Report of the southern right whale aerial surveys - 2021
Els Vermeulen, Chris Wilkinson, Matthew Germishuizen
Since 1969, annual aerial surveys have been condicted along the South African coast to monitor the recovery of the South African population of southern right whales (Eubalaena australis) from extensive whaling. From 1979, these annual surveys incorporated a photo-identification component, resulting in an uninterrupted 42-year survey series of photo-identification history. Although these surveys have revealed a steady population increase since the protection of the species from commercial whaling, recent results have indicated substantial changes including a marked decline in the prevalence of unaccompanied adults since 2010, an increase in the reproductive cycle from the normal 3-year cycle to 4- and 5- year calving intervals since 2010, and an enormous fluctuation in the number of cow-calf pairs along the South African shore since 2015.
The 2021 annual photo-identification aerial survey, covering the area between Nature?s Valley to Muizenberg, was flown between 3 and 5 October 2021 in a westward direction, with the aim to count all southern right whales and photograph all females with calves as well as individuals with a brindle or grey blaze colouration. A total of 16 hours and 33 minutes were flown in an Airbus EC120B helicopter, chartered by Silvercross Helicopters, to complete the survey. Of these, 12 hours and 6 minutes were flown as search effort, and 4 hours 27 minutes were flown in transit to and from the survey start and end-points. In total, 191 cow-calf pairs of southern right whales (382 animals) and 32 unaccompanied adults were observed, leading to a total of 414 southern right whales. Although this number is an increase from the low number of cow-calf pairs seen during the 2020 annual aerial survey (n=67), it remains far below the numbers to be expected based on the projected population growth rate. For unaccompanied adults, numbers remain similar to 2020 (n=31) and therefore marks the fifth-lowest count since the commencement of the aerial surveys.
In total, 191 cow-calf pairs and 18 unaccompanied adults were photo-identified during the survey. Subsequent data analysis indicated the presence of 12 duplicates in the dataset, and 2 individuals with a lack of good quality photographs, leading to the identification of 195 unique individual southern right whales, including 177 cows. Of these, 86 could be matched to previously known females giving results on 75 new inter-calving intervals. The observed calving intervals indicated the majority of cows calving in 2021 had a normal 3-year (19%), 6-year (17%) and 7-year (16%) calving interval.
Similar to the 2020 survey, this year?s photo-identification analyses showed the low frequency of re-identifications of the older females known in the catalogue. In fact, the vast majority (71%) of females (re-)identified in 2021, had been added to the catalogue only in the past decade, and only 17 cows photographed in 2021 were known to the catalogue since prior the year 2000. Whether or not this is a result of a non-random component of the population being captured on the October survey, remains to be determined.
Prior to the annual helicopter survey, two aerial count surveys were conducted, with the sole purpose of counting cow-calf pairs in the coastal area between Hermanus New Harbour and Witsand (covering the main nursery grounds). This survey was flown in an eastward direction on 3 August 2021 and 15 September 2021 using an autogyro. Results of this survey indicated the presence of 169 cow-calf pairs and 52 unaccompanied adults, and 211 cow-calf pairs ?and 35 unaccompanied adults respectively, along this limited stretch of coastline.
These data continue to indicate a peak of cow-calf pair numbers in September rather than October, and is believed to reflect a reduced residency time on the South African coast.
The incessant low number of unaccompanied adults remains a concern in regard to the country?s whale-watching industry (legislation prohibits the approach of cow-calf pairs < 300m), as accumulative effects of approaching vessels on cow-calf pairs could have energetic consequences for both cow and calf. Furthermore, the observed demographic fluctuations in the South African southern right whale population reiterates the extreme value of this long-term dataset. An uninterrupted continuation of these survey series is therefore crucial, not only to monitor southern right whales in their breeding ground off South Africa, but also to improve our understanding of the changing state of the larger marine ecosystem these whales inhabit.