Southern right whale (Eubalaena australis) body condition and glucocorticoid levels at the South Africa breeding ground
Terriann Thavar, Fredrik Christiansen, Andre Ganswindt, Mariano Sironi, Marcela Uhart, Lars Bejder, Els Vermeulen
Southern right whales (Eubalaena australis) in South Africa have been extensively studied since 1979 through annual photo-identification surveys. The resulting database revealed an increased rate of reproductive failure in the last decade. As reproductive success is mediated through body condition, this study aimed to assess the body condition and physiological indicators of stress of southern right whales on the South Africa breeding ground at present, and compare it to historical data and other populations. For this, aerial photographs of southern right whales were collected using an unmanned aerial vehicle in September 2019 from which body condition was assessed. Additionally, blubber biopsy samples were collected for glucocorticoid (GC) analysis and compared to the body condition estimates of individual whales. To assess temporal change, analogue aerial photographs taken in coastal South Africa in 1988 and 1989 were selected and digitized for body condition measurements, and compared to the 2019 data set. To determine population differences in body condition, the 2019 data set was compared to body condition data from breeding grounds in Argentina and Australia collected the same year. We found a positive relationship between body condition of lactating southern right whales of the South African population and their blubber GC levels, albeit in a small sample size. The temporal comparison revealed a 24% (SE=5.31) decrease in body condition between 2019 and 1988/1989. Furthermore, lactating females in South Africa were in significantly poorer condition compared to those in Australia and Argentina, at a magnitude of -8.1% (SE=3.07) and -7.1% (SE=3.31), respectively. The reduced maternal body condition in the South African population of southern right whales is of grave concern, as it is likely the main reason behind the reduced reproductive success that is decelerating population recovery.