The fronts of the Prince Edward Islands

The fronts of the Prince Edward Islands

Every summer, approximately 300,000 macaroni and 80,000 rockhopper penguins return to the Prince Edward Islands to breed. During this period, their need to return regularly to the island to provision offspring constrains the distance they can travel to find resources. This makes them dependent on allochthonous prey i.e. originating from somewhere else.

Macro- and mesoscale processes in the vicinity of the Prince Edward Islands are largely driven by fluctuations in the positions of the sub-Antarctic Front (SAF) and Antarctic Polar Front (APF). During years when the fronts are close to the islands, currents are stronger. During years when the fronts are farther away, currents are weaker. The proximity of the SAF and APF to the islands determines the composition of the zooplankton species. For example, when the SAF is closer to the islands, increased intrusions of warmer waters carry sub-tropical and sub-Antarctic species. Conversely, when the SAF is farther north, Antarctic species are more common. These processes influence the type of resources available to penguins during the breeding season.

The animation below represents daily maps of sea surface temperature anomalies. In 2011/12 and 2013/14, conditions were much warmer than in 2012/13. Note the positions of the fronts.

Interestingly, the dive depths of penguins were consistently to 40 to 60 m in 2011/12 and 2013/14, but during 2012/13, when cooler conditions persisted, dive depths were considerably deeper and penguins ate more fish. This highlights the role that the frontal systems play in delivering food to the Prince Edward Islands.

Eddies and filaments

Eddies and filaments

Imagine you are standing on a rock at the edge of the sea, gazing out at the seemingly infinite expanse of ocean that stretches to the horizon. You are hungry, really really hungry. In fact, you haven't eaten for three weeks! Now, jump into that ocean and try find some food. Where would you go? How would you know where you are when you can no longer see land? It's a pretty mindblowing thought to think that there are millions of animals out there that do that all the time. Part of my PhD thesis involves trying to understand how macaroni and rockhopper penguins manage to find food during the resource-limited winter months. During this time they spend six consecutive months at sea, never returning to land.

Thanks to satellites and the work of organisations like Aviso, anybody can find out what the ocean was doing anywhere in the world at any given time. For instance, I wanted to find out whether macaroni and rockhopper penguins were using mesoscale eddies and/or submesoscale filaments. These oceanographic processes are known to aggregate and sustain elevated concentrations of zooplankton and fish, and thus act as important foraging areas for many marine predators. Nobody, however, has investigated whether crested penguins utilise these structures during the winter period. The graphic below shows the movements of a macaroni penguin in relation to sea level anomaly (SLA - eddies) and finite-size Lyapunov exponent (FSLE - submesoscale filaments). Notice how the penguin encounters the eddy on May 26th and only leaves on July 21st. That's nearly two months at the same eddy. Also take notice of how the penguin moves in relation to the filaments.