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Report of the southern right whale aerial surveys 2020
Els Vermeulen, Chris Wilkinson, Gideon van den Berg, Sanne Paarman
Annual aerial surveys have been carried out since 1969 to monitor the South African population of southern right whales (Eubalaena australis). From 1979, these annual surveys incorporated identification using photography of natural markings, resulting in an uninterrupted 41-year survey series of photo-identification history. Although these surveys have revealed the steady population increase since the protection of the species from commercial whaling, recent results of these surveys have indicated substantial changes in the prevalence of southern right whales on the South African breeding ground. These changes include 1) a marked decline of unaccompanied adults since 2010, 2) an increase in the reproductive cycle from the normal 3-year cycle to 4- and 5- year calving intervals since 2010, and 3) an enormous fluctuation in the number of cow-calf pairs along the South African shore since 2015.
The 2020 annual photo-identification aerial survey was flown between the area of Natureâ€™s Valley and Muizenberg in late September (27 to 29 September 2020) in a general 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. For this survey, an Airbus EC120B helicopter was used, chartered by Silvercross Helicopters. A total of 12 hours and 53 minutes of flight operations were required to complete the survey, of which 9 hours and 27 minutes were flown as search effort, and 3 hours 26 minutes were flown in transit to and from the survey start and end-points. In total, 67 cow-calf pairs of southern right whales (134 animals) and 31 unaccompanied adults were observed, leading to a total of 165 southern right whales. This the second-lowest number of cow-calf pairs along the South African coastline since 1986 (after the extreme low numbers of 2016 (55 pairs)). For unaccompanied adults, this marks the fourth-lowest count since the commencement of the aerial surveys.
In total, 67 cow-calf pairs and 19 unaccompanied adults were photo-identified during the survey. Subsequent data analysis indicated the presence of 4 duplicates in the dataset, leading to the identification of 82 unique individual southern right whales, including 64 cows. Of these, 30 could be matched to previously known females, giving results on 28 new inter-calving intervals (2 females were previously identified without a calf). The observed calving intervals indicated the majority of cows had a normal 3-year (68%), 6-year (11%) or 9-year (7%) calving interval. Considering a 6- and 9-year interval is assumed to be a combination of two and three normal 3-year calving intervals respectively, this result is remarkably different from the recently observed shift to a 4- and 5-year calving cycle since 2010. Photo-identification analysis further showed that 84% of the identified have been added to the catalogue post-2010 and thus presumably under the age of 20. Only 4 cows re-identified in 2020 were known to the catalogue since prior the year 2000.
Due to COVID-19 limitations, only one aerial count survey could be flown prior to the photo-identification survey, with the sole purpose to count cow-calf pairs in the area of Hermanus New Harbour and Witsand (covering the main nursery grounds). This survey was flown in an eastward direction on 21st August 2020 using an autogyro. Results of this survey indicated the presence of 71 cow-calf pairs and 11 unaccompanied adults in this limited stretch of coastline.
The decrease in whale numbers from the August count survey to the October photo-identification survey re-confirms a shift in peak presence from early October (when the annual photo-identification survey is flown) to earlier in the year. This is the 4th consecutive year in which an apparent shift in peak presence was shown, with hypothesis formulated on a reduced residency time of cow-calf pairs on their South African breeding ground. Whether or not the observed age and calving interval of the females photo-identified on the annual aerial survey this year is a result of a non-random component of the population being captured, remains to be determined.
The incessant low number of unaccompanied adults remains a concern in regards to the countryâ€™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. However, considering the COVID-19 limitations on the South African whale-watching industry, these effects would have been negligible in 2020.
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.