If I remember right, most fish have two types of muscle, one type for routine work which is the main mass (meat) of the fish and another type for burst or short term use as in attack/escape useage. This second or short term use muscle is that dark layer along the side next to the skin. I've never tasted any difference in it on salmon, at least not enough to worry about however I have heard that on some fish, like tuna, you'd might want to cut it out and discard it as it tastes very strong.