The carbon in the fish otolith originates from two sources. It comes partly from the water in the form of dissolved inorganic carbon (blue balls) and partly from fish the diet that is being burned as part of the metabolism (red balls). The carbon from these two sources mix in the blood of the fish. The proportion of dietary carbon increase when metabolism increase - the number of "red balls" increase - in the blood that delivers the carbon to the growing otolith in the inner ear of the fish. In this way, the growth layers of the otolith continuously stores information on the metabolic rate of the fish that can be recovered as the so called delta 13C value. Illustration: Ming Tsung-Chung.