Swimming muscles
There are two types of skeletal muscles: white and red. Most of the muscles in a fish consist of white muscles. In most salmon species these are pink due to a pigment salmon get from their diet. The red muscle is in most fish located as a band along the side of the fish (see the figure). The red muscle contains a lot of myoglobin, capillaries and also a lot of glycogen and lipids. The red muscle mass is somewhere between 0.5 to 30% of the total muscle mass in a fish, depending on the species. Active fish, such as bluefin tuna, have a higher proportion compared to sedentary species, like catfish.

The red muscles are aerobic while the white muscle is mostly anaerobic. As long as a fish swims within the sustained swimming speed only the red muscles are used, while in prolonged swimming at high swim speeds, some of the white muscles are used, and this is what eventually leads to fatigue. During burst swimming the white muscles are used at maximum capacity, and this leads to a rapid fatigue. For a refresher on sustained, prolonged and burst swimming, see the preceding pages.