The dietary response of otters (Lutra lutra) to introduced ruffe (Gymnocephalus cernuus) in Loch Lomond, Scotland

In this study the diet of otters Lutra lutra was examined following the establishment of a population of non-native ruffe Gymnocephalus cernuus in Loch Lomond, Scotland. Otter spraints (n=453) were collected at monthly intervals from four sites during 2002. Overall, 98% of spraints contained at least one species of fish, invertebrates were found in 49%, 17% contained amphibians, while mammals and birds were found in 9%. The most frequent prey from all sites were ruffe, which were found in 69% of spraints. Eel Anguilla anguilla, cyprinids (roach Rutilus rutilus, dace Leuciscus leuciscus and minnow Phoxinus phoxinus) and Salmo spp. were also relatively common and were recovered in 60%, 35% and 33% of spraints, respectively. Northern pike Esox lucius (14%), perch Perca fluviatilis (12%), stickleback Gasterosteus aculeatus (9%), powan Coregonus lavaretus (14%) and lampreys Lampetra spp. (2%) were less common. The fact that ruffe were the most frequent prey in otter spraints in all regions and in all seasons of the year indicated that they are one of the most important prey for otters in Loch Lomond at the present time