Analysis of the Approaches, Timing, Methods and Methodologies for the Study of the Eurasian Beaver (Castor fiber L.) and Its Habitats in Bulgaria

After we accidentally detected the Eurasian beaver (Castor fiber) in Bulgaria in 2021, by now, as a result of popularization, aroused public interest and targeted studies, we found beavers in more than 50 localities with more than 250 families throughout the entire 470-kilometer border coastal area of the Danube and its river system. The studies of the newly found mammal necessitated the development of a system for informing the locals – how to identify and where to report its presence. With minimal finances, we organized a team to record and account family groups. Depending on the size and type of the habitat, the survey timeframes and methodologies vary greatly, and their calibration requires field experimentation. Our experiment proved the morpho-ecological and ecological-statistical methods as successful for monitoring, and pointed out the statistical and the family “power” methods as good express methods. The techniques we applied have shown to be successful, inexpensive and highly effective. We surveyed the Danube and its islands in August–September 2022 and September 2023 and 2024, and medium and small rivers – annually after 2021 in each season. We installed cameras near two family groups and collected the data each week, in order to identify specific features in beavers’ biology and ecology and to verify our field surveys data. Our attempts to use a drone proved infeasible and futile given the peculiarities of the beavers’ riverine ecological form. The only appropriate way to establish the number of individuals, family groups, and occupied territories is by traversal of the entire riverside sector, or by applying sampling transects and subsequent biological extrapolation.