Most of the isotopic enrichment between the blood and the otolith was associated with the movement of C from blood to endolymph, while enrichment associated with the precipitation of otolith aragonite from the endolymph was small. Net isotopic enrichment between sources and the otolith was therefore slightly negative. There was substantial depletion in 13C of blood relative to C sources (ε bloodsources = ≡6.9 ± 1.1 SE), but substantial enrichment in 13C in otolith relative to blood (ε otoblood = 13.3 ± 1.3 SE). We also estimated isotopic enrichment factors associated with physiological carbon transformations by measuring δ 13C of blood and endolymph (which closely tracked otolith δ 13C). At the activity levels and total metabolic rates characteristic of fish in this study, 17% (☓% standard error, SE) of otolith C was metabolically derived, while >80% was derived from DIC in ambient water. We manipulated the δ 13C of food and ambient DIC in a factorial design with juvenile rainbow trout ( Oncorhynchus mykiss). Unfortunately, uncertainty regarding the proportion of otolith carbon that derives from metabolized food versus dissolved inorganic carbon (DIC) in the water currently limits utilization of this marker. Otolith stable carbon isotope ratios provide a unique and widely applicable environmental record.
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