LATE HOLOCENE METABOLIC RATE OF FRESHWATER FISH: EVIDENCE FROM STABLE CARBON ISOTOPE VALUES OF FRESHWATER DRUM OTOLITHS

Otolith stable oxygen isotope values are known to represent equilibrium precipitation with respect to environmental water. However, the significance of stable carbon isotope values has remained controversial. High-resolution milling of carbonate from otoliths reveals detailed seasonal and lifetime variation in carbon and oxygen stable isotope values. It is hypothesized that intra-otolith variation in carbon stable isotope values of fish otoliths is a proxy for metabolism that varies with climate. We present data from otoliths of freshwater drum acquired from an archaeological site in Tennessee to assess a mass balance model where otolith carbon stable isotope values represent carbon derived from metabolic processes and to a lessor extent dissolved inorganic carbon of the ambient water. We find that the metabolically derived carbon is the dominant control on ontogenetic variation in carbon stable isotope values. Thus, the total intra-otolith range in carbon isotope values of mature fish is postulated to reflect the total change in metabolic rate over the lifetime of a fish. The total intra-otolith range in carbon isotope values of otoliths negatively co-varies with calculated oxygen isotope values of ambient water suggesting that the metabolism of mature fish is higher during periods characterized by warm winters, longer growing seasons and/or a decrease in the summer/winter precipitation. However, the total intra-otolith range in carbon isotope value did not significantly correlate with maximum summer temperatures. A proxy for metabolic rate preserved in otoliths would facilitate the understanding of evolutionary history in physiological traits of fish.