Researchers from the Universities of Auckland and of Pretoria, South Africa, carried out research on the southern right whales that winter along the coast of South Africa. Southern right whales feast during summer and fast during winter when they give birth and nurse their young on coastal wintering grounds.
Scientists used microchemical markers called stable isotopes found in small samples of the whale’s skin. Stable isotopes vary predictably across the Southern Ocean and can be matched to those in the whale’s skin to identify where the whales feed during the summer months.
During the 1990s, the South African southern right whale population was growing well and stable isotope data suggested they were feeding mainly on krill from South Georgia/Islas Georgias del Sur. In contrast, over the last 5 years, the whales have been having babies less often and the stable isotope data suggest they are now feeding much further north.
Like other whales such as humpback whales, southern right whales are reliant on getting enough food during the summer months to sustain themselves through the winter breeding months. The slow-down in recovery of the South African population, at the same time as the shift in feeding behaviour, suggests their preferred prey may have declined.
The study suggests this shift represents a response to changes in preferred habitat or prey – for example a decrease in abundance and southward range contraction of Antarctic krill.
By linking reproductive decline to changing foraging strategies for the first time in southern right whales, the study shows that altering foraging strategies may not be sufficient to adapt to a changing ocean.
“At first we were happy to see that the whales were shifting their feeding grounds as there was concern that they would stick to their traditional areas,” says Gideon van den Berg who led the new study. “But this work suggests that even changing their behaviour may not be enough to keep pace with the changing ocean.”
University of Auckland Rutherford Discovery Fellow Dr Emma Carroll says in the southern ocean near New Zealand, right whale populations also seem to have slowed down their population growth.
“This work suggests that even though we have stopped hunting whales, our actions are now indirectly impacting their populations through climate change.”
Dr Carroll is leading research to determine where and on what New Zealand southern right whales – tohorā nō Aotearoa – are feeding, and relate this to their recovery and to the effects of climate change.