As Scott says, this was explained back in 2010 on this forum: "The yellow spot is called a chromataphore. A chromataphore is like a birth mark. If you look closely at a spring salmon, you can notice the gold pigmentation tones in its skin color." As Spring Velocity notes, chromataphores are usually noticed on the bellies shortly before the fish return to spawn. Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in contrast, have a class of cells called melanocytes for coloration.