Marine or freshwater food? The dilemma of Eurasian otter (Lutra lutra) along Bulgarian Black Sea coast

BACKGROUND: The Eurasian otter (Lutra lutra Linnaeus, 1758) is an endangered species in Bulgaria and is protected under national legislation. Under harsh environmental conditions, as well as the increasing anthropogenic pressure, knowledge about this species is fundamental for conservation planning. The otter occupies a broad range of freshwater habitats, and the species has also been recorded along the Bulgarian Black Sea coast. The contribution of both components of this complex habitat-freshwater and marine environments-to the otter's diet has been demonstrated. RESULTS: The study confirmed that the otter is primarily piscivorous, consuming predominantly freshwater fish during the cold season, and - marine fish during the warm season. Non-marine arthropods were found as an essential secondary food for the otter with a relative frequency of occurrence (RFO) of 22.2%. Each of the other food categories (birds, anurans, marine-arthropods, rodents and reptiles) accounted for less than 10% RFO in the diet. Seasonality in consumption of reptiles, birds, fish, non-marine arthropods was established. CONCLUSIONS: Eurasian coastal otters used both marine and inland habitats for foraging along the Bulgarian Black Sea coast and were predominantly piscivorous overall. Among identifiable fish remains, freshwater fish occurred more frequently during the cold season, whereas marine fish were more frequent during the warm season. However, a substantial proportion of fish remains could not be assigned to either marine or freshwater origin.