IAP-24-006

The tactics of rheotaxis: movements of higher vertebrate predators in relation to Southern Ocean currents

Movement is an important aspect of the lives of migratory animals and those that roam from a fixed central place, such as a colony or den/nest site. It underpins individual and population distributions and influences access to spatially structured resources and their exposure to risks. These may in turn influence their body condition, survival and reproductive success. Insights into the processes influencing movement may therefore lead to a mechanistic understanding of factors that influence how animals travel across their environment and the consequences for their distributions, demography and conservation status.

Animal movement is heavily affected by the permeability of the environment as well as the distribution of the resources it offers. For animals that inhabit fluid mediums, flows can either assist or impede movement, depending on the individual’s relative orientation; a process known as rheotaxis. Negative rheotaxis or “going with the flow” involves drifting or actively swimming downstream, which requires little effort but may result in advection to unsuitable localities. However, animals can wait until their movements coincide with flows that take them in the intended direction, although this incurs time penalties. Travelling across strong flows may deflect the animal from its intended goal, unless energy is exerted, and travel distance sacrificed, in compensating for drift. Positive rheotaxis or “swimming against the tide” is energy demanding but can maintain the animal’s position in favourable habitat and will constantly replenish a drifting food supply, which may make such investment worthwhile. Hybrid patterns of rheotaxis may also arise, where animals swim into the flow to feed, but at a rate slower than the current, so they are advected backwards with the flow. Animals can apply these different strategies tactically during foraging trips or migrations to trade-off energy intake and expenditure, based on the environments they encounter.

Particularly interesting trade-offs may arise where animals must return to a central place, such as a colony, at phenologically fixed intervals to complete their annual cycles. Some animals, such as Atlantic salmon or grey-headed albatrosses can use circular flows of the North Atlantic Gyre or the Southern Ocean winds to complete such journeys using negative rheotaxis. However, in many locations, flows that can be circumnavigated within the time-period dictated by the animals’ phenology may be unavailable. Here, animals are obliged to adopt positive and negative rheotaxis on either the outbound or return legs of their trips, which raises interesting questions over how they tactically adapt their paths and behaviour to optimise travel costs and access to resources.

This project will investigate the movement behaviour for a range of higher predators (penguins and marine mammals) in relation to the Antarctic Circumpolar Current, which has the strongest ocean flows on the planet. It will seek to identify fundamental patterns that affect the travel of these animals relative to ocean currents using telemetry data and how their trajectories are shaped by traits such as central place constraints, body size, life stage and guild. The project will further explore the implications of their findings for conservation of marine predators and their habitats.

The student will conduct a systematic literature review of approaches used to quantify movements of large nektonic vertebrates (i.e. fish, penguins, sea turtles and marine mammals) relative to currents. They will then model the movements of satellite-tagged king penguins (juvenile and adults, originating from the Falklands, South Georgia and Crozet) in relation to currents estimated from remotely sensed oceanographic data. They will build models to understand how currents affect speed and energetic costs of movement and how king penguins utilize currents to access foraging areas. The student will go on to study a range of predator species within the Scotia Sea using open access datasets (from the Retrospective Analysis of Antarctic Tracking Data (RAATD) project), including various penguin species, seals and cetaceans. They will compare and contrast the patterns of movement relative to currents in the context of animal size/power, central place constraints or other aspects of their ecology. The study will conclude by considering the significance of the findings for conservation, such as how movement of animals relative to currents affects recognition or prediction of important areas, exposure to spatially and temporally variable threats and the design of dynamic ocean management. This methodology provides a framework for students to work within but there is scope to develop their own ideas, interests and approaches, with guidance from the supervision team.

Year 1

Inductions to host institutions. Systematic Review and meta-analysis. Test and develop modelling methods.

Year 2

Further methodological development and application to king penguin data set; ecological interpretation and write up.

Year 3

Extend methods and ecological insights/hypotheses to a range of Southern Ocean taxa; explore fundamental patterns and exceptions. Write up. Attend one international conference to present results.

Year 3.5

Complete synthesis and conclusions. Develop papers for submission. Domestic conference and lecture tour of East Anglian and Scottish Universities.

The student will gain skills in the analysis, modelling and visualization of data using a range of cutting-edge methods, implemented in the R software environment. They will also develop skills in ecological interpretation, delivering presentations and scientific writing. These skills will be gained from training by their supervision team, CASE partner and collaborators. Formal training courses will be arranged as appropriate. There may be potential for fieldwork in the Falklands or Antarctica, conditional on additional logistic support and funding, which will provide training in animal handling and the management and deployment of telemetry devices.