South African southern right whale aerial survey
Els Vermeulen, Christopher Wilkinson, Meredith Thornton, Ingrid Peters, Ken Findlay
International Whaling Commission
The South African southern right whale (Eubalaena australis) population has been monitored through annual aerial surveys across the southern Cape coast since the early 1970’s. From 1979 onwards these annual surveys have incorporated identification using photography of natural markings, resulting in an uninterrupted 38-year survey series of photo-identification history. In recent years, these surveys showed a marked decline in the presence of southern right whale unaccompanied adults (since 2010) and cow-calf pairs (since 2015) along the South African coast. Reasons for this decline remain speculative. In order to continue monitoring this population and further investigate the observed trend, a series of aerial surveys were conducted in the whale calving and nursing season (June to December) of 2017.
The annual photo-identification aerial survey was flown as usual in October with the aim to count all southern right whales and photograph all females with calves and individuals with a brindle or grey blaze colouration between the area of Nature’s Valley and Muizenberg. This year, the survey was flown coastwise over the period 2 to 9 October 2017 and in a general westward direction using an Airbus EC120B helicopter (only the first day was flown in an eastward direction). A total of 29 hours and 30 minutes of flight operations were required to complete the survey, of which 23 hours and 44 minutes was flown as search effort and 5 hours and 46 minutes was flown in transit to and from the survey start and end points. In total, 179 groups comprising 180 cow-calf pairs of southern right whales (360 animals) and 65 groups comprising 135 unaccompanied adults were observed. Considering the decrease in southern right whale sightings in recent years, and the extension of the survey in 2016 survey to Lambert’s Bay on the west coast of South Africa, this year’s survey was also extended between Muizenberg and Lambert’s Bay on 10 October 2017. Within this extended area, 3 groups of 3 cow-calf pairs (6 animals) and 17 groups of 26 unaccompanied adults were encountered, bringing the total to 182 groups comprising 183 cow-calf pairs of southern right whales (366 animals) and 82 groups comprising 161 unaccompanied adults observed during the entire survey. Regarding cow-calf pairs, this reflects an increased presence along the South African coast when compared to the 55 cow-calf pairs encountered in the 2016 aerial survey, yet levels remain the second-to-lowest count in the last 17 years of survey (Findlay et al., 2017). Regarding the unaccompanied adults, this year’s count reflects among the highest presence since 2010, although levels remain significantly lower than those observed pre-2010.
In order to investigate the seasonal increase and decrease of cow-calf pairs along the southern Cape coast during whale season, additional aerial surveys were flown in July, September and November between the area of Hermanus New Harbour and Witsand (covering the main calving areas), with the sole purpose to count cow-calf pairs. These surveys were flown along the coast using an autogyro (due to financial limitations) in an eastward direction, and each survey was undertaken in less than 3hs of flight time. Results of these surveys suggest a peak presence of cow-calf pairs along this stretch of coastline in early September, opposed to the previously believed peak in October (when the annual photo-identification survey is conducted). Although numbers may not be fully comparable due to different survey methods, this seasonal abundance warrants further investigation in future years.
Additionally to the aerial surveys, all photo-identification data available from the 2014 through to the 2017 annual aerial surveys were processed using the Hiby-Lovell automated computer based image recognition system. Results indicated an increasing occurrence of 4 and 5 year calving intervals post-2014. Sighting histories are being analysed to assess the consequences of these increased calving intervals on the population’s demographic parameters.
Although the reason for the continued low presence of southern right whales on the South African breeding ground remains speculative, currently available data point towards two working hypotheses, including a temporal shift in seasonal presence and a decreased calving success (indicated by an increasing calving intervals). However, both hypotheses require further investigation. Therefore, the continuation of the aerial survey series and an in-depth assessment of the resulting demographic parameters is of crucial importance to monitor and investigate the recent changes in coastal presence of southern right whales in their breeding grounds off South Africa, and its effect on the dynamics of the population.