Fine-scale behavior of a benthic predator: first insights into the ecology of an angel shark (Squatinidae) revealed by tri-axial biologging

Abstract

Background: Angel sharks (Squatinaspp.) are among the most threatened group of sharks worldwide, with half of all recognized species classified as Endangered or Critically Endangered by the IUCN. Despite a coastal distribution, their cryptic nature has limited our understanding of their ecology and fine-scale behavior. Here, we report on the first deployment of a high-resolution acceleration data logger on an Atlantic angel shark (Squatina dumeril). Results: The biologger was deployed on an Atlantic angel shark in Delaware Bay, USA, for a period of 88 hours in August 2023. The shark, which was only swimming during 6% of the tag deployment, alternated between long stationary periods (13.4 26.5 minutes) and shorter periods of swimming (51 44 seconds).Although the shark exhibited movement throughout the diel period, it primarily moved at night but not during crepuscular periods. Heading values derived from magnetometer data combined with currents estimated by the Doppio ocean circulation model indicated that the shark predominantly faced into the tidal currents when lying on the substrate, switching between approximate southeastward and northwestward body orientations with the tide, which suggests that the Atlantic angel shark may display positive rheotaxis relative to tidal flow when stationary. Such a countercurrent pattern was also observed when swimming, but to a lower extent, suggesting other competitive drivers of body orientation when swimming. Conclusions: Though based on a single individual, this study offers new insights into the ecology and behavior of squatinids, specifically regarding their patterns of activity. We found this angel shark individual to display a highly sedentary benthic lifestyle with diel and tidal variations of behavior, as well as a body orientation consistent with positive rheotaxis in response to tidal flow when lying on the substrate. While more research is needed to better understand how ourresults may extend to whole species and to squatinids in general, this study uncovers the potential of using the Atlantic angel shark as a surrogate species to overcome some of the challenges related to conducting research on endangered and elusive angel sharks.