Acoustic deep scattering layers as dynamic prey landscapes for air-breathing deep-diving Antarctic predators

• Main Author: Le Guen, Camille Melanie Marie-Anne
• Other Authors: Brierley, Andrew, Boehme, Lars, Antarctic Circumnavigation Expedition Foundation, Natural Environment Research Council (NERC), Trans-Antarctic Association
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of St Andrews 2021
• Subjects: Antarctic; Diving predators; Acoustics; Foraging ecology; Penguins; Seals; Deep scattering layers; Myctophids; QH91.8E3L4; Echo scattering layers > Antarctic Ocean; Lanternfishes > Antarctic Ocean; King penguin > Food > Case studies; Elephant seals > Food > Case studies; Antarctic Ocean; Southern Ocean; The Antarctic; Antarc*; Elephant Seals; King Penguins; Mirounga leonina; Southern Elephant Seals
• Online Access: https://hdl.handle.net/10023/21604; https://doi.org/10.17630/sta/41

This PhD addresses the central hypothesis that acoustic Deep Scattering Layers (DSLs) are a prey landscape for deep-diving air-breathing Southern Ocean predators. In the open ocean, mesopelagic fish (including myctophids), zooplankton and other animals migrate down from the surface at dawn to the mesopelagic zone (200-1000 m) to avoid visual predators during daylight. There, they form layer-like aggregations known as Deep Scattering Layers that can be detected using echosounders. A large component of DSL biomass is comprised of myctophids, which are both a potential resource for fisheries and important in the diets of several iconic Antarctic predators such as King Penguins (Aptenodytes patagonicus) and Southern Elephant Seals (Mirounga leonina). Although these two predator species are amenable to bio-logging, there are very few simultaneous observations of DSLs and their foraging behaviour. Therefore, the importance of DSLs to Antarctic air-breathing diving predators is unknown. This is problematic given the predicted changes in DSLs in response to climate change and to the increasing interest shown in DSL harvest by commercial fishers. The 2017 Antarctic Circumnavigation Expedition (ACE), which is the first scientific expedition around the Antarctic continent stopping at most subantarctic islands to investigate a range of aspects of the Southern Ocean, provided a unique opportunity to simultaneously observe DSL characteristics acoustically from the ACE ship (at 12.5 kHz) and the foraging behaviour of predators using bio-logging. King Penguins and female Southern Elephant Seals appeared as good candidates to study the link with DSLs as they both mainly feed on myctophids, are both deep-diving predators potentially capable of reaching the depth of DSLs and are both known to dive deeper during the day compared to night time (like the Diel Vertical Migration (DVM) pattern of the components of DSLs), several clues that initially suggest that DSLs could be a prey landscape for them. I compiled a dataset of DSL depth ...