Although great white sharks have few natural enemies, orcas have been known to prey on them. A new study now shows that due to attacks by just two orcas, great whites are staying away from parts of South Africa.
Located about 100 km (62 miles) east of Cape Town, the Gansbaai coast has long been known for its large population of great white sharks.
Since 2017, however, a total of eight dead sharks have washed up on the shore, bearing distinctive bite wounds that were matched to two local male orcas. The orcas, named Port and Starboard, are also readily identifiable by their drooping dorsal fins.
In seven of the shark carcasses, the orcas had eaten the liver and sometimes also the heart. This makes sense, as the liver is a highly nutritious organ. It is believed that the orcas have likely killed many other sharks in order to eat their livers, but the carcasses haven't been spotted by humans, as they didn't wash ashore.
The recently published study, led by scientists from South Africa's Dyer Island Conservation Trust, was conducted over a five-and-a-half-year period. In that time, 14 tagged great white sharks were tracked fleeing areas where the orcas were present. Additionally, great white sightings in those areas have dropped significantly.
"Initially, following an orca attack in Gansbaai, individual great white sharks did not appear for weeks or months," said the lead scientist, Alison Towner. "What we seem to be witnessing though is a large-scale avoidance (rather than a fine-scale) strategy, mirroring what we see used by wild dogs in the Serengeti in Tanzania, in response to increased lion presence. The more the orcas frequent these sites, the longer the great white sharks stay away."
While fewer great whites in the area may make things safer for human swimmers, the decline might not be good news for all species.
"Balance is crucial in marine ecosystems," said Towner. "For example, with no great white sharks restricting Cape fur seal behavior, the seals can predate on critically endangered African penguins, or compete for the small pelagic fish they eat […] To put it simply, although this is a hypothesis for now, there is only so much pressure an ecosystem can take, and the impacts of orcas removing sharks are likely far wider-reaching."
It is thought that Port and Starboard may be members of a relatively rare shark-eating orca subtype, whose behaviour was altered due to a reduction in prey such as other types of sharks.
The research is described in a paper that was recently published in the African Journal of Marine Science.
Source: Taylor & Francis
