Spatial Distribution of Emys orbicularis (L., 1758) and Mauremys rivulata (Valenciennes, 1833) in the Lower Veleka River, Bulgaria: First Observations

The Veleka River in SE Bulgaria is amongst the northernmost localities of coexistence of Emys orbicularis and Mauremys rivulata, the two native freshwater turtles. This linear structure provides a rare opportunity to study whether the species separate across a seemingly similar habitat. In 2010–2014, we sampled by boat 5.5 km of the river near its mouth. We collected 64 localities with observations of single or multiple individuals of E. orbicularis and 86 of M. rivulata. Although both species co-inhabited large portions of the study site, they displayed a tendency to occur more frequently in different parts. Emys orbicularis was predominantly found in the lower to middle portions of the study site (interquartile range, IQR of the distance to the bridge: 467–1,867 m), while M. rivulata was mostly observed in the middle to upper sections (IQR: 1,287–2,820 m). The medians of the calculated distances in the two species are significantly different (Mann–Whitney U test: U = 1,664.00, Z = -4.13, p < 0.001; E. orbicularis: median = 1,234.94 m, range = 3,335.98 m; M. rivulata: median = 1,911.09 m, range = 4,122.04 m). These preliminary results need to be supplemented with further studies to aid in our understanding of the species’ biology and their successful conservation.