with Creamy Fennel-Tomato Sauce, it's recommended that you remove the blood
line, a blood-rich muscle that runs through the center of some species of fish,
especially strong swimmers like swordfish as well as tuna and mackerel.
choosing fish, a look at the blood line helps determine freshness. If
it's still bright red, the fish is fresh.
As the fish
deteriorates, the blood line turns dull and brownish. While it's
perfectly safe to eat, it has a strong, fishy taste, which is why recipes often
recommend cutting it out.