When an ‘invasive’ fish species fails to invade! Example of the topmouth gudgeon Pseudorasbora parva

A major problem in evaluating biological invasions is the lack of information on failed non-native species introductions, with invasiveness determined purely on establishment successes. This is the case of topmouth gudgeon Pseudorasbora parva, an Asiatic cyprinid fish that now occurs throughout most of Europe and is said to be highly invasive. Although the species has established itself in many locations, and often in high densities, not all topmouth gudgeon invasions are successful. In this brief commu-nication, the appearance and disappearance (following pond drain down and repeated electrofishing depletion) of topmouth gudgeon Pseudorasbora parva from a small pond in northeast London (England) is described, along with two other cases elsewhere in Europe.