplets, and myoglobin, and it powers long, steady contraction. White muscle fills most of the body, has few mitochondria, lacks myoglobin, and gets its energy without oxygen in short bursts [4].
In that study, red muscle had about 2.3 times the activity of citrate synthase, an enzyme used in oxygen-based burning, and white muscle had about 3.2 times the activity of lactate dehydrogenase, one used in the oxygen-free route, which fits the idea of one muscle for cruising and one for sprinting [4]. A simple way to picture it is a person who can walk all day but sprint only a few seconds. Fish muscle is built differently from ours, so it is only a picture.
So "red flesh" means a fish with a lot of red cruising muscle. The name reflects how much oxygen-rich cruising muscle the fish has, and the color is a clue to it.
4. The Dark Stripe in a Fillet Shows How Much Cruising Muscle a Fish Has
The cruising muscle shows up in a fillet as the dark red stripe called chiai in Japan, often "dark muscle" or "bloodline" in English. A Japanese encyclopedia entry says some thin dark muscle lies under the skin in nearly every fish, but it is well developed in fish that swim actively near the surface and barely developed in fish living on the sea floor. A thick layer deep beside the backbone appears only in medium and large open-ocean travelers such as tuna and bonito [5].
The entry says dark muscle's share varies widely by species, from a few percent up to over 20 percent, and is usually about 10 to 15 percent of a fish's meat. A second entry puts it at 15 percent or more in red-flesh fish like tuna but generally only a few percent in white-flesh fish, not above 10 percent. It names bonito, tuna, mackerel, sardine, and saury as fish with a lot of it, and sea bream, cod, and flounder as fish with little [5].
Now the puzzle resolves. Japanese Wikipedia counts salmon as having little of the fish's own red muscle pigment, and its pink is borrowed from food. Sorting by the fish's own muscle puts it on the white side. Popular articles often quote a precise cutoff, 10 milligrams of pigment per 100 grams of flesh, but the number traces back to a weak source, so it is better treated as a rule of thumb than a measured line [1].
5. Some Red-Flesh Fish, Like Tuna, Use Their Red Muscle to Keep Warm
A lot of red muscle can pay off in unexpected ways. Most fish are about as warm as the water around them. Bluefin tuna and bonito are exceptions: their blood vessels run in a countercurrent pattern, so heat made by working red muscle moves back into the muscle instead of leaking away at the gills. Warm muscle can swim harder, which lets these fish travel into cooler water [5].
This ability is not there from birth. University of Tokyo researchers reported in 2025 that Pacific bluefin tuna develop the power to stay warmer than the water at a body length of roughly 20 to 40 centimeters (8 to 16 inches). Both making more heat and holding it in matter. The measurements that rose at that stage included swimming metabolic rate, the weight of the dark muscle, and the weight of the heart's main chamber [6].
It is a special case, though. It describes tuna and a few relatives, not every red-flesh fish [5]. For the main question, it shows how far a swimming lifestyle can reshape a body.
6. How to Check a Fish's Swimming Style by Looking at a Fillet
Next time you see fish at a market or on a plate, look for the dark band in the fillet. Is it thick or thin? Does it sit deep by the bones or just under the skin? Compare a tuna or mackerel with a cod or flounder if you can. A thicker band hints at a fish that spends its days cruising.
Then compare the band with the overall color of the flesh. In salmon, the pink and the dark band are two different things. Leave any cutting to an adult, and look without touching the knife.
A question to explore further: the pigment cutoff in Japanese lists has no clear origin. Could you find a fisheries textbook or an official standard that explains where the line between red-flesh and white-flesh fish is drawn?