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.

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