Background. Introduction of the common carp, Cyprinus carpio Linnaeus, 1758, may cause a cascade of alterations in trophic webs, among other negative effects, indirectly due to its feeding behaviour. A simplified model site, covenient for studying such interrelations is Lago de Pátzcuaro, México, a shallow lake where carp was first introduced in 1974. The aim of our study was to characterize diet composition, diet breadth and trophic position of C. carpio and local fish fauna, and to assess diet overlap between fish fauna from sites with different habitat characteristics during the wet and the dry seasons. Materials and Methods. We assessed the diet of C. carpio and local fish fauna in six sites during the wet and the dry seasons. Stomach content analysis was carried out separately for each length class. The importance of each item was estimated by the index of relative importance (IRI) and the omnivory index was used to assess the feeding behaviour. The trophic guild and trophic position were determined using the TrophLab software and stable isotope analysis. Diet breadth was calculated by Levin’s Index and diet overlap by the Horn’s Index. Fish dorsal muscle and water hyacinth tissues were obtained from nitrogen isotope signature analysis. Results. We found that C. carpio is an omnivorous fish that feeds in shallow areas with water hyacinth cover and rooted macrophytes, consuming mainly detritus and plant debris (>60%). However, δ 15N values indicate that C. carpio is also a secondary consumer because it also ingests invertebrates associated with plant debris. The carp diet overlaps with native blackfin, Goodea atripinnis Jordan, 1880, at most sites. Small carp (<120 mm SL) overlapped with the Chirostoma spp. diet, as well as the introduced Oreochromis spp. at shallower sites with surface water hyacinth and with plant detritus at the bottom. Conclusion. Lago de Pátzcuaro is facing trophic food web reduction that seems to be a more serious problem than feeding competition between the common carp and other fish species.
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