Ecological aspects related to reintroductions to avert the extirpation of a freshwater fish from a large floodplain river

The factors leading to reintroduction success are rarely determined in species translocation programmes attempting to prevent extirpations. Examining interactions between translocated fish and ecological aspects will provide information to increase success of future reintroduction efforts. The zooplankton are a key food source for small-bodied fishes; therefore, the examination of prey availability may contribute useful insight. Fish that are ecological specialists may have preference for invertebrates associated with specific habitat conditions. Food availability for ecological specialists also may be influenced by the presence of alien fishes, through competition for prey and habitat exclusion, including invasive Eastern Gambusia ( Gambusia holbrooki ) which is linked to the decline of numerous small-bodied fishes. The Murray–Darling Basin in south-eastern Australia is a highly regulated floodplain system where native fishes have declined over recent decades, especially ecological specialists adapted to ephemeral wetlands with natural flow regimes. Yarra Pygmy Perch ( Nannoperca obscura ) is an ecological specialist that was extirpated in 2008 during a prolonged drought. The objective of this study is to examine dietary aspects of reintroduced Yarra Pygmy Perch as related to zooplankton prey availability and the potential for competition with Eastern Gambusia. The study demonstrated that prey was being consumed by Yarra Pygmy Perch within 24 h of release and provides evidence of overlap in diet with cohabiting Eastern Gambusia. The findings provide direction for further study and a new understanding regarding reintroduction ecology of threatened small-bodied wetland fishes to aid species recovery and persistence.