## 3 Materials and Methods  
### Study Area and Field Sampling  
The fish samples were collected at two locations from Catalonia, Xerta (40\({}^{\circ}\)54\({}^{\prime}\)14\({}^{\prime\prime}\) N, 0\({}^{\circ}\)29\({}^{\prime}\)36\({}^{\prime\prime}\) E) and Tortosa (40\({}^{\circ}\)48\({}^{\prime}\)56\({}^{\prime\prime}\) N, 0\({}^{\circ}\)31\({}^{\prime}\)14\({}^{\prime\prime}\) E), both in the last part of the Ebro River. The climate in the lowland of the River Ebro is typically Mediterranean, with rainfall concentrated in autumn and spring (200-300 mm) and intense summer drought (<50 mm). The flow regime is pluvio-nival because of the left-margin tributaries from the Pyrenees. The average annual temperature ranges between 10 and 15 \({}^{\circ}\)C. The lowest temperatures occur in winter and can reach down to 0 \({}^{\circ}\)C, while the highest in summer can reach over \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline
**Control** & 3124 & 140.7 & & \(<\)LOQ & 284.2 & & 9686.0 & 315.7 & 63.87 \\ \hline
**DLT1** & & & & 1773 & & 67.73 & & & & \\ \hline
**DLAX1** & & 31.53 & 997.8 & 14,462 & \(<\)LOQ & 108.5 & 203.3 & \(<\)LOQ & 354.7 & 5259 & \\ \hline
**DLX2** & 15,115 & 74.81 & & \(<\)LOQ & 87.83 & & \(<\)LOQ & 227.7 & 924.2 & 38.24 \\ \hline
**SGT1** & & & & 1708 & 2362 & & 147.3 & & & \\ \hline
**AAX5** & 186.9 & & & & & & & & \\ \hline
**AAX7** & 859.8 & & & & & & & & 76.51 \\ \hline
**AAX8** & & & & & & & & & & \\ \hline
**CAT3** & & & & & 498.4 & & & & \\ \hline
**ICT1** & & & & 900.4 & & & & & \\ \hline
**ICT2** & & & & & \(<\)LOQ & & & & & \\ \hline
**RRX2** & & & & & & & & & & 29.10 \\ \hline
**SLT1** & & & & & & & & & 109.7 & \\ \hline
**CYT1** & & & & & & 866.8 & & & & \\ \hline
**CYX1** & 3096 & 106.4 & 572.9 & 5453 & \(<\)LOQ & 59.12 & 129.6 & 181.6 & 1342 & \\ \hline
**LIFT1** & & & & 10,173 & & & 678.1 & & & \\ \hline
**LIFT2** & & & & 2657 & 174.1 & 1163 & & & & \\ \hline
**LIFT3** & & & & 249.7 & & & & & \\ \hline
**LIFT4** & & & & 56.29 & \(<\)LOQ & & & & & \\ \hline
**LIX1** & & 99.64 & 30.747 & & \(<\)LOQ & 172.5 & \(<\)LOQ & \(<\)LOQ & 1463 & 423.1 & 83.45 \\ \hline
**LIX2** & 28,319 & 147.7 & & \(<\)LOQ & & & & & & \\ \hline MCT1 & & & & 1191 & & & & & 2191 & & \\ \hline
**MCT2** & & & & & 1344 & & & & & \\ \hline
**MCT3** & & & 3003 & & & & & & \\ \hline
**MCX1** & 664 & 953.0 & 65,499 & \(<\)LOQ & 281.3 & 143.4 & \(<\)LOQ & 1765 & 667.3 \\ \hline
**rcp** & & & & & 41.09 & 32.48 & & 1185 & & \\ \hline \end{tabular}  
* Compounds quantified using an equivalent standard.  
\end{table} 
Table 2: Concentration values for the highest-scored compounds. 40 \({}^{\circ}\)C. The substratum in the area is mainly calcareous, with Cenozoic limestones, gypsum and alluvial sediments. The sampling campaign was carried out in April 2019, the peak season for rice and crop activities and the initiation of touristic activities in this area (i.e., the annual peak for the highest level of water pollution). A total of 40 fish samples was collected, 20 from each sampling site. Specifically, fish species collected were _Alburnus alburnus_ (6), _Cyprinus carpio_ (2), _Carrassius auratus_ (6), _Dicentrachus labrax_ (3), _Ictalurus punctatus_ (2), _Izia_ sp. (6), _Lucibarbus graelsii_ (1), _Mugil explains_ (5), _Pseudorashora para_ (1), _Rutilus rutilus_ (2), _Silurus glanis_ (1) and _Squalius laietanus_ (5). Table S3 of the Supplementary Materials section provides a summary of the samples. We selected native species that are abundant in each river transect. This facilitated the capture and guaranteed the representativeness of the sample. Moreover, all selected fishes were large enough to facilitate the GIT extraction and sample manipulation. Furthermore, they were species that did not present any cataloguing in conservation matters, so the sampling did not inflict any damage on the viability of wild populations at risk.