Trophic ecology of two non-native cyprinid fish, the common carp Cyprinus carpio and the bleak Alburnus alburnus.

The introduction of non-native fish species poses a significant threat to biodiversity. In Spain, the common carp Cyprinus carpio (Linnaeus, 1758) and the bleak Alburnus alburnus (Linnaeus, 1758) are both established and sometimes co-occurring non-native species. Here, we investigated the trophic ecology of these species using an integrative approach, combining dietary and stable isotope analyses, to understand seasonal variations in their feeding ecology and potential niche overlap. We found a variable pattern for the gonadosomatic index of the two species in spring and autumn, suggesting the occurrence of extended reproductive periods for both A. alburnus and C. carpio. The gut content and isotopic niche analyses showed seasonal niche shifts, as well as interspecific niche partitioning and potential feeding competition. Notably, we found remains of small fish in the gut of both species and one entire individual of Gambusia holbrooki (Girard, 1859) in the carp gut, highlighting the potential of both species to prey also upon native fish fry or juveniles. Moreover, A. alburnus showed an ontogenetic shift toward prey from higher trophic levels and from the littoral/benthic environment, while C. carpio showed an increase in the proportion of prey from the pelagic environment with size. [ABSTRACT FROM AUTHOR]