What factors influence fin-nipping damage by the invasive Gambusia holbrooki (Poeciliidae) on native fishes in riverine systems?

Gambusia holbrooki is arguably the most widely introduced and ecologically damaging freshwater fish in the world. Although aspects of its aggressive behaviour have been studied in lentic environments and ex situ experiments, the physical damage to native freshwater fishes in riverine systems caused by this behaviour remains relatively unknown. This study quantified the spatial and temporal patterns of abundance of G. holbrooki and the prevalence of fin-nipping damage on freshwater fishes in river systems of south-western Australia, a globally endemic hotspot with a high proportion of threatened species. It then determined the environmental factors influencing the abundance of G. holbrooki and the prevalence of fin-nipping damage on native fishes. Caudal fin damage differed significantly among four endemic native fishes, with damage being most prevalent in the percichthyid Nannoperca vittata, which has a similar size and ecological niche to G. holbrooki. Fin-nipping damage of native species also showed significant seasonal variation; occurring most commonly during summer and autumn when many rivers in this region cease to flow and contract to refuge pools, probably increasing interactions between G. holbrooki and the native fishes. Moreover, the environmental variables that best explained both the density of G. holbrooki and the prevalence of fin-nipping were broadly similar and were characteristic of more degraded habitats. We anticipate that habitat degradation, river regulation and severe declines in surface flows resulting from climate change will benefit G. holbrooki to the detriment of native fishes. Restoration of riverine environments along with public education campaigns to prevent the further spread of G. holbrooki or the introduction of additional species is crucial to mitigate their effects on aquatic ecosystems.