What Are Dark Spots on Swordfish Steak?

Swordfish are among the most striking of all commercial seafoods, a large and streamlined predator resembling an unusually large mackerel, but with a signature long bill and unusually large eyes. Their flesh is firm and meaty, like that of other large, active predators such as king mackerel and bluefin tuna, and is usually sold as steaks.

Swordfish are among the larger commercial fish, with market specimens averaging 200 to 300 pounds. Because of their size, they're seldom sold whole or filleted at retail, but rather as fresh or frozen steaks, usually cut to a thickness of approximately 1 inch. The pattern of the swordfish's powerful muscles will be plainly visible on these steaks. Those steaks that are cut from along the fish's center line will show dark triangular or Y-shaped spots between the two main muscle groups..

In general, fish are made up largely of white meat muscle fibers. These are intended for bursts of quick activity, which is how most fish swim. The layer of darker tissue visible in some fish, usually under the skin and between larger muscle groups, is a section of red meat muscle fibers intended for long-term marathon swimming. It's most prominent in powerful swimmers such as large tuna or swordfish, where it's referred to as the "bloodline." This area has a slightly richer, stronger flavor than the rest of the steak.

Swordfish aren't sold whole at retail, so most of the usual freshness tests aren't available to the consumer. If you're buying swordfish steaks fresh, rather than frozen, the bloodline is one of your best indicators of freshness. In newly-harvested swordfish, the bloodline will have a deep red color. Over time, it fades to a rusty brown as its iron-bearing myoglobin oxidizes. The other top indicator of freshness is its smell. Fresh swordfish should smell faintly of clean, fresh salt water. If there's a fishy smell, the swordfish isn't fresh or has been handled poorly.

The swordfish industry has been closely scrutinized for decades, because of the steep decline in stocks between the beginning of the 1960s and the end of the 1970s. Currently, American and Canadian stocks are well managed and sustainably harvested, but that isn't necessarily the case with imported swordfish. As a so-called "apex predator," occupying the top of the food chain, swordfish are also among the species highest in mercury. Swordfish shouldn't be eaten more than once or twice a month, not at all if you're pregnant or nursing.