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, stable isotope values in liver tissue were depleted in 13C, leading to more negative values of δ13C and variation at each time point was high (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 -0.1 ‰ to 1.9 and for Δ15N from 0.2 to 2.0 ‰ (Table 1, Figure S2c, d). For muscle tissue, size-corrected Δ13C differed significantly between weight classes (ANOVA: F4,28: 6.289, P= 0.001), with significantly lower TDF in juvenile individuals (i.e., weight class <20 g at the start of the experiment) and 20-30 g individuals caught in the pelagic zone that were significantly different to the higher values found in individuals of 40-50 g. In addition, size-corrected Δ13C in juveniles differed significantly from size-corrected Δ13C in 20-30 g individuals caught in the pelagic zone, as well as from 30-40 g individuals. Size-corrected Δ15N in muscle tissue differed significantly between weight classes (ANOVA: F4,28: 8.781, P< 0.001), with significant higher Δ15N observed in <20 g individuals compared to all other weight classes (Figure 1b). For liver tissue, size-corrected Δ13C differed significantly between weight classes (ANOVA: F4,27: 3.028, P= 0.039), with significantly lower TDF observed in juveniles compared to 20-30 g individuals caught in the pelagic zone (Figure 1c). No significant differences were found in the size-corrected Δ15N of liver between weight classes (Figure 1d).
SMR between the individuals varied substantially 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: 11.322, P< 0.001). SMRs were highest in <20 g individuals and 20-30 g individuals caught in the pelagic zone that were significantly different to the low values found in individuals of 40-50 g (Figure 2). In addition, SMR in juveniles differed significantly from SMR in 20-30 g individuals caught in the pelagic zone, as well as from 30-40 g individuals (Figure 2).
For muscle tissue, linear regression showed that SMR had a significant negative effect on the size-corrected Δ13C (t=- 3.375, P= 0.003, Figure 3a). For size-corrected Δ15N in muscle, a positive trend could be observed, but regressions were not significant (Figure 3b). Furthermore, no significant relationship could be identified between SMR and size corrected Δ13C, or size corrected Δ15N in liver tissue (Figure 3 c, d).