Cut through the flesh along the backbone from the tail to just behind the head. Cut down to the collar bone. Turn the knife flat and slide it along the ribs to the tail, cutting the flesh away from the backbone. Repeat on the other side. Thoroughly wash the dressed fish, steaks, or fillets under plenty of cool, running tap water.
Many large fish can have tapeworms, which are usually found in the tail quarter of the fish.
To remove them, fillet the fish in the usual fashion. The affected area will have a reddish tinge, unlike the whitish, unaffected areas. The tapeworms themselves are white. Although there should be no danger from eating this portion of the fish, most people will probably want to remove and discard the affected section. If you plan to eat your fish fresh, wrap it in clear plastic or place it in zip-top storage bags.