## 3 Results
### Niche Metrics and Diet of Anguilla anguilla  
The isotopic signatures and niche metrics of _Anguilla anguilla_ varied among lakes (Table 2, Figures 2-4; Kruskal-Wallis, Hc least 12.06, \(p\)-value \(<\)0.001). The highest \(\delta^{15}\)N values were observed in HP (Table 2; Mann-Whitney with Bonferroni correction in cases of multiple comparisons, U at least 0.1, \(p\)-value always \(<\)0.001). The Carbon Range increased with eutrophication (Figure 4, Table 2) and the largest Nitrogen Range was observed in the eutrophic lake (Table 2). Overall, no correlation between the body length and \(\delta^{13}\)C (%o) of _Anguilla anguilla_ was observed in any lake (Pearson correlation, \(p>\)0.05). _Anguilla anguilla_ had 5 ITUs in LP and HP and 12 ITUs in IP, where the eel-resource ITU linkage density was highest (Table 2). ITU-based mixing models showed no differences between lakes in terms of the overall contribution of invertebrates to the eels' diet (Figure 5a). By contrast, the consumption of basal resources increased and piscivory decreased with increasing levels of pollution (i.e., from LP to HP; Table 3 Figure 5a). _A. anguilla_ showed a generalist diet including 20 different categories of food source (Table 3, Figures 6 and 7). Some of these were common to the three lake populations (e.g., Actinoptergyii, Bivalvia, Gastropoda, Decapoda and Polychaeta) but their consumption varied. The Bray-Curtis index (BC) applied to diet showed a lower similarity between the HP population and the others (76% similarity between LP and IP vs. 41% between LP and HP, and 54% between IP and HP). Specifically, in LP the diet of _A. anguilla_ was mostly based on Actinoptergyii (34.76% \(\pm\) 1.90), Decapoda (27.84% \(\pm\) 4.60) and Gastropoda (13.06% \(\pm\) 0.40), in IP on Actinoptergyii (30.65% \(\pm\) 0.50) and Decapoda (26.13% \(\pm\) \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{_Anguilla anguilla_} & \multicolumn{3}{c}{_Dipololololous annularis_} \\ \hline  & **LP** & **IP** & **HP** & **LP** & **IP** & **HP** \\ \hline \(\delta^{13}\)C (\%o) & \(-1229\pm 0.19\) * & \(-15.01\pm 0.53\)* & \(-17.20\pm 1.12\)* & \(-13.00\pm 0.49\)* & \(-14.00\pm 0.10\)* & \(-15.57\pm 0.16\)* \\ \(\delta^{15}\)N (\%o) & \(9.08\pm 0.32\)* & \(9.50\pm 0.36\)* & \(12.96\pm 0.51\)* & \(7.73\pm 0.17\)* & \(9.59\pm 0.29\)* & \(13.87\pm 0.16\)* \\ CR & 1.55 & 4.66 & 6.19 & 2.17 & 0.35 & 0.65 \\ NR & 2.54 & 2.59 & 2.86 & 0.67 & 0.96 & 0.74 \\ L & 30 & 84 & 33 & 28 & 13 & 14 \\ S & 19 & 42 & 22 & 21 & 10 & 11 \\ L/S & 1.6 & 2.0 & 1.5 & 1.3 & 1.3 & 1.3 \\ SEAc & 1.46 & 9.62 & 4.84 & 1.55 & 0.45 & 0.32 \\ TNW & 1.81 & 2.06 & 2.32 & 2.15 & 2.28 & 1.98 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._  
Figure 4: Boxplot of the distribution of \(\delta^{13}\)C (**a**) and \(\delta^{15}\)N (**b**) isotopic signature of _Anguilla anguilla_ and _Dipolous annularis_ in each lake: LP, IP and HP: low, intermediate and high eutrophication. For each lake, the thick horizontal line represents the median of the distribution, the box includes 50% of the data, the symbol (x) represents the mean and the whiskers reach the highest and lowest value within 95% of the distribution. Different letter (a, b, c) within panels indicates differences among lakes (Mann-Whitney test with Bonferroni correction in cases of multiple comparisons; \(p<\)0.05).