Fish size selection and diet composition of Eurasian otters (Lutra lutra) in salmonid streams: Picky gourmets rather than opportunists?

Knowledge on predator diet and drivers of prey selection is particularly of interest for an efficient management of predator and prey populations where predators potentially compete with humans for resources. Actual or perceived predation by Eurasian otter (Lutra lutra) on fish stocks generates conflicts in many countries. Recently, conflicts are heating up in riverine habitats, where multiple stressors affect stream fish populations. We combined dietary analysis of otter faeces and prey fish availability in three Austrian streams to assess spatial and seasonal differences in diet composition, the extent of (salmonid) fish consumption and the selection for specific salmonid fish sizes relative to their availability. Otters in upper reaches of temperate salmonid streams occupied a narrow trophic niche. Overall, otters fed predominantly on fish with salmonids dominating diet, both in terms of frequency and ingested biomass measures. Within the category of salmonids, otters selected for specific size classes. Concurrently, otters also displayed an opportunistic feeding behaviour, and seasonally and locally non-fish prey and other fish species than salmonids became key resources. Diet composition and salmonid size selection varied significantly within and between streams, which we relate to spatio-temporal variations of prey community composition and stream-specific habitat characteristics affecting prey vulnerability.