The role of prey abundance and flow regulation in the marking behaviour of Eurasian otters in a Mediterranean catchment

Abstract

Scent marking is a common behaviour used to delineate territories by carnivores, yet it remains insufficiently understood as there are several alternative hypotheses, one being that sites are marked where resources are abundant, thus guaranteeing their exclusive use. Small Mediterranean rivers are highly variable environments throughout the year, representing an ideal scenario to test this theory, as prey availability changes as a function of the spatial and temporal variability of the flow regime. We studied seasonal variation in the habitat selection of latrine (i.e. scent-marked) sites, prey availability and prey selection of the Eurasian otter, Lutra lutra, between two contrasting fluvial systems, the regulated mainstem of River Bullaque and four unregulated tributaries (River Guadiana basin, central Spain). In 2010, we measured habitat variables, collected otter spraints and used electrofishing to compare the diet with prey abundance. Latrines were more frequent at shallow sites, and otters did not discriminate between fluvial systems across seasons. Also, latrines were associated with sites where the main prey, the calandino, Squalius alburnoides, was more abundant in the tributaries during spring and summer. However, this abundance was not translated into prey selection, with calandino being preyed below its availability in the mainstem. This observation suggests that otters mark places where fishing is especially easy (i.e. shallow sites with proportionally more fish). Calandino was substituted by red swamp crayfish, Procambarus clarkii, which was easier to catch but of lower energetic value, where higher water levels made fishing more difficult (i.e. the regulated mainstem).