Tissue-specific turnover rates of the nitrogen stable isotope as functions of time and growth in a cyprinid fish

Ecological applications of stable isotope data require knowledge on the isotopic turnover rate of tissues, usually described as the isotopic half-life in days (T (0.5)) or the change in mass (G (0.5)). Ecological studies increasingly analyse tissues collected non-destructively, such as fish fin and scales, but there is limited knowledge on their turnover rates. Determining turnover rates in situ is challenging, with ex situ approaches preferred. Correspondingly, T (0.5) and G (0.5) of the nitrogen stable isotope (delta N-15) were determined for juvenile barbel Barbus barbus (5.5 +/- 0.6 g starting weight) using a diet-switch experiment. delta N-15 data from muscle, fin and scales were taken during a 125 day post diet-switch period. Whilst isotopic equilibrium was not reached in the 125 days, the delta N-15 values did approach those of the new diet. The fastest turnover rates were in more metabolically active tissues, from muscle (highest) to scales (lowest). Turnover rates were relatively slow; T (0.5) was 84 (muscle) to 145 (scale) days; G (0.5) was 1.39 x body mass (muscle) to 2.0 x body mass (scales), with this potentially relating to the slow growth of the experimental fish. These turnover estimates across the different tissues emphasise the importance of estimating half-lives for focal taxa at species and tissue levels for ecological studies.