The decline of common bottlenose dolphin (Tursiops truncatus) presence and group size in Montenegrin waters

Abstract

The Mediterranean subpopulation of common bottlenose dolphins (Tursiops truncatus) is classed as vulnerable with a decreasing population trend. Yet, research effort in the Mediterranean varies with relatively little being known of the Southern Adriatic Sea’s bottlenose dolphins, which face increasing numbers of anthropogenic threats, mainly marine traffic, tourism, fishing and seismic activity. To investigate trends in bottlenose dolphin presence, group size and sub-adult presence in Montenegrin waters, 576 land and boat surveys were conducted from September 2016 to August 2020. Chisquared tests showed a significant decline in bottlenose dolphin presence over the four years. Changes in group size over the four-year period were also assessed using Chi-squared tests, and results showed that in Year 1 (September 2016 to August 2017) significantly more medium (3-4 individuals) and large (over 5 individuals) groups were observed than expected. Whereas, in the following three years more small groups were observed (1-2 individuals). Neither bottlenose dolphin presence nor group size was found to be associated with season. Multinomial logistic regression was used to assess the relationship of sub-adult presence with year, season and group size. Sub-adults were most likely to be present in the summer (June, July and August) and when group size was large. Montenegrin waters are of significant importance to the wider Adriatic. Therefore, urgent action is required to identify the cause of the declining trends and to protect Montenegro’s bottlenose dolphin population from anthropogenic pressures. A dedicated management strategy must be quickly and effectively implemented, while continuous study is required to monitor and assess trends moving forward.