Alien prey in the diet of the indigenous Eurasian otter in Vistula River, Poland

Several non-native fish species were recorded in the studies of Eurasian otter Lutra lutra diet in south European and English rivers.To document the role of alien species as a prey of this aquatic top predator in central Europe, the diet of the otter was studied by spraint analysis in the Vistula, a large European river.Fresh spraints were collected at the study site in central Poland from 2012 to 2016, and in south Poland in 2015.At both sites, fish were the staple food of otters, constituting over 90% of prey consumed.The food niche breadth of otters was relatively high.Among 20 fish species registered in the diet, there were four alien species: prussian carp Carassius gibelio, common carp Cyprinus carpio, topmouth gudgeon Pseudorasbora parva, and round goby Neogobius melanostomus.Besides alien fish species, the invasive spiny cheek crayfish Orconectes limosus was present in the diet of otters at both localities.Altogether, the five invasive species constituted a large proportion of the diet of otters at both sites (over 30%).According to the literature data and results of this study, at least 18 non-native fish species were preyed on by otters in Europe.It is suggested that alien prey will constitute an increasing fraction of the diet of the indigenous Eurasian otter.