The spread of exotic fish species in Italian rivers and their effect on native fish fauna since 1990

The introduction of exotic fish species into Italian river systems presents an additional threat to freshwater native fish species, a wildlife group already under conservation pressure from myriad other anthropogenic factors. Few large-scale and long-term monitoring programmes exist that adequately describe the impact of exotic fish species on native fish. To fill this gap, we analysed the bulk of available data (1985–2014) on fish collected at a national level within the BioFresh Project with the aim of investigating the effects of human-introduced exotic fish species on native species. The results showed that both the Continental and Mediterranean biogeographic regions are home to fish communities which, in conservation terms, are a cause for concern, while Alpine Regions are less so. Generally, the richness of exotic species is still growing in Italy, with about 50% of them having a positive trend while native species’ trends are in the opposite direction–that is, the spread of many exotic species is positively correlated with the decline in many co-occurring native species. Key policy insights Worrying increase in the richness of exotic species between the 1985 and the 2014. A large segment of the current exotic fish contingent is self-sustaining. Common bream, Black bullhead, Goldfish, Largemouth bass, Rainbow trout, Roach, Pike perch and European catfish negatively affect the presence probability of at least 50% of co-occurring native species over time. The restoration of habitat characteristics and native fish assemblages as well as more rigorous controls during population reinforcement for recreational fishing are needed to prevent or counteract the further spread of exotic fishes.