1. Diet analyses using C and N stable isotopes commonly focus on mean isotopic signatures; however, isotopic variance among individuals is likely to also contain useful information including details of omnivory. 2. Changes in isotopic signature as a result of dietary shifts are not instantly manifest in the isotopic signature of consumer tissues, but lagged over a period of time required for equilibration. Tissue turnover times have not previously been described in terms of variance in isotopic signature among individuals, and variance among individuals following equilibration with a constant diet is limited. 3. Temporal changes in d15N and d13C variance in juvenile European Sea Bass (Dicentrarchus labrax) muscle, heart and liver were monitored following a shift from a wild diet to two single-source diets administered under seminatural conditions in captivity. Exponential decay functions of the standard deviation of d15N and d13C among individuals were used to model changes in variance over time. 4. All tissues exhibited a similar rate of tissue turnover using variance. However, variance among individuals within tissue types differed once fishes were equilibrated with the laboratory diet. The coefficients of variation of d13C and d15N were smallest in muscle and greatest in liver and greater among sampling dates than within. 5. Analysis of d15N and d13C in different tissues will not therefore provide equivalent power to detect differences in diet or to track changes in patterns of omnivory. Analysis of omnivory should be restricted to variance from a single tissue type. Of the tissues considered here, white muscle is most appropriate for this purpose. 6. Variance estimates derived here provide minimum values expected for a highly specialist feeding population. Departure from these values can be used to describe the degree of omnivory within a population.