Invasive gibel carp use vacant space and occupy lower trophic niche compared to endangered native crucian carp

The introduction of invasive species increases interspecific competition with native species, especially if the invasive fish have a similar ecological role in the ecosystem. The vacant niche hypothesis postulates that an invasive species may be a stronger competitor if it has, additionally to a native species niche, access to a food unavailable to native species. However, there are very few model examples of nearly identical invasive and native species differing in trophic niche utilization. The once common crucian carp (Carassius carassius) has become endangered or extirpated in many regions of Europe mainly due to the invasion of gibel carp (C. gibelio). To estimate the trophic niche divergence between gibel and crucian carp living in syntopy, a non-lethal method of stable isotope analysis (SIA) of fish scales (delta C-13 and delta N-15) was employed. Samples were collected from four sites in the Czech Republic to determine the overlap and sizes of the trophic niches of these two species. The results showed that at two sites, gibel carp had significantly lower delta N-15 than crucian carp, indicating its lower trophic position. The gibel carp also significantly higher delta C-13 at two sites indicating higher utilization of littoral sources compared to crucian carp. In addition, isotopic niches partially overlapped at the four study sites, with the most divergent trophic niches found in the macrophyte-rich site. Finally, the gibel carp had higher probability to occur within the crucian carp niche space than vice versa. Our results provide support for the vacant niche hypothesis, indicating that invasive gibel carp gain a competitive advantage over the native crucian carp via feeding on plant material that is underexploited by native crucian carp. Furthermore, data suggest that more "natural" environmental conditions, such as a rich littoral zone, may help to decrease isotopic niche overlap between the two species. Lower trophic position and higher reliance on unexploited food sources seem to contribute to the competitive superiority of the invasive gibel carp over the native crucian carp.