Comparative diving ecology across Southern Ocean marine predators

• Main Author: Roncon, G
• Format: Thesis
• Language: English
• Published: 2020
• Subjects: physiology; diving; seals; penguins; southern ocean; foraging; Antarctic; Indian; Southern Ocean; Weddell; Antarc*; Aptenodytes forsteri; Arctocephalus gazella; Mirounga leonina; Pygoscelis adeliae; Sea ice
• Online Access: https://eprints.utas.edu.au/35861/; https://eprints.utas.edu.au/35861/1/Roncon_whole_thesis.pdf

The Southern Ocean is home to a great diversity of marine predators (cetaceans, pinnipeds, flying and diving seabirds), many of high conservation value, and all well adapted to exploit their underwater habitats including coastal shelf, sea ice and oceanic zones. Marine divers are particularly interesting for studying the underlying principles related to foraging behavior and diving physiology. Their need to acquire enough food resources (determined by prey distribution, abundance, quality) is balanced against their physiological constraints (e.g., oxygen stores, body mass, diving capacity). This interplay between need and constraint is reflected in what is directly observable, that we can measure, i.e., their diving behavior, byusing simple telemetry devices like time-depth recorders. This thesis examines the diving behavior of Southern Ocean marine predators, with a focus in the Indian sector. To do this I use dive datasets available for key Antarctic seal (Antarctic fur Arctocephalus gazella; southern elephant Mirounga leonina; Weddell Leptonychotes weddellii) and penguin (Adélie Pygoscelis adeliae; emperor Aptenodytes forsteri; king A. patagonicus) species. This thesis is organised into three main chapters as follows: (1) a systematic literature review presenting common approaches for addressing physiological and foraging questions. This is followed by two chapters employing a comparative analytical approach to (2) examine the underlying factors, particularly body mass, that influence diving behavior, and (3) evaluate the dive capacity of air-breathing birds and mammals, and in particular their capacity to adapt their dive behavior when actively foraging. (1) A systematic literature review synthesizing approaches for addressing physiological and foraging questions. Increasingly sophisticated electronic logging devices record behavioral, physiological and habitat variables, providing insight into the diving physiology and foraging behavior of marine mammals and seabirds. However, a variety of methods have been developed for dive data making comparative studies and syntheses difficult even amongst closely-related species. Adopting a question-driven orientation, I conducted a systematic literature review using dive telemetry data gathered in the Southern Ocean. I focused on the years 2006–2016, as this was a period of considerable study when both well-established sensors (e.g., time-depth recorders) and newly developed devices (e.g., accelerometers, animal-borne cameras) were employed. I identified key research questions emergent across Southern Ocean species, and explored two major sections focussing on the foraging and physiological inferences obtainable using diving data. Finally, I discuss key emergent areas in which dive telemetry data are being upscaled and more quantitatively integrated with movement and demographic information to link to population level consequences. This work is important because it highlights the benefits of a standardized approach and paves the way for more integrative multi-species meta-analyses. (2) Investigating diving patterns and body mass scaling within and across six marine predators in the Indian sector of the Southern Ocean. Despite our greatly increased ability to study how marine predators regulate their dive cycle, proximate (e.g. limited oxygen stores, metabolic rate) and ultimate (dive capacity) influences controlling the diving behavior of individuals are still poorly understood. In my comparative analysis of diving data of three penguin and three seal species in the southern Indian Ocean, I examined the influence of body size on dive performance and the interdependencies of dive parameters. Across species, my results support the well-established expectations that dive duration and dive depth are tightly linked, and that mass is an important determinant of dive capacity. However, the body size effect within a species was not the same as the between103 species relationship, and more importantly the relationship varied amongst thespecies. Furthermore, unlike dive depth and duration, post-dive surface intervalswere not influenced by body size within a species. This suggests that at the species level dive depth and dive duration are not simply driven by physiological allometry, but probably also by other ecological factors. Ultimately, my examination of the interdependencies of diving parameters showed support for both between- and within-species effects. These results were more consistent than for the size-based analyses described above, suggesting universal principles to potentially at play. (3) Behavioral plasticity and observed limits of underwater dive behavior of marine predators during intense foraging. In this chapter, I extend my characterization of the diving capacity of the six Southern Ocean marine predators. I performed a comparative analysis of relationships between basic dive parameters. Using quantile regressions, I described diving limits of marine vertebrates in terms of these components. I then used a hunting time metric to identify dives as hunting (foraging) or other dives, and observed how diving performance may vary during different levels of activities (foraging vs non-foraging dives; short vs long hunting time dives). My results showed that most marine mammal and seabird species were able to adjust their dive cycle when foraging, generally diving deeper and for longer. Deeper dives corresponded with longer bottom time, but different species displayed different strategies to reduce their transit time. Moreover, most of the species were able to lengthen the duration of their shallower forage dives, but potentially showed less capacity to do so for the deeper, intensive hunting dives. My results quantifying dive limits of Antarctic marine predators showed that their dive plasticity is associated with their taxonomic position, the environmental conditions, and a species’ life history traits. General discussion. This study has provided important new insights into the diving ecology of Southern Ocean marine predators. Assembling high-resolution diving data across various species of marine mammals and seabirds, I developed and applied systematic approaches for dive-based indicators to make inferences about diving behavior, foraging and physiology. These multi-species comparative analysis of dive patterns and performances of Antarctic animals help identify which intrinsic and extinct factors may constrain animals’ diving ability. Understanding what determines an animal’s dive ability is essential to elucidating its feeding ecology; foraging is a fundamental requirement of all animals and has implications for the distribution, growth and persistence of wild populations. This study has shown how within their morphological and physiological specializations, some species may have considerable plasticity in response to changes in their energetic needs, while others seem to operate at their maximum diving capacity and are thus less likely to have the capacity to increase their foraging effort. The variations in capacity and ability could be used as input into ecological models, and for answering broader ecological questions regarding ecosystem energy flow. Marine predators have been recognized and monitored as indicators of ecosystem changes in the Southern Ocean for many years. The Southern Ocean is one of the most seasonally dynamic oceans on our planet, and Antarctic marine life has already showed a radical response to a range of climate stressors. Developing an integrated and synthetic view of marine mammals’ and seabirds’ diving ecology is an important first step to enable the development of predictive models that will improve our understanding of how future climate change will affect this unique biota.