2025 Annual Southern Right Whale Aerial Survey

For more than five decades, the recovery of South Africa’s southern right whale population from the impacts of commercial whaling has been closely tracked through annual aerial surveys. These surveys, conducted each year since 1969, involve counting every female whale with a calf along the coast between Nature’s Valley and Muizenberg. From 1979 onwards, the surveys were expanded to include photo-identification, a technique that uses overhead photographs of the unique callosity patterns on each whale’s head to recognise individuals. Thanks to this effort, we now know the population contains around 6,500 whales and is still increasing at roughly 6.5% per year. With 55 years of continuous records, the survey represents one of the world’s longest- running, uninterrupted datasets for any marine mammal species.

The exceptional length of this dataset means it provides insights far beyond simple population growth. It enables researchers to investigate how these whales respond to longer-term environmental changes, such as conditions in their Southern Ocean feeding grounds. One key example is examining how shifts in ocean productivity influence the whales’ annual calving success.

The 2025 southern right whale aerial survey is scheduled to begin, weather permitting, on Monday 29 September and will be carried out following established protocols. The coastline between Nature’s Valley and Muizenberg will be flown using a Eurocopter AS350 (Squirrel) helicopter at an altitude of about 300 m. Flights are restricted to suitable survey conditions between 08h00 and 16h00, as low light and glare outside these hours reduce the quality of survey photographs. During the survey, sightings of all whale species are logged. Particular focus is given to southern right whale females with calves, as well as individuals with distinctive brindle colouration or other unique markings. These whales are photographed from above, capturing both head and back images. The callosity patterns on the head, and in some cases pigmentation patterns on the back, allow individual identification. Photographing each group typically takes fewer than five minutes, though occasionally up to 15 minutes, as the helicopter descends to hover 150–200 m above the whales. Once adequate images are secured, the aircraft climbs back to 300 m to continue scanning the coast, although in areas of high whale density it may move directly from one group to the next at the lower altitude.

After the survey, all images and associated records are processed. The clearest photographs of each individual are selected and compared within the year to eliminate duplicates, and then cross-referenced against the Whale Unit’s identification catalogue of over 2,600 recognisable adults compiled across the past 43 surveys. This is done using computer-assisted image recognition software, with final confirmation made by eye. These analyses build up detailed sighting histories of known whales, enabling researchers to track individual movements and distributions, as well as the reproductive histories of females. The resulting data feed into broader assessments of key population parameters, including abundance, growth rate, survival, calving intervals, and age at first reproduction. Together, these metrics allow the long-term demographic modelling of the population and provide invaluable insights into its recovery and ongoing response to environmental change.

A preliminary count survey conducted in early September this year indicated a total of 154 females with associated calves between Hermanus and Witsand, similar to what was observed in 2021. When the survey is finalised, we will provide an update on the precise number of whales counted and photographed on our survey and put it in perspective with the numbers observed over the past 4 decades.

The 2025 survey will be flown in association with Silvercross Helicopters (Figure 1), and with the support of the De Hoop Collection, Denel Overberg Test Range, Grootbos Foundation and private citizens along the route. The survey is carried out under a permit from the Department of Forestry, Fisheries and the Environment to approach whales, and under specific Marine Protected Area permits from the relevant conservation authorities. We thank TotalEnergies for providing the needed funds for this survey.

From 6 October, the MRI Whale Unit at the University of Pretoria will again conduct boat-based fieldwork in Walker Bay to deploy satellite transmitters on adult southern right whales. Subsequently, starting 10 November, we will relocate to Yzerfontein to deploy satellite transmitters on humpback whales. These tags allow researchers to follow the whales’ long-distance migrations and study their feeding ecology in the Southern Ocean.

The goal of this work is to better understand where South Africa’s southern right and humpback whales travel to feed, and how successful they are in building up the energy reserves needed for migration and calving. As “capital breeders,” both species rely entirely on the fat stores they accumulate during the summer feeding season in the Southern Ocean to sustain them through the winter, when they fast while giving birth and nursing calves. A female’s ability to reach southern Africa and reproduce therefore depends directly on her body condition.

Decades of research by the MRI Whale Unit have revealed worrying trends, especially for southern right whales. Since the late 1980s, maternal body condition has declined by about 23%, reflecting reduced foraging success. At the same time, whales are altering their migration patterns, often travelling far greater distances than previously recorded. These shifts strongly suggest that environmental changes in the Southern Ocean feeding grounds are reducing food availability, which in turn underpins both the observed decline in reproductive success and the large year-to-year fluctuations in whale numbers along the South African coast.

While much of our long-term research has focused on southern right whales, tagging humpback whales provides crucial comparative data. By tracking their migrations and feeding behaviour, we can better understand how environmental changes in the Southern Ocean affect multiple baleen whale species, and whether similar challenges in foraging success and body condition are influencing their reproductive outcomes and population trends along the South African coast.

Therefore, the 2025 tagging programme will deploy six new satellite transmitters (SPOT tags) on adult southern right whales between 6 and 20 October, and 8 satellite transmitters on humpback whales between 10 and 20 November. Each tag can relay the whale’s location for up to one year, enabling researchers to track their journeys across thousands of kilometres. Last year, 16 southern right whales were tagged, producing remarkable data on their movements (see Figure 2).

Members of the public can follow this year’s tagged whales in real time via the Whale Unit website:
https://www.mammalresearchinstitute.science/whale-unit

Because the deployment of satellite tags is a sensitive and highly specialised task, this year’s field team will once again include world renown expert Dr Alexandre Zerbini from the National Oceanic and Atmospheric Administration (NOAA), the University of Washington’s Cooperative Institute for Climate, Ocean and Ecosystem Studies (CICOES/UW), and Marine Ecology and Telemetry Research (MarEcoTel).

We are also proud to announce that the Whale Unit now has an in-house tagging expert. Our technical manager, Chris Wilkinson, has completed a year of intensive training in satellite tagging of large baleen whales with Dr Zerbini, and will be applying these skills during the 2025 field season. This new capacity marks an important milestone for South African-led whale research.

We gratefully acknowledge funding from the South African Polar Research Infrastructure (SAPRI) and thank Southern Right Charters for their invaluable logistical support.

Follow our progress on social media, where we will share frequent updates throughout the fieldwork.

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For further enquiries please contact Dr Els Vermeulen (Research Manager) on 060 971-4301 or Chris Wilkinson (Technical Manager) on 083 580-8247