Great white sharks are being exposed to deadly chemicals as a result of the fight against malaria.
Skin samples taken from 15 great whites near Dyer Island, off Gansbaai in Western Cape, contained high levels of DDT - a carcinogenic insecticide.
Researchers from Stellenbosch University and universities in Siena and Calabria in Italy said this was "probably a result of the recent introduction of DDT to fight malaria in the KwaZulu-Natal region".
DDT is banned in many countries and in 1996 its use was halted in KwaZulu-Natal.
But it was reintroduced in northern parts of the province in 2000 due to a surge in malaria cases.
The provincial department of health said at the time it had become necessary because "cases of malaria in the province had doubled from 14000 in 1996 to 28000 by 1999".
SA Medical Research Council malaria researcher Rajendra Maharaj said between 2000 and 2010 "a large decrease of malaria cases was noted in KwaZulu-Natal. Malaria has serious economic impacts in Africa, slowing economic growth and development and perpetuating the cycle of poverty for those in poorly constructed dwellings that offer few barriers against mosquitoes."
DDT was reintroduced because mosquitoes had become resistant to sprays.
Another worrying finding in the study was the presence of a carcinogenic organic pollutant known as PAH, which was found in the skin samples, as a result of "South Africa being one of the most frequented oil shipping routes, with 28% of the Middle East oil passing along the coastline".