Individual variability in ontogenetic dietary shifts and their influence on the trophic interactions of native and non-native freshwater piscivorous fishes.

Abstract

Introduced fishes for enhancing sport angling tend to be large bodied and of high trophic position, raising ecological concerns on their impacts on prey communities and interactions with native fishes in the same functional guild. The piscivorous pikeperch Sander lucioperca is invasive in several European countries, including in England where it occupies the same functional guild as native Northern pike Esox lucius. Both species undergo ontogenetic dietary shifts from macroinvertebrates to piscivory during their juvenile life-stages. The influence of this dietary shift on their trophic positions and niche sizes was assessed across three sites in an invaded lowland river basin in Western England. Stable isotope metrics revealed pikeperch switched to piscivory at smaller body sizes than pike (albeit with high inter-individual variability); stomach contents analyses revealed pikeperch were piscivorous from 31 mm. Irrespective of ontogeny, there was low overlap in the isotopic niches of the two species, with the niche of pike generally more enriched in δ13C but with minimal differences in δ15N. The prey resources of invasive pikeperch and native pike were thus largely differentiated, suggesting the ecological consequences of pikeperch invasion include increased predation pressure on aspects of prey populations that are usually not depredated upon by pike.