By Wendell Roelf
CAPE TOWN (Reuters) - South Africa’s great white sharks could die out due to human interference, ocean pollution and a limited gene pool, a new study released on Wednesday showed.
There are 350-520 great white sharks left off the South African coast, 50 percent fewer than previously thought, according to a six year study carried out mainly in Gansbaai, a shark hotspot 160 kilometres from Cape Town.
"South Africa’s white sharks faced a rapid decline in the last generation and their numbers might already be too low to ensure their survival,” said Sara Andreotti, research leader and marine biologist at the University of Stellenbosch.
Scientists say there are still thousands of great white sharks off the coast of Australia, Canada and the east coast of the United States.
Thousands of tourists travel to South Africa's Western Cape each year to catch a glimpse of the ocean's top predator from underwater cages, but human interaction has made the largest contribution to declining local shark numbers.
Shark nets used to protect swimmers and surfers killed more than 1,000 great whites off the Durban coast in the 30 years up to 2008, while trophy hunting and pollution also killed off large numbers of a species which can trace its lineage back 14 million years.
South African great white sharks also have the lowest genetic diversity of all white shark populations globally, making breeding more problematic and the likelihood of illness higher, the study, which included documenting individual sharks by their dorsal fins, showed.
There are only 333 great whites capable of breeding in South African waters, below the 500 usually needed to prevent "inbreeding depression", the study found.
"We are already in a situation where our number of breeders is below the minimum level required for a population to survive," Andreotti told reporters.
Losing great white sharks, which have no natural predators, would have a knock-on effect on ocean ecology. Common prey, such as the Cape fur seal, could flourish in their absence and reduce fish numbers.
South Africa helped pioneer great white shark conservation and in 1991 became the first in the world to declare the predator a protected species, with other countries including the U.S. and Australia following suit.
(Editing by Joe Brock)