The Mammal Research Institute’s Whale Unit at the University of Pretoria will commence its annual aerial survey of southern right whales on 1 October. This is the 42nd annual southern right whale photo-identification aerial survey, making it one of the longest uninterrupted datasets on any marine mammal worldwide.
“All encountered whale species will be recorded, with a special focus on southern right whales,” says Dr Els Vermeulen, the unit’s Research Manager. “The data collected will be used to investigate the vital parameters of the population, including abundance estimation, population growth rate, survival, calving intervals and age of first parturition (when a female has her first calf). This allows researchers to accurately model the population demographic parameters over the long term.”
The survey will be flown between Nature’s Valley on the Garden Route in the Western Cape and Muizenberg in Cape Town in association with SilverCross Aviation, and with the support of the De Hoop Collection and Denel Overberg Test Range. An Airbus H120B helicopter will be used to survey the coast at an altitude of about 300 metres.
“Southern right whale females with calves as well as those with distinctive brindle colouration or markings will be photographed to allow for individual identification,” Dr Vermeulen explains. “Vertical images of the heads and backs of the animals will be taken, which will allow for recognition of the pattern formed by the wart-like callosities on their heads and, in some cases, of the white and grey pigmentation patterns on their backs.”
The best images will be selected and compared to the unit’s catalogue of identification photographs of just over 2 400 recognisable adults from the previous 41 surveys. These analyses allow for sighting histories of known individuals to be compiled and an investigation of individual movement and distribution patterns as well as the reproductive/calving histories of females.
Sightings of females with calves decreased dramatically in 2015, 2016 and 2017, increased above normal levels in 2018 and dropped substantially in 2019 and 2020. Sightings of unaccompanied adults (males and noncalving females) decreased drastically in 2010 and have not yet returned to normal levels. “This suggests that only pregnant females that are about to give birth complete their migration to the South African coast to increase their calf’s survival chances, whereas most of the others are not undertaking their migration to the full extent,” Dr Vermeulen explains. “Additionally, data continues to show that females are giving birth to a calf every four to five years instead of every three years.”
The successful migration and calving in southern right whales rely heavily on adequate body condition (blubber thickness), which is directly influenced by the amount of food they consume during their foraging season in the Southern Ocean. “Results from our research confirm that over the past few decades, our whales have changed their feeding location and the prey they target, indicating that they are responding to environmental changes in their feeding grounds,” Dr Vermeulen says. “Additionally, a study we recently completed indicated that our southern right whale mothers’ body condition (or “fatness”) has decreased by 24% since the late 1980s, indicating that their feeding has been less successful. We believe this lack of energy reserves is the basis of the increased calving interval and a reduced migration towards our shores.”
Southern right whales received their name from whalers who believed them to be the “right whale” to hunt, as they are predictable in distribution, generally slow swimming and float when killed. The species was heavily whaled in the past, which reduced the global population from about 70 000 individuals to a mere 60 reproductive females at the termination of southern right whaling in 1935.
However, since their international protection in 1935, the three main breeding populations in the coastal waters of Australia, Argentina and South Africa have been increasing at about 6.5% per year. Currently, the regional (southern African) abundance is estimated at just over 6 000 individuals, with a global population size of just under 15 000 individuals.
“Another objective of the annual aerial survey is to monitor this recovery and the new challenges these whales face,” Dr Vermeulen says.