Hidden in the gut: Metabarcoding reveals overlooked predation by the invasive European catfish (Silurus glanis).

Morphological and metabarcoding analyses provided new insights into the diet of the European catfish (Silurus glanis) in an invaded area in the Iberian Peninsula. Out of 264 S. glanis examined, data were obtained for 173 individuals through metabarcoding of intestinal contents and for 110 through morphological analysis of stomach contents. Metabarcoding consistently identified more prey taxa per sample than morphological analysis, yielding a total of 34 and 21 taxa, respectively, that primarily included fish and crustaceans. Morphological analysis demonstrated a higher detection of crustaceans than metabarcoding, which detected more fish. Both methods identified native Anguilla anguilla, Chelon spp., Luciobarbus spp. and Atyaephyra desmarestii as well as non-native Lepomis gibbosus as the most frequent prey. However, metabarcoding detected A. anguilla at more than twice the frequency found by morphological analysis, and other endangered taxa that were missed by morphological analysis. Metabarcoding showed that prey composition was influenced by both season and predator total length, suggesting dietary shifts related to prey availability and foraging habits. Taken together, results indicate that metabarcoding may be important for assessing the full range of prey diversity and uncover patterns in prey use, particularly fish, providing more information from fewer samples compared to morphological analysis and can contribute to conservation management. Metabarcoding results suggest that predation by the S. glanis on endangered and economically valuable fish such as A. anguilla, Alosa spp. may be stronger than previously considered.These findings reinforce the urgent need for targeted management actions to mitigate the impacts of S. glanis in the Iberian Peninsula, where many native and endemic species are currently threatened.