Drone-based assessment of humpback and fin whale vessel-strike vulnerability

Collisions with vessels (vessel-strikes) are one of the largest threats to the recoveries of baleen whale populations worldwide. Vessel-strike vulnerability is complex, depending on a vessels size and speed, and the presence, position, visibility, and movement of whales. To date, discrete behaviour states have not been considered in strike risk analyses. Here we used unoccupied aerial systems (UAS; drones) to conduct focal follows of humpback whales ( Megaptera novaeangliae ) and fin whales ( Balaenoptera physalus ) during the summer in a North-East Pacific foraging ground to assess daytime near-surface depth distributions, fine-scale behaviour parameters, and visibility of each species. We generated lethal-zone occupancy estimates for both species with respect to 11 vessel classes ranging from small pleasure crafts to large fuel carriers. We estimated that fin and humpback whales spend on average 70.2 and 73.5% of their time at or above a depth of 17 m, respectively, and within the lethal zone of large fuel carriers. We showed that lethal-zone occupancy, visibility, swim speed, and travel linearity vary by species and behaviour state, and that resting and surface traveling behaviours increase vessel-strike vulnerability. Our results indicate that behaviour state is a critical facet to vessel-strike vulnerability that requires consideration in future strike-risk assessment models.