The humpback whale (Megaptera novaeangliae) is one of the most well-studied species of baleen whale and occurs worldwide in all ocean basins. These whales are migratory and typically follow the usual baleen whale paradigm of moving between summer feeding grounds in high latitude regions and winter breeding grounds in low latitude regions.
Different populations of humpback whales, which are more or less geographically separated, exist in different ocean basins. There is also little to no genetic interchange between populations in the northern and southern hemispheres due to limited trans-equatorial movement, as well as a six month difference in their breeding seasons.
In the southern hemisphere, there are estimated to be more than 140 000 humpback whales which comprise of seven different breeding stocks. Two of these breeding stocks, breeding stock B and C, utilize the waters of South Africa as a migratory corridor, and also as a summer feeding ground in the case of breeding stock B.
Breeding stock B occurs along the west coast of Africa, with breeding grounds off West Africa around Gabon and southwards. It is believed that this breeding stock is split into two sub-stocks, with sub-stock B1 breeding off Gabon, the Congo, Angola and north to the Bight of Benin and sub-stock B2 possibly breeding more south of sub-stock B1. Overall population estimates for the stock have been hampered by the complexity of this breeding stock’s population structure and also the lack of data collected between Walvis Bay and northern Angola. However, sub-stock B2 has been estimated to comprise of between 350 and 500 individuals and is considered as ‘Vulnerable’ on South Africa’s Red Data List.
The humpback whales seen off Plettenberg Bay belong to breeding stock C which occurs along the east coast of Africa, with breeding grounds off Mozambique, Madagascar and Tanzania. This breeding stock is split into three sub-stocks, with sub-stock C1 breeding off the eastern coasts of South Africa and Mozambique, sub-stock C2 breeding off Madagascar, and sub-stock C3 breeding off western Indian Ocean island groups such as Mayotte and the Comoros. Each of these sub-stocks are estimated to consist of over 7000 individual humpback whales.
Both the B and C stocks migrate south to high latitude feeding grounds in the Southern Ocean, usually around Antarctica, for the summer months, where they feed almost exclusively on Antarctic krill (Euphausia superba). However, a small component of breeding stock B, sub-stock B2, feeds along the west coast of South Africa and Namibia in the Benguela upwelling system. Here, they have recently been observed forming “super-groups” of between 20 and 200 whales as they feed on another krill species (Euphausia lucens), hyperiid amphipods (Themisto gaudichaudii), mantis shrimp (Pterygosquilla armata capensis) and clupeid fish.
Written by Danielle Conry
For further reading:
- BARENDSE, J. & CARVALHO, I. 2016. A conservation assessment of Megaptera novaeangliae. In: The national Red List of mammals of South Africa, Swaziland and Lesotho, (eds) M. F. Child, E. Do Linh San, D. Raimondo & H. Davies-Mostert. South African National Biodiversity Institute.
- BEST, P. B. 2007. Whales and dolphins of the southern African subregion. Cambridge University Press, Cape Town, South Africa.
- FINDLAY, K. P., SEAKAMELA, S. M., MEYER, M. A., KIRKMAN, S. P., BARENDSE, J. CADE, D. E., HURWITZ, D., KENNEDY, A. S., KOTZE, P. G. H., MCCUE, S. A., THORNTON, M., VARGAS-FONSECA, O. A. & WILKE, C. G. 2017. Humpback whale “super-groups” – A novel low-latitude feeding behaviour of Southern Hemisphere humpback whales (Megaptera novaeangliae) in the Benguela Upwelling System. PLOS ONE. https://doi.org/10.1371/journal.pone.0172002