Gram for gram fish have more muscle than any other vertebrate, a male salmon or tuna can be nearly 70% muscle, which is one reason why fish are so good to eat. The muscles of fish are layered rather than bundled as in the other vertebrates. Each segment, or sheet, of muscles is called a myomere or myotome and is separated from its neighbor by a sheet of connective tissue. Other sheets of connective tissue, called septa (pl =septum) occur along the vertical midline of the body separating the muscles of the left and right sides of the body, and horizontally separating the muscles of the upper and lower halves of the body. The muscles of the upper half of the body are called 'epaxial' and those of the lower half are called 'hypaxial'. Salmon are a particularly good fish for studying the basic shape of fish muscle because the coloration of their muscles allows you to see the edges of each myotome, as in the picture above and the photos below.
The myomeres are not flat sheets of muscle, but are folded into a 3-d shape. In their placement along the body they are angled against the line of the body, with the innermost edge nearer the front of the body and the outermost edge nearer the tail. This means that if you cut a fish in half across the long axis of the body you will cut through a number of myomeres. This angling, in combination with the complex folded structure of the myotome means that the muscles fit into each other along the fish's body with the outer edge looking a little like a W on its side and the extended edges of the fold forming cone like projections. By looking at the two photographs of sections of a salmon's muscles seen below (longitudinal section on the left (this is rotated 90 degrees to the right so that the epaxial muscles are to the right and the hypaxial to the left) and transverse on the right) while keeping in mind the diagram of a single myotome to the left it is possible to get a good idea of basic structure of a fish's musculature. Notice how the muscles appear to be zigzag in the LS photo and a series of inset ellipses in the TS. However cooking a small salmon, or wild caught sea-trout, and then carefully teasing out the individual myotomes is a very informative exercise.
Fish muscles come in three different types, red, pink and white. Most fish have a mixture of two, or all three, types of muscle, but keep the types in discrete groupings, however in the salmonid fishes the red and white muscle types are mixed to form a mosaic type of muscle. The colours these muscles show is related to the amount of haemoglobin present in the muscles, with red muscle having plenty of haemoglobin present and white very little, if any. However when looking at fish muscles it is well to be aware that some fish, that feed on crustaceans, particularly salmonids, develop a pink colour to their muscles as a result of a carotenoid pigment they acquire from their food, in the same way that flamingoes get their pink colour from the crustaceans they eat.
Red muscle, also known as slow muscle, is red because it has a high number of capillaries present in it and thus has a high haemoglobin content. Being well supplied with oxygen red muscle is used for steady, constant-effort swimming and is found in active fish, particularly those that live in the open waters of seas and oceans, nevertheless red muscle seldomly makes up even as much as 20% of a fish's total muscle mass. White muscle, or fast muscle, has thicker fibres than red muscle and possesses many less capillaries, and so it has a much reduced blood flow, and therefore, a reduced oxygen availability. Most white muscle activity is anaerobic (glycogen is converted to lactate). White muscle fibres can produce tensions that are up to 2.7 times greater than those of red muscle, but they are more energetically wasteful and therefore the cost to the animal is higher. Finally white muscle can only work for short periods of time, a couple of minutes maximum is not unusual, before they exhaust their supply of glycogen and need to rest. All this means that white muscles are convenient for short quick bursts of movement, in which capacity they out compete red muscle easily, but that they are no good for prolonged swimming. Pink muscle is intermediate between the two, and is good for continued swimming efforts lasting a few tens of minutes at a relatively high speed. Of course, like all animals, fish use all their muscles in concert as they go about their daily lives.