UC Santa Barbara

Ontogenetic habitat shifts are a common feature of many marine species, including sharks, which face conservation threats when their distributions overlap with human resource extraction and habitat modification. White sharks (Carcharodon carcharias), for example, exhibit a distinctly coastal phase as juveniles, with a limited distribution compared to the basin-scale range of adult white sharks. Unoccupied aerial vehicles (UAVs) are a promising tool for studying coastal shark aggregations, especially in instances where sharks’ proximity to human activity has both conservation and public safety implications. In this study, we conducted UAV survey flights over a white shark (Carcharodon carcharias) aggregation site in Southern California from 2019 to 2021. We determined the density of observed individuals, their location within the aggregation site, and their body length using photogrammetry. We also determined how temporal and oceanographic factors affect white sharks at different developmental stages. We observed a year-over-year increase in the average density of white sharks. We also noted that the temporal pattern of density within each year was not consistent across years. White shark detections were much higher in the immediate inshore area of the surveyed aggregation site. The proportion of individuals ≥ 3m total length (TL) increased in 2021 relative to previous years, comprising 40% of observations - a prevalence of larger sharks higher than was previously observed at these sites. White shark density was highly variable within and across years, and more sharks were observed at later hours of the day. Sharks < 3 m TL were observed more often during periods of colder water temperature, while the density of larger sharks did not appear to be affected by water temperature. Low visibility and higher tides reduced shark density, while chlorophyll-A levels, sea state, and swell height were not associated with density. Our study highlights the dynamically variable use of near-shore habitat by this top predator and suggests that juvenile white sharks may be more responsive to changes in oceanographic conditions, namely water temperature, than larger individuals. Understanding how temporal patterns and oceanographic predictors of density change over time, and as white sharks age, can help us better predict how this species uses coastal habitats and when they may be more likely to share space with humans and be more exposed to anthropogenic threats. We demonstrated that using UAVs to monitor white shark aggregations can produce useful insights which have both pure and applied value when attempting to better understand how these and other sharks utilize these especially dynamic coastal habitats.