Impacts of SCUBA divers on thresher sharks’ (Alopias pelagicus) behaviour were investigated at a coastal seamount in the Philippines. Sharks visit daily the seamount to benefit from the services of cleaner wrasse, which attracts intense SCUBA diving tourism. In situ observation and remote video cameras were used to examine sharks’ presence and behaviour in the presence and absence of divers respectively. 53 dives were achieved in presence of divers between January and April 2019, during which 226 sharks and 1240 divers were counted. 54 replicates of 10 minute observations were gathered in absence of divers, during which 96 sharks were counted. The analysis revealed that divers and their number have an impact on sharks’ behaviour, which manifests in shorter time spent at the cleaning stations, in fewer cleaning behaviours displayed by the sharks, and in fewer occurrences of interactions with cleaner wrasse. Predictions allowed to estimate that up to 14 divers present at a cleaning station, the probability of observing a shark displaying cleaning behaviour is greater than 50%. This study provides evidence that divers have an effect on interactions between sharks and cleaner wrasse. Thus, regulation of divers must be considered to complement the existing site management measures.
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