Co-occurring fisheries and tourism synergistically drive behavioural shifts in endangered dolphins

Intensifying human activities are reshaping marine ecosystems worldwide, and yet their impact on wildlife behaviour and survival remain poorly understood. While fisheries-related mortality is the primary threat to cetaceans globally, many dolphin species still forage in the proximity of fishing vessels. In addition, coastal populations also face increasing interactions with tourism-related activities. Few studies have simultaneously examined the complex impact of these factors on dolphin behaviour. We therefore conducted drone-based focal follows of 90 groups of the Endangered Indian Ocean humpback dolphin Sousa plumbea in Goa, India, to quantify how tourism and fisheries jointly impact dolphin behavioural states, as well as transitions between states. We used integrated machine learning to extract information on dolphin tracks and behaviour, and covariates such as group size, number of boats and distances to boats/fishing nets. Using multinomial regression models, we found that both human activities triggered distinct behavioural transitions absent in undisturbed groups. Dolphins expressed avoidance behaviours exclusively near tourist boats and were more likely to transition into foraging near fishing nets (mostly purse-seines). Smaller groups reacted more strongly to tourist boats and interacted more frequently with fishing nets. Interactions with fishing nets decreased when tourist boats approached dolphin groups, but simultaneously, dolphins were also less likely to avoid tourist boats when interacting with fishing nets. Our findings offer new insights into the behavioural mechanisms underlying the co-occurrence of coastal dolphins with humans. Our work also demonstrates the application of a novel method for studying fine-scale behaviours in free-ranging animals.