Do non-native fish as prey favour the conservation of the threatened indigenous Eurasian otter?

1. Biological invasions are considered a major threat to biodiversity. Most research has focused on the distribution, biology and impacts of non-native species on native fauna and flora. However, few studies have explored their role as prey for native predators of conservation concern. 2. To assess the incidence and intensity of predation by the Eurasian otter Lutra lutra on established non-native fish species, data were collated from the published literature. To be selected, studies had to cover at least 1 year, analyse more than 100 spraints and report the study period and percentage relative frequency (%RF) of all prey fish species. 3. To permit reliable, time-related comparisons with %RF of non-native fishes in otter diet, we also reviewed available information about both the distribution of non-native fishes and history of their introductions to European countries, revealing a decrease with longitude in the number of naturalised non-native fishes taken (ranging between 5 and 34) and their percentage in each fish assemblage. 4. Our selective criteria were met by 30 dietary studies from 44 study areas in 15 European countries during 1970–2010. The extent to which otters rely on non-native fishes was almost negligible (mean %RF = 4.8), with the number of non-native fishes preyed upon by otters decreasing with both latitude and longitude. 5. The %RF of non-native fish in the diet increased slightly with time, with otters preying significantly more on non-native fish in study areas where alterations of the fish assemblage had been highlighted in the reference papers. No relationship was found between otter diet breadth and the occurrence of non-native fishes in their diet. 6. The current role of non-native species in otter diet suggests that effective otter conservation management plans should focus on the maintenance and⁄or enhancement of native fish assemblages.