We examine patterns of intra-otolith variation in δ13C values of fossil Aplodinotus grunniens (freshwater drum) otoliths recovered from an archeological site in northeast Tennessee. We find three repeatable patterns: an initial increase early in ontogeny followed by relatively stable δ13C values as the fish ages, an initial strong covariation between seasonal δ18O and δ13C values, and a decrease with age in the magnitude of seasonal change in δ13C values. These last two observations are illustrated by seasonal least-squares linear regressions between δ13C and δ18O values that tend to progressively decrease in r2 value and slope with fish age. These patterns are evaluated by using a mass balance model in which otolith δ13C values are derived from dissolved inorganic carbon of ambient water mixing with carbon derived from metabolic processes. The proportion of metabolically derived carbon is found to be the dominant factor controlling intra-otolith variation in δ13C values.
Thus, the difference between maximum and minimum δ13C values from a single otolith (δ13Cmax–min) is postulated to reflect the total change in metabolic rate over the lifetime of a fish. δ13Cmax–min values significantly and negatively covary with average δ18O(CaCO3) values, suggesting either a higher total change in metabolic rate over the lifetime of a fish in cooler climates characterized by shorter growing seasons, or a decrease in summer/winter precipitation ratio. A proxy for metabolic rate preserved in otoliths would facilitate the understanding of evolutionary history in physiological traits of fishes and improve our understanding of bioenergetics.