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Metabolic rate of late Holocene freshwater fish: evidence from ¹³C values of otoliths

¹ Christopher M. Wurster* and William P. Patterson.* Department of Earth Sciences, 204 Heroy Geology Laboratory Syracuse University, Syracuse, New York 13244
² Present address: Department of Geological Sciences, 114 Science Place, University of Saskatchewan, Saskatoon S7N 5E2, Canada. cmw619{at}mail.usask.ca

We examine patterns of intra-otolith variation in ¹³C 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 ¹³C values as the fish ages, an initial strong covariation between seasonal ¹⁸O and ¹³C values, and a decrease with age in the magnitude of seasonal change in ¹³C values. These last two observations are illustrated by seasonal least-squares linear regressions between ¹³C and ¹⁸O 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 ¹³C 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 ¹³C values.

Thus, the difference between maximum and minimum ¹³C values from a single otolith (¹³C_max–min) is postulated to reflect the total change in metabolic rate over the lifetime of a fish. ¹³C_max–min values significantly and negatively covary with average 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.