Some species, especially oily fish, have a strip of dark, blood-rich muscle running down the centre of the fillet; it is edible, but has a stronger taste than the paler flesh. Some people enjoy the stronger, fishy flavour and are happy to eat it, others prefer to remove it for visual or taste reasons. Cutting it out is also a quick and easy way to remove the pinbones that run down the centre of most fillets. The bloodline is a good indicator of freshness, it’s bright pink-red in very fresh fish, becoming duller and turning brown as the fish deteriorates. For this reason it is sometimes left on sashimi-grade fish to show freshness, however eating it raw is an acquired taste, so many people choose to remove it if serving fish raw or rare. The choice is yours.