Results:
Over the course of the experiment, δ13C of muscle tissue gradually decreased from an average of -25.9 ‰ ± 0.3 (SD) measured after 8 weeks of feeding on a chironomid diet to -26.9 ‰ ± 0.4 (Figure S1a), while the δ15N of muscle tissue gradually increased from 14.2 ‰ ± 0.6 to 14.9 ‰ ± 0.2 (Figure S1b) during the same time interval. Compared to muscle tissue, lipid-normalized stable isotope values in liver tissue were more depleted in 13C, leading to more negative values of δ13C (Figure S1c). δ15N of liver decreased from 16.5 ‰ ± 0.4 to 15.5 ‰ ± 0.3 and liver tissue was generally more enriched in 15N compared to muscle tissue (Figure S1d).
Δ13C of perch muscle ranged from 2.6 ‰ to 4.4 ‰ and for Δ15N from 0.5 to 3.0 ‰ (Table 1, Figure S2a, b). Δ13C of perch liver tissue ranged from 0.0 ‰ to 2.0 and for Δ15N from 0.2 to 2.0 ‰ (Table 1, Figure S2c, d). For muscle tissue, Δ13C differed significantly between weight classes (ANCOVA: F5,28: 4.584, P= 0.008), with significantly lower TDF in juvenile individuals (i.e., weight class <20 g at the start of the experiment) compared to all other weight classes. In addition, muscle Δ13C of 20-30 g individuals caught in the pelagic zone was significantly lower than in individuals of 40-50 g (Figure 1a). Δ15N in muscle tissue differed significantly between weight classes (ANCOVA: F5,28: 18.365, P< 0.001), with significantly higher Δ15N observed in <20 g individuals compared to all other weight classes (Figure 1b). In this ANCOVA, final weight had a significant effect on the Δ15N of muscle tissue (ANCOVA: F4,28: 7.403, P= 0.012), while this covariate was not significant for the Δ13C of muscle tissue. No significant differences were found in Δ13C and Δ15N of liver tissue between weight classes using final weight as a covariate (Figure 1c,d).
SMR between the individuals varied substantially among perch groups and ranged from 55.6 to 106.5 mg O2 kg -1hr-1 (Table 1). SMR differed significantly between perch of the different weight classes (ANOVA: F4,26: 4.685, P= 0.008) and final weight as a covariate was not significant. SMRs were highest in <20 g individuals, but Bonferroni-adjusted pairwise comparisons showed that only differences between high SMR in 20-30 g individuals caught in the pelagic zone and the low SMR in individuals of 40-50 g were significant (Figure 2).
For muscle tissue, linear regression showed that SMR had a significant negative effect on the Δ13C (t=- 4.424, P< 0.001, Figure 3a) and a significant positive effect on Δ15N (t= 2.657, P= 0.014, Figure 3b), respectively. In contrast, this relationship was not significant for Δ13C or Δ15N in liver tissue (Figure 3 c, d).