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White Shark carcasses found along the Western Cape Coast

10 May 2017

 

Over the past week, there have been several reports of Great White Shark carcasses washing ashore. Of the three recovered carcasses, two were in Gansbaai and one in Struisbaai. Examination of the carcasses showed that all three had had their livers removed and had bite indentations that were consistent with a killer whale predation. Further to this, small pods of Orcas have been spotted in and around Gansbaai and False Bay in the preceding weeks.

The fact that only the livers of each of these animals were consumed is quite a common occurrence in nature, where quality is more important than quantity. Other examples of this are found in seals predating of seabirds where they often remove and consume only the stomach/abdominal content and not the rest of the carcass. In this case, the squalene (oil and fat that assists sharks with their buoyancy) in the sharks liver is highly nutritious in pound for pound comparison with muscle tissue.

Further to the white shark predations, Alison Kock, Marine Scientist for Shark Spotters, reported recovering several cow shark carcasses in a similar condition with their livers removed, in False Bay subsequent to a series of Orca sightings. Sharks, and White Sharks specifically, have been documented to register Orcas as a threat and elicit avoidance behaviour. White Shark Cage Diving operators in False Bay and Gansbaai also reported declines and the absence of White Shark Sightings proximate to Orca sightings.

The Department of Environmental Affairs, in collaboration with shark scientists and marine mammologists, is focusing on collating and documenting scientific information on these incidents, as they relate to tourism, management and conservation priorities. However, the department would like to urge media and citizens to take cognisance that this is a natural phenomenon and should be taken in the context of an anticipated shift that occurs in an ecological system due to seasonal/temporal incidence differing predator and prey regimes.

Killer whales are widely distributed mammals, extending from the Arctic/Antarctic, into the tropics, in both coastal and oceanic waters, but are typically more abundant in cooler water. Orcas are common to the region, with them frequenting South Africa’s Southern Ocean territories and being documented ephemerally along the mainland coastline. To date, there have been 785 documented incidents of occurrence on the South African coast ranging from the Western Cape all the way to Northern KwaZulu-Natal. The sightings of Orca pods appears to be increasing in South Africa, but this may be an artifact of increases in the number of water users and the extent of access along the coastline.

Killer whales are apex predators and while we are accustomed to viewing Great White Sharks as occupying the top of the food-chain in our waters, Orcas are much more specialised hunters and consider almost anything in the ocean as potential prey. These mammals have highly developed social groupings and interactions which lend organised and strategic dimension to their hunting behaviour, which is backed by a large brain, capacity to learn and specialised physiological tools such as echolocation, resulting in very formidable predators.

Their diet is varied, with whales, dolphins, and seals being the predominant prey species, however, they are opportunistic and will make use of anything that may be locally abundant. Records of species in their diet include dugongs, sea otters, bony fish, sharks and rays, birds, turtles, and squid. Records of land animals in their stomach content allude to a scavenging/opportunistic shift from its normal predatory modus operandi, making it quite a generalist in its approach to feeding. Orcas also tend to harass and injure other species without necessarily having the motivation to feed. This has been observed with pods of orcas drowning large whales, flipping and ramming dolphins and porpoises, or engaging in play with sea birds sharks and other cetaceans often resulting in their death.

For media inquiries contact:

Zolile Nqayi
Cell: 082 898 6483/
E-mail: znqayi@environment.gov.za