Report of the southern right whale aerial surveys -2019
Dr Els Vermeulen, Christopher Wilkinson, Gideon Van den Berg
International Whaling Commission
Annual aerial surveys to monitor the South African population of southern right whales (Eubalaena australis) have been carried out since 1969. From 1979, these annual surveys incorporated identification using photography of natural markings, resulting in an uninterrupted 40-year survey series of photo-identification history. Over time, these surveys have shown a steady population increase since the protection of the species from commercial whaling. However, recent results of these surveys indicate strong changes in the prevalence of southern right whales on the South African breeding ground, including a marked decline of unaccompanied adults since 2010 and extreme fluctuations in the number of cow-calf pairs since 2015. Additionally, female sighting histories show an overall increase in calving interval from a normal 3-year cycle to 4- and 5-year calving intervals.
The 2019 annual photo-identification aerial survey was flown in early October (30 September to 5 October 2019) 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 between the area of Nature’s Valley and Muizenberg. As in previous years, an Airbus EC120B helicopter was used. A total of 14 hours and 17 minutes of flight operations were required to complete the survey, of which 10 hours and 8 minutes were flown as search effort and 4 hours and 9 minutes were flown in transit to and from the survey start and end points. In total, 92 groups comprising 94 cow-calf pairs of southern right whales (188 animals) and 12 unaccompanied adults were observed, leading to a total of 200 southern right whales. This marks the second lowest number of cow-calf pairs along the South African coastline in October since 1995 (after the extreme low numbers of 2016 (55 pairs)) and is a huge decrease from last year’s all-time record of 536 cow-calf pairs along the same stretch of coastline. For unaccompanied adults, these numbers reflect the second lowest count since the commencement of the aerial surveys.
Due to financial limitations, only one additional aerial 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 15 August 2019 using an autogyro. Results of this survey indicated the presence of 129 cow-calf pairs and 24 unaccompanied adults in this limited stretch of coastline, suggesting an apparent shift in peak presence from early October (when the annual photo-identification survey is flown) to earlier in the year. Although numbers may not be fully comparable due to different survey methods, this is the 3rd consecutive year in which an apparent shift in peak presence was shown, and warrants further investigation and consideration in future years.
This year, additional photo-identification data was collected in early September in Walker Bay and Saint Sebastian Bay using an Unmanned Aerial Vehicle (UAV), during fieldwork investigating southern right whale body condition. In total, 53 individuals, 45 of which were cows, were photo-identified during this period. Of these, only 7 cows were re-photographed during the annual aerial survey, suggesting 38 cows and 8 unaccompanied adults were missed or left the South African breeding ground in the 2-week time period between the boat-based survey and the annual aerial survey. If the latter, this would be considered a large movement of female right whales, and may be related to the apparent temporal shift in the peak presence of calves along the South African coast. If cow-calf pairs leave the nursery ground before the calf has reached its critical size, calf survival could be affected. Further investigation should assess this situation, as it will impact population dynamics in the long term. Consideration should therefore be given to conduct additional photo-identification work earlier in the season to assess whether (1) there has indeed been a shift in the peak presence of cow-calf pairs and (2) to provide more detailed information related to the timing and magnitude of movement of cow-calf pairs on the South African coastline.
In total, 93 cow-calf pairs and 7 unaccompanied adults were photo-identified during the annual aerial survey. Photo-identification data analysis showed the presence of 14 duplicates in the dataset, leading to the identification of 86 unique individual southern right whales, including 79 cows. Of these, 52 could be matched to previously known females. The observed calving intervals indicated the majority of cows had a 6-year calving interval (22%), followed by a 5-year interval (20%) and a 3-year interval (16%). Considering a 6-year interval is assumed to be a combination of two normal 3-year calving intervals, this result is slightly different from the recent observed shift from a 3-year to a 4-year calving cycle. Nonetheless, the proportion of females calving at a 5-year interval remains high, and the incessant low numbers of unaccompanied adults remains a concern, especially as they form the basis of the country’s whale watching industry (legislation prohibits the approach of cow-calf pairs < 300m). In the absence of unaccompanied adults and law enforcement, accumulative effects of approaching vessels on cow-calf pairs could have energetic consequences for both cow and calf. Without more specific data, the precautionary approach should be adopted, and so it is strongly recommended that the South African permitting authority and the South African Boat based Whale-watching Association engage with researchers in order to properly manage the activities with this valuable natural resource, especially considering the likely low energetic reserves of the nursing cows.
The observed demographic changes 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.