The effect of nature-based tourism on wildlife has been the focus of much attention. Studies have demonstrated how boat-based cetacean-watching tourism can cause both short-term and long-term effects on targeted populations. However, limited attention has been given to the effect of swim-with activities on humpback whales (Megaptera novaeangliae). This study qualified whale responses to swim-with activities off Reunion Island during the 2018 humpback whales breeding season. We used both under- and above-water videos collected from social media outlets, commercial whale-watching operators, and audiovisual professionals. We documented a high rate of agonistic whale behaviors (during 42.1% of all observations;n= 164) towards swimmers within videos containing swim-with events. We documented seven agonistic behaviors including threat, attack, or defense behaviors that were predominantly exhibited by mother/calf groups (73.8%;n= 121) and by singletons (16.5%;n= 27). Pectoral shears (27.4%) and fluke thrashes (23.2%) were the most exhibited agonistic whale behaviors aimed towards swimmers, both of which pose a danger and serious injury to swimmers. During swim-with attempts whales changed their behavioral state (82.3%,n= 159) and used avoidance tactics to avoid swimmers (56.1%,n= 92). Whales exhibited a higher rate of agonistic behaviors when swim groups were active and dispersed, in contrast to when they were quiet and compact. To mitigate whale disturbance and improve swimmer safety, we recommend avoiding swimming with whale groups containing calfs. Our findings support the implementation of strong regulations and educational tools to ensure a sustainable practice of whale watching off the Reunion Island.

Despite increased attention over the last decade on Mesophotic Coral Ecosystems (MCEs)