New study finds that adult female southern right whales visiting Cape Town are 25% lighter than 30 years ago, and are giving birth less often.

The southern right whale’s name is reminiscent of the heyday of the whaling industry in the first half of the 19th century, when they were said to be the ‘right whales’ for hunting because of their docility and pattern of swimming close to shore.

As a result, southern right whales (Eubalaena australis) and their two closest relatives — the even-more endangered North Atlantic right whale and the North Pacific right whale — only narrowly escaped extinction. But while the numbers of southern right whales have stuttered upwards over the past decades, climate change may be kicking the whales while they’re still recovering.

In fact the breeding females visiting Cape Town and surrounding shores of South Africa for their annual calving migration have dropped 23% of their body weight since the 1980s, according to a paper published in Scientific Reports. They are also giving birth less frequently, dropping from every three years to every four to five years.

The findings are based on a photogrammetry analysis in which researchers compared aerial images taken in 1988 and 1999 with those in 2019 and 2021. Scientists were able to calculate the whales’ body conditions from estimates of body length, width and height.

The Whale Unit of the Mammal Research Institute (MRI) at the University of Pretoria first started a simple headcount of the southern right whales (SRWs) in 1969 using a helicopter, but then evolved to identifying individual whales and using drones and autogyro for counting and photogrammetry. “A strong focus is given to females with a calf for reasons of population monitoring,” explains Els Vermeulen, the research manager of the Whale Unit and lead author on the study.

Over the years, the Whale Unit has amassed a catalogue of over 2,500 females, estimated to account for about 70% of the total number of females in the South African population. Based on this data, the researchers have also concluded that the females are giving birth less often, which could be attributed to either additional resting years between calving periods to build up fat reserves, or the loss of a foetus. The weight loss could be the result of shrinking food sources, notably krill in the Antarctic. It’s a theory Vermeulen and others first proposed in a 2020 paper titled “Decadal Shift in Foraging Strategy of a Migratory Southern Ocean Predator.”

This would explain why, as recent findings with the Cape Town SRWs indicate, whales are also leaving coastal areas earlier than in previous seasons to go back to eat as they don’t have enough energy or food to sustain them.

Read more about right whales

This article first appeared in Nature Africa on 26 March 2024