Phenotypic responses to piscivory in invasive gibel carp populations

Abstract

The establishment of introduced fishes can be inhibited by the biotic resistance from species in the receiving environment, including strong consumptive resistance from specific piscivorous fishes. In response to predation pressure, prey fish population responses include predator-induced morphological changes, where an extreme example is seen in the crucian carp Carassius carassius, which forms deep-bodied morphs in predator presence that reduces individual predation risk. As its congener, gibel carp Carassius gibelio is a highly invasive fish across in its non-native range in Eurasia. Here we test whether their introduced populations also respond to the presence of piscivorous fishes by altering their body shape and trophic ecology by testing differences across 16 non-native lentic populations in Turkey that provided groups of piscivorous fish presence versus absence. In piscivore presence, gibel carp had a higher ratio of body length-to-depth than in piscivore absence, but with their body condition factor being higher in absence. Stable isotope mixing models predicted that gibel carp had diets that were more animal based (gastropods and zooplankton) in piscivore absence, but plant based in piscivore presence. Moreover, diet predictions of piscivore diet suggested gibel carp were consistently consumed less than other prey fishes. These results suggest that these alien gibel carp were responding to piscivory as per crucian carp, reducing their predation risk at the individual level by forming deep-bodied morphs. We suggest these morphological responses then decrease the strength of the biotic resistance against their invasion at the population level.