Background Biologging studies have revealed a wealth of information about the spatio-temporal movements of a wide range of vertebrates large enough to carry electronic tracking tags. Advances in autonomous underwater vehicles (AUVs or UAVs) and unmanned aerial vehicles (commonly known as drones), which can carry far larger payloads of sensor technologies, have revealed insights into the environment through which animals travel. Some AUVs have been used to film target animals, but are generally limited to periods as long as a drone operator can actively follow an animal. In the present study, we use an AUV, the REMUS-100 SharkCam, paired with a custom transponder tag attached to the shark, to autonomously follow three basking sharks for a cumulative total of 10.9 h to collect video and environmental data on their sub-surface behaviour. The basking shark is the second largest fish in the world and is endangered globally, but despite being subject to various biologging studies, little is known of this species breeding ecology and their mating grounds remain unknown. Results We detail the first successful autonomous tracking of basking sharks, comprising three missions that filmed basking sharks in mid-water and close to benthic habitats. Sharks spent very little time feeding, and travelled relatively close to sandy, rocky and algae-covered benthos. One basking shark was observed defecating. Conspecifics were not observed in the three missions, nor were courtship or breeding behaviours. AUV offset distances for videography were determined iteratively through tracking. These offsets varied depending on the trade-off of between water clarity and proximity of the AUV for obtaining useful video data and directly influencing shark behaviour. Conclusions The present study is the first successful use of an AUV to gain insight into the sub-surface behaviour of basking sharks.

Basking sharks, the world’s second largest fish, are endangered globally following two centuries of large-scale exploitation for their oily livers. In the northeast Atlantic, they seasonally gather in key sites, including the western Scottish Isles, where they feed on plankton, but their breeding grounds are currently completely unknown. Using high-resolution three-axis accelerometry and depth logging, we present the first direct records of breaching by basking sharks over 41 days. We show that basking sharks breach both during the night and day, starting at approximately 20 m depth and can breach multiple times in short succession. We also present early evidence of potential lateralisation in basking sharks. Given the energetic nature of breaching, it should have an important biological function, but this remains unclear.

While biologging tags have answered a wealth of ecological questions, the drivers and consequences of movement and activity often remain difficult to ascertain, particularly marine vertebrates which are difficult to observe directly. Basking sharks, the second largest shark species in the world, aggregate in the summer in key foraging sites but despite advances in biologging technologies, little is known about their breeding ecology and sub-surface behaviour. Advances in camera technologies holds potential for filling in these knowledge gaps by providing environmental context and validating behaviours recorded with conventional telemetry. Six basking sharks were tagged at their feeding site in the Sea of Hebrides, Scotland, with towed cameras combined with time-depth recorders and satellite telemetry. Cameras recorded a cumulative 123 hours of video data over an average 64-hour deployment and confirmed the position of the sharks within the water column. Feeding events only occurred within a metre depth and made up ¾ of the time spent swimming near the surface. Sharks maintained similar tail beat frequencies regardless of whether feeding, swimming near the surface or the seabed, where they spent surprisingly up to 88% of daylight hours. This study reported the first complete breaching event and the first sub-surface putative courtship display, with nose-to-tail chasing, parallel swimming as well as the first observation of grouping behaviour near the seabed. Social groups of sharks are thought to be very short term and sporadic, and may play a role in finding breeding partners, particularly in solitary sharks which may use aggregations as an opportunity to breed. In situ observation of basking sharks at their seasonal aggregation site through animal borne cameras revealed unprecedented insight into the social and environmental context of basking shark behaviour which were previously limited to surface observations.