The common bottlenose dolphin (Tursiops truncatus) is the most regularly sighted cetacean in the Adriatic. However, anthropogenic and environmental pressures are accelerating the decline of this species. Prior to this study, there was a lack of long-term monitoring in Montenegrin waters; much like other Southern Adriatic countries which are considered data deficient. To identify population trends, data was collected over a period of four years from September 2016 to August 2020, through both land and boat surveys. The current study reveals no significant effect of season on the sighting rate or group size of the bottlenose dolphins. Yet, group size shows significant variation under the effect of year (p=0.0034), as fewer large groups were observed than expected from 2017 to 2020. Current results highlight a significant 15% decline in the presence of bottlenose dolphins between 2016 and 2020. A multinomial logistical regression was used to test the effect of year, season, and group size on sub-adult presence. Initial tests showed that the decline in sub-adult presence was evident but not significant, so year was removed from the final model. However, season and group size affected sub-adult presence. Sub-adults were most likely to be present in the summer, and in larger groups (18 times more so than small groups). This result, in conjunction with the apparent decline in observed group size is concerning, as the presence of sub-adults in larger groups is thought to be associated with alloparental care. Further monitoring of bottlenose dolphins in Montenegro will allow trends to be recognised and suitable protective measures to be implemented. Well established conservation actions will encourage population recovery.