#01 2024

Satellite tagging whales to study climate change in the Southern Ocean

By Dr Els Vermeulen, Senior Lecturer and Research Manager, Mammal Research Institute Whale Unit, Department of Zoology and Entomology, University of Pretoria

A more than 40-year time series of southern right whale reproductive rates has revealed significant changes to the population in the last decade, including a reduction in reproductive rates. In addition to this, mothers have become skinnier over the past two decades, and information on their diet by looking at isotopes in their skin indicates that they have focused their foraging efforts further north when compared to historical data.

As capital breeders, these data point towards a decreased feeding efficiency, which in turn raises questions on prey availability and the environmental conditions of their foraging ground.

These changes have prompted an effort to develop a better understanding of southern right whale migratory and foraging behaviour by deploying satellite tags to track the whales during their migration to the Southern Ocean where they feed. So far, a total of 24 whales have been tagged over 2021, 2022 and 2023. These include 11 tags provided by the South African Polar Research Infrastructure (SAPRI) and successfully deployed at the end of the 2023 season.

The tags are providing valuable data, already showing that the whales shifted their main foraging efforts further north compared to historical data (see Figure 1). Additionally, a few exceptionally long migrations have been documented, including individuals which travelled more than 15 000 km in a single year.

Figure 1. Satellite tagging tracks from 15 whales tagged in 2021 and 2022 compared to historical whaling and tagging data.

When assessing sea ice trends over the period of these changes, data has shown a dramatic reduction in Marginal Sea Ice over the area where most of the global Antarctic krill stock resides (see Figure 2). This is relevant because Antarctic krill, one of the southern right whale’s primary food items, lives under sea ice during the winter for protection from predation, and feed on under-ice algae.

Figure 2. A large area of reduced sea ice concentrations over the most important region for Antarctic krill during the period of population-level changes in the South African southern right whale population (2009–2015).

Southern right whales are shifting their migratory behaviour in response to environmental change, with detrimental effects on foraging efficiency and their reproductive rates (Photo: Shutterstock)

It is clear that southern right whales are shifting their migratory behaviour in response to environmental change, but with detrimental effects on foraging efficiency and, consequently, their reproductive rates. The combination of time series data through annual monitoring of the population, satellite tracking data and health monitoring can all be used in furthering our understanding of the impacts of climate change in the Southern Ocean.

The data also reveals the potential effects climate change will have on recovering baleen whale populations through further alterations to their foraging grounds. It is therefore vital to continue advancing our understanding of whale migratory behaviour, both as a tool to further our knowledge of the Southern Ocean, and to ensure the continued recovery of baleen whale populations.