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).