We investigated the short-term effects of non-targeted tourism on the behaviour of bottlenose dolphins Tursiops truncatus in the South Adriatic off the coast of Montenegro, by comparing dolphin group behaviour during impact (the presence of non-targeted tourism vessels) and control (absence of all marine vessels) scenarios. Tourism vessel and dolphin behavioural data were collected through systematic weekly land-based surveys. Using instantaneous focal ‘group’ scan sampling, the predominant behaviour of bottlenose dolphin groups was determined. To quantify the effect of vessel interactions on the behavioural budget of the dolphins, we followed a stepwise modelling approach. A first-order Markov chain was used to calculate the transition probabilities between behavioural states before a Monte Carlo simulation estimated the behavioural budgets of dolphins during impact and control situations. In the presence of non-targeted tourism, dolphins were found to be less likely to remain diving (during which dolphins were assumed to be feeding), whilst milling-socialising and surface feeding were completely absent from their behavioural repertoires. Nevertheless, the behavioural budgets demonstrated an increase in resting behaviour in the presence of non-targeted tourism vessels. No significant changes to travelling behaviour were observed. The decrease in foraging behaviour (both surface feeding and diving) could result in a reduction in energy intake for dolphins, which over time (assuming repeated disturbance) could negatively affect body condition, and ultimately survival and reproduction. Regulations on non-targeted tourism should therefore be considered to minimise potential long-term negative effects on dolphins within Montenegrin territorial waters.

Several studies indicate that unregulated nautical tourism can have negative implications on cetacean behaviour. In recent years, dolphin watching activities (DWA) have increased off the West coast of Istria, Croatia, a region in which the NATURA 2000 site: ‘Akvatorij zapadne Istre’ has been proposed to be designated for bottlenose dolphins (Tursiops truncatus M.). For data collected between 2016 and 2019, we compared dolphin group behaviours from this region during impact (presence of nautical tourism boats (NTBs)) and control (absence of NTBs) scenarios, as well as providing descriptive analysis on the displacement of individuals in the presence of NTBs. Throughout the study years, 48.5 % of NTBs were observed within 15m of the dolphin focal groups and 97 % were observed within 50 m distance. The greatest rates of displacement in dolphin focal groups occurred when NTB numbers were greatest per individual dolphin. Markov chain analysis were used to quantify the short-term effects of NTB presence on dolphin behaviour. In the presence of NTBs, dolphins were more likely to spend time milling and less time foraging. Cumulative behavioural budgets, derived by accounting for the time bottlenose dolphins spent in the presence or absence of NTBs, indicated that vessel exposure levels of 14 % and 25 % were enough to statistically affect milling and foraging behaviours respectively. To lessen the lack of sustainable DWA, the implementation of relevant guidelines, e.g. Global Best Practice Guidance for Responsible Whale and Dolphin Watching (50 m no approach and 300 m caution zone) is therefore crucial to mitigate any long-term consequences the actions of NTBs may have on this key species. To date, 162 bottlenose dolphins have been photo-identified off West coast of Istria and cumulative interference to this population could affect direct ecosystem functioning.

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