Inference of foraging success in a top Southern Ocean predator

• Main Author: Arce Gonzalez, F
• Format: Thesis
• Language: English
• Published: 2020
• Subjects: Foraging success; Southern Ocean; Elephant seal; diving behaviour; Antarctica; body condition; Antarctic; Indian; The Antarctic; Antarc*; Elephant Seal; Elephant Seals; Mirounga leonina; Sea ice; Southern Elephant Seals
• Online Access: https://eprints.utas.edu.au/35875/; https://eprints.utas.edu.au/35875/1/Arce_Gonzales_whole_thesis.pdf

Acquiring resources is the foremost activity that any organism must succeed at, given all other aspects of an animal’s life-history (growth, survival and reproduction) depend upon how successful an individual is at foraging. Despite the central importance of foraging, quantifying foraging success remains difficult in the wild and especially so in the marine environment. Here, direct observations or real-time measurements are particularly challenging. It is often not possible to get in-situ information, especially from highly migratory species - like seals – that spend many months in remote oceanic realms. Most of the knowledge about at-sea feeding behaviour is generated by instrumenting free-living organisms and drawing inference from their tracking and/or diving data. Many different types of behaviours, including those associated with foraging events, can be inferred but the actual foraging success remains elusive. This study is concerned with the quantification of foraging success for southern elephant seals (Mirounga leonina), a large marine predator widely distributed in the Southern Ocean. Elephant seals are an ideal study organism because they spend most of their lives at sea feeding but return to their natal sub Antarctic islands to breed and moult, enabling tagging and in some cases recovery of electronic instruments that record their at-sea behaviours. Importantly, southern elephant seals are one of the few species known to perform a special dive type, called drift dives, which allow changes in body condition to be quantified. Drift dives are characterized by a long, inactive phase; during this phase, the rate of change in depth is determined by the buoyancy of the seal, ultimately linked to the ratio of lean:fat tissues. If these drifting segments are correctly identified, then the in-situ body condition and its changes through time become tractable. However, since in many cases dive information is actually transmitted through satellite systems, this data must be summarized by on-board algorithms before transmission. This loss of resolution hampers identification of drift dives; furthermore, changes in summarization algorithms have limited the development and widespread use of drift rate as a measure of body condition from summarized dive datasets that are now commonly available for this species. In this thesis, I present my original research on: (1) Drift dive identification and drift rate estimation. By developing a flexible, hierarchical approach for detecting drift dives and estimating the body condition of southern elephant seals from summarized dive profiles, I overcome a long-term challenge due to changes in dive summarization algorithms. Validation with independent datasets shows the method retains approximately 3% of all dives, of which 88% are true drift dives, and the drift rate estimates are unbiased. The trend of the drifting time-series match expectations for capital breeders, showing the lowest body condition commencing foraging trips and a progressive improvement as they remain at sea. This method to is now publicly available to the research community in the form of an open source R package, enabling at-sea changes in condition to be robustly estimated for large existing and ongoing data collections with potential for application across other drift diving species. (2) The importance of the polynyas as a foraging habitat for Antarctic predators. Southern elephant seals forage in a diverse range of ocean habitats and recent research has highlighted their usage of Antarctic coastal polynyas (recurrent areas of open water in the sea-ice). By implementing the method developed in part 1 above, I develop body condition indices and provide for the first time quantitative evidence of enhanced foraging success for those southern elephant seals (primarily males) targeting the polynya areas, in comparison to those animals using the rest of the shelf and surrounding Antarctic waters. My results confirm the relative importance of polynyas as specific areas of the Antarctic region that are regularly targeted by seals and other air-breathing marine predators (3) Predicting drift dive changes in relation to diving behaviour. I investigate the correspondence between drift-rate indices and a suite of dive metrics traditionally related to the body condition and buoyancy. I have found similar results to those obtained via high resolution time-depth recorders and accelerometer data, but using an extensive data-base of tracked elephant seals from the Indian sector of the Southern Ocean of compressed dive information. The relationships found between the body condition and different dive metrics expand previous research by improving the ability to infer the body condition from simple dive metrics. The results also demonstrate the apparent benefits of seals being near neutral buoyancy, as predicted by the literature. Considering elephant seals as ‘surfacers’ (i.e., well adapted air-breathing organisms that in fact spend most of their time at sea, deep diving and returning only briefly to the surface to breathe), I argue they should be expected to maximize their diving efficiency, irrespective of whether they are foraging or not. However, elephant seals are found increase their diving efficiency under favourable foraging scenarios.