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 (δ¹³C_max–min) is postulated to reflect the total change in metabolic rate over the lifetime of a fish. δ¹³C_max–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.