Subsurface observations of white shark predatory behaviour using an autonomous underwater vehicle

Abstract

Investigations of animal habitat use and behaviour are important for understanding the ecology of animals and are vital for making informed conservation decisions. Most of what is known about shark behaviour comes from direct observations at shallow depths, captive studies, baited and chance encounters, and inferences from tracking and tagging data. Over the course of the last two decades, new technologies have been developed to track the movements of marine animals over multiple spatial and temporal scales, but they do little to reveal what these animals are actually doing. It is well established that the white shark, Carcharodon carcharias, is a top predator of marine mammals and fishes, but virtually all published observations of white shark predatory behaviour are based on surface interactions with pinnipeds at well-studied white shark aggregation areas. Guadalupe Island off the coast of Mexico is a seasonal aggregation site for white sharks, which are presumably drawn to the island to feed upon pinnipeds, yet predation has rarely been observed. In this study, an Autonomous Underwater Vehicle (AUV) was used to test this technology as a viable tool for directly observing the behaviour of marine animals and to investigate the behaviour, habitat use, and feeding ecology of white sharks off Guadalupe Island. During the period 31 October – 7 November 2013, six AUV missions were conducted to track one male and three female white sharks, ranging in estimated total length (TL) from 3.9-5.7 m, off the northeast coast of Guadalupe Island. In doing so, the AUV generated over 13 hours of behavioral data for white sharks at depths up to 90 m. The white sharks remained in the area for the duration of each mission and moved through broad depth and temperature ranges from the surface to 163.8 m (mean ± SD = 112.5 ± 40.3 m) and 7.9-27.1 °C (mean ± SD = 12.7 ± 2.9 °C), respectively. Video footage and AUV sensor data revealed that two of the white sharks being tracked and eight other white sharks in the area approached (n=17), bumped (n=4), and bit (n=9) the AUV during these tracks. In this study, it was demonstrated that an AUV can be used to effectively track and observe the behaviour of a large pelagic animal, the white shark. In doing so, the first observations of subsurface predatory behaviour were generated for this species. At its current state of development, this technology clearly offers a new and innovative tool for tracking the fine-scale behaviour of marine animals.