We used bulk tissue analyses of δ13C fin clips and potential prey to estimate diet composition of experimental individuals. Freeze-dried and homogenized samples were analysed in WasserCluster Lunz, Austria. Isotope ratios are reported relative to the international Vienna PeeDee Belemnite carbonate standard. δ13C values were mathematically corrected for lipid content following methods as described by Post et al. (2007). The baseline 13C value was further corrected to account for variability in basal resources across the sampling sites (Olsson et al. 2009; Musseau et al. 2020) using the following equation: