Stable isotopes and gut contents indicate differential resource use by 8 coexisting asp (Leuciscus aspius) and pikeperch (Sander lucioperca)

Abstract
Differential use of habitat and prey resources is an important mechanism that may allow
coexistence of sympatric species. Unlike interactions between smaller cyprinid and percid
fishes, the resource use by coexisting predatory asp (Leuciscus aspius) and pikeperch (Sander
lucioperca) is relatively unknown. Here, gut content and stable isotope analyses were used to
study ontogenetic dietary shifts and interspecific trophic niche overlap between asp and
pikeperch coexisting in two reservoirs. The hypothesis that both species show an ontogenetic
dietary shift from small invertebrates to large fish prey, but at the same time use different prey
resources to reduce potential competitive interactions, was validated. The isotopic niches of
the two predators showed no, or only a moderate, degree of overlap (0–65%). The ontogenetic
changes in the degree of interspecific isotopic niche overlap were different in the two
reservoirs, suggesting that trophic segregation can be dynamic and variable among systems.
Gut contents revealed that small (<100 mm standard length) asp consumed mostly terrestrial
invertebrates and emerged aquatic insects, whereas small pikeperch foraged on zooplankton,
larval and pupal stages of aquatic insects and fish. Larger individuals (>100 mm) of both
species were predominantly piscivorous, with asp consuming more cyprinid prey and
pikeperch more percid prey. Coexisting asp and pikeperch populations are able to utilise
different prey resources, thereby reducing potential negative competitive interactions.

Keywords: dietary ontogeny, foraging strategy, interspecific competition, piscivory, stable
isotopes