Coast KZN

Durban – Research conducted by the University of Pretoria (UP) has shown that the dramatic decline in the number of southern right whales that usually visit the South African coast may be due to female whales calving less often and because the mammals lack the energy to do so. The large number of southern right whales that usually visit the coast has been decreasing since 2010.

“We have seen a huge drop in numbers along our coast, which includes females with calves, males, resting females, receptive females – so-called ‘unaccompanied adults’ – and juveniles,” said Dr Els Vermeulen, research manager of UP’s Mammal Research Institute Whale Unit.

Whales are capital breeders, meaning they feed for half of the year and fast during the other half. The success of reproduction depends on the success of feeding. Also, migration requires a higher level energy.

“We think there might be something wrong with their feeding,” said Vermeulen, “and that they may not have the energy to migrate, or to maintain their pregnancies.”

Southern right whales calve every three years, but according to the data, this has shifted to every four to five years. Data from the 1990s compared to 2015 – 2019 also shows a drastic shift northward in foraging behaviour.

“It looks like the whales are trying to compensate for a change. If the foraging was successful, they would not have changed their behaviour.”

Southern right whales forage in a vast region in the southern Indian and Atlantic Oceans, mainly on krill and sometimes on copepods, the latter being lower in protein content. Vermeulen said there could be a large ecosystem change in sub-Antarctic waters. These whales usually arrive in South African waters between July and the beginning of December.  While the whales are trying to adapt, it seems they have been unsuccessful, because pregnancy rates have dropped.

“The adaptation is not sufficient at this point,” Vermeulen said. She said there was a peak in calves in 2018, but those were the females who should have given birth in 2016 and 2017.

“This year there were very few whales to be seen.”

Pregnant females are motivated to migrate to the South African coast to give birth because the waters are shallow and there are not many predators. But they need to expend energy to travel. The rest of the whales will not migrate if they don’t have the energy, which is what is happening.

“We are seeing only 20 to 30 unaccompanied adults, when we should be seeing 300 to 400. We don’t know where they are if they are not migrating to South Africa.”

Next year satellite trackers will be attached to some of the whales to monitor where they are going.