When plotting the d13C against the C:N ratio for the
digestive gland, we observed a clear correlation (r = 0.79,
p < 0.001, Fig. 4). The correlation between the C:N ratio and the
d13C has been described previously by Post et al. (2007) as an effect
of lipid content, prompting us to apply a mathematical correction
on the raw d13C values to estimate lipid-free isotopic ratios. This
led to noticeable corrections (up to +2.5% in August when the lipid
content was higher).