Patterns of diet overlap between populations of non-indigenous and native fishes in shallow ponds

No significant differences in the diet composition were detected for any of the populations of four non-indigenous fish species (brown bullhead Ameiurus nebulosus , pumpkinseed Lepomis gibbosus , topmouth gudgeon Pseudorasbora parva and eastern mudminnow Umbra pygmaea ) and two native fish species (gudgeon Gobio gobio and roach Rutilus rutilus ), between two small shallow ponds and between habitats within the ponds, during late summer. Based on diet composition, groups of size classes within species ('functional groups') were distinguished. For most functional group combinations of exotic fishes, diet overlap values were low. Although chironomid larvae formed the most important food source, differential consumption of chironomid size classes allowed an important degree of niche differentiation between non-indigenous fishes. In contrast, high diet overlap was found between the functional groups of indigenous gudgeon and of several non-indigenous fishes, indicating a high potential for interspecific exploitative competition. The diet of roach consisted almost entirely of non-animal remains (detritus and plant material). The high proportion of such low-energy food in the diet of this species may be indicative for a competition induced niche shift to suboptimal food sources.