That brownish-grey strip running down the middle of the salmon fillet is the bloodline (also called the lateral line or It’s a muscle area rich in blood vessels that help the fish with swimming It’s completely safe to eat, but it can have a stronger, fishier flavor than the pink flesh.
The longer the fish has been dead the darker the blood line will be. Can be a good indicator of how fresh the fish is that you are being bought
Many people trim or scrape it off before cooking, especially when preparing raw dishes like sushi. For cooked salmon, you can leave it—it often mellows in flavor when baked or grilled.
Its dark color is normal and not a sign of spoilage, unless it looks slimy.
