First evidence of feeding on micromammals and fish by a cyprinid species (Squalius carolitertii) in lacustrine food webs

The consumption of resources at multiple trophic levels reflects the omnivorous behaviour of various species. Despite the importance of omnivory in food web structure and stability, there are still few observational data to assess the omnivorous role of species in food webs. We first assessed the feeding of two cyprinid [northern Iberian chub (Squalius carolitertii) and northern straight-mouth nase (Pseudochondrostoma duriense)] and one salmonid (brown trout Salmo trutta) species in a mountain lake of the Iberian Peninsula, and then we studied the omnivorous behaviour and niche partitioning of the cyprinid species. A noteworthy result was the consumption of micromammals by northern Iberian chub. This species showed a high trophic flexibility feeding on diverse food types (detritus, vegetal rests, zooplankton, aquatic insects, terrestrial insects, micromammals and small fish), whereas northern straight-mouth nase fed mostly on detritus. We observed that the consumption of zooplankton decreased with increasing fish length in northern Iberian chub, whereas no evidence for ontogenetic dietary shifts were observed for northern straight-mouth nase. Brown trout fed only on cyprinids, acting as top predator in the studied mountain lake. Despite high dietary overlap between omnivorous cyprinids, our analyses suggested that omnivorous behaviour and ontogenetic dietary shifts of northern Iberian chub can be an important adaptive feature that may reduce food competition and enable species coexistence in mountain lakes. Our study shows that endemic omnivorous cyprinids can exploit alternative energy pathways and underlines the importance of omnivorous species as promoters of stability in lacustrine food webs. We highlight that the importance of occasional pulses of allochthonous energy sources fueling lacustrine food webs, such as micromammals and terrestrial insects, represent a particularly promising area for future research.