Stable carbon isotope evidence for reduced feeding of Gulf of Mexico sturgeon during their prolonged river residence period

Stable carbon isotope ratios were used to delineate food sources for Gulf o f Mexico sturgeon (Acipenser oxyrinchus de sotoi), an anadromous fish that migrates between Gulf of Mexico and the coastal rivers in south-east U.S.A. The large difference in isotope ratios (similar to 11 parts per thousand) between freshwater food sources and fish muscle tissue suggests that the Gu lf of Mexico sturgeon do not feed significantly in fresh waters. Isotope ra tio data from this study and also from the literature indicate that the gro wth of Gulf of Mexico sturgeon is almost entirely supported by coastal mari ne food sources. It is likely that Gulf of Mexico sturgeon use the cool spr ings that seep into the river as a thermal refuge during their river reside nce in summer and that thermal barriers may prevent the fish from exploitin g the rich food sources available in the warmer portions of the Suwannee Ri ver. (C) 2001 Academic Press.