In dark-meat fish such as bonito, mackerel, torik (large bonito), and tuna, the dark red-brown strip running through the center of the fillet is a continuously working muscle. It is safe to eat, but it has a distinctly stronger flavor, a sharper smell, and a fattier texture. Remove it for a milder result; leave it in if you want the fish’s full character. Make the choice based on both the recipe and the fish’s freshness.
This strip is the group of muscles that runs along the fish’s lateral line and works continuously as it swims. Because the muscle is used constantly, oxygen-carrying compounds and fat build up in it, making it dark. The same principle explains the difference between heavily used and less-used muscles in red meat. In migratory, fast-swimming species, this muscle is much thicker; in bonito and Atlantic bonito it can reach the width of a finger, while in sedentary white fish it is barely visible.
The strip tastes different from the rest of the fish: stronger, fattier, and sharper after cooking. If the fish is very fresh, this difference can come across as a pleasant intensity, and some people seek it out. If the fish has been stored for 1 to 2 days, this tissue is the first to spoil; its fat turns rancid quickly, and the bitterness spreads through the whole dish. Make the decision based on freshness: the strip can stay in very fresh bonito, but it should be removed from fish that has been stored.
If you are removing it, do so after filleting the fish. Place the fillet on the board skin-side down; the dark strip will appear as a slightly raised line through the middle. With a sharp, thin knife, make two shallow V-shaped cuts, one on each side of the strip, and remove it intact. Do not cut too deeply, or you will remove more meat than necessary. This is not usually done with small fish because the strip is thin and difficult to separate from the fillet. You can discard the strip or add it to fish stock, where its flavor is diluted and is less noticeable.
If you leave the strip in, balance its flavor. Acid is the best accompaniment: lemon, pomegranate molasses, or a vinegar-based sauce will temper the smell. A marinade made with onion and bay leaves does the same. Cooking method matters too: the strip’s smell is stronger with dry-heat methods such as grilling and roasting, and milder when the fish is steamed or cooked in a tomato-based fish stew. Leaving the strip in is much safer if the fish will be cooked the same day.
Do not confuse this strip with the dark line scraped along the backbone when cleaning the belly. That tissue is the fish’s kidney and is removed from every fish; the strip discussed here is muscle inside the fillet, and removing it is optional. This muscle is generally already separated in canned fish. In lakerda and similar salt-cured fish products, however, the strip is always removed because it is the first part to turn bitter during long storage. With thick grilled bonito steaks, the strip is often left in because trying to remove it from a cross-cut steak can tear the meat apart.
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