Cape Town - Scars left by killer whales and cookiecutter sharks are helping scientists unravel the mystery of humpback whale migration routes.
Researchers know humpbacks migrate between their polar feeding grounds and warmer waters where they breed, but the exact routes they take are not known.
Now, in a paper published in the Journal of Mammalogy, researchers from the University of Pretoria’s Mammal Research Institute say seeing the kind of scars on humpback whales, and how recently they were made, has enabled them to get a pretty good idea of the routes the whales took before arriving in their breeding grounds off the west coast of South Africa, Namibia and Gabon.
The reason is that cookiecutter sharks prefer warmer water.
Researcher Tess Gridley said by looking at the patterns of scarring on the humpbacks from cookiecutter sharks and killer whales, and comparing these with the distribution of these two predators, they got a better understanding of the waters the humpbacks had travelled through before reaching our coast.
“We are reasonably confident that cookiecutter sharks, which look particularly gruesome and have razor-sharp teeth for gouging out discs of flesh, prefer living in warmer and temperate waters and avoid the shallower, colder waters of the Benguela ecosystem on the western coast of South Africa and Namibia,” Gridley said.
In contrast, killer whales, which regularly attack and kill humpback whales, especially calves, are found all over the ocean. “So if whales have lots of cookiecutter shark bites, there’s a good chance they have recently passed through warm water in offshore areas, and only recently reached the coast.”
Another of the researchers, Simon Elwen, found that the humpbacks he photographed off the coast of Namibia during the winter months had a lot of fresh cookiecutter shark bites. This suggested that the whales had arrived recently, and had come directly from the waters offshore of the Benguela.
The whales photographed off the South African west coast by another researcher, Jaco Barendse, during summer had no fresh bites, suggesting the animals had spent a long time migrating through the cold Benguela, giving the bites time to heal.
Elwen said: “In Namibia almost all the whales had fresh bites, suggesting they were coming straight across from the western ocean, but there are no fresh bites on the whales in South Africa.”
Cookiecutter sharks are so-called because they gouge out chunks of flesh from other sharks, fish, whales and dolphins, leaving open wounds just as if a super-sharp cookie cutter had dug into the animals’ flesh. A cookiecutter shark attaches itself to its prey with suctorial lips, anchors itself with its upper teeth and slices into the animal’s flesh with its lower teeth.
It then rotates its body to make a circular cut.