A saltwater fish, related to the tuna, with an oily, soft, pale flesh, that is sometimes pink, which when cooked, becomes flaky and firm with an off-white color. The rich flavor will vary according to the oiliness of the fish, which varies with the seasons and with different species. Steaks or fillets of mackerel are available fresh or frozen and can be substituted with tuna, marlin, or swordfish in many cases. To check the fish for doneness, use the tip of a sharp knife and cut through the thickest part of the fillet. If the fish has been properly cooked, the meat will appear opaque but will still be moist.