side of the body, usually right under the skin. The cross section of a salmon steak, for example, has small triangles of dark meat nestled against the skin on either side. These are the slow-twitch fibers that the salmon uses for most of its regular swimming. If fish never needed to swim fast, they’d all be narrow shafts. The rest of a fish’s muscle—from 60 to 90 percent of it, depending on the species—
