Tracking population breakthrough of ‘elusive’ sharks
MORE than 1000 great white sharks call the waters off South Australia home, new research shows.
The CSIRO looked for brothers and sisters among juvenile white sharks, which have provided the final pieces of information needed to estimate the size of populations in Australian waters.
Rather than relying on observation alone, the research used DNA markers and statistics to determine the parents of each shark sampled.
Until now, information about adult white sharks had been elusive, because adults were very difficult to sample.
Thanks to a breakthrough in genetic and statistical methods, this problem has been solved. The breakthrough means scientists have been able to estimate adult shark numbers without having to catch or even see any adult white sharks. Instead, they located the telltale marks of the parents in the DNA collected from juveniles.
In a small population, more juveniles share a parent than in a large population, and vice versa. As more juveniles are sampled over time, the parental marks detected also reveal patterns of adult survival.
Australia has two white shark populations, an eastern population ranging east of Wilson's Promontory, Victoria, to central Queensland and across to New Zealand, and a southern-western population ranging west of Wilson's Promontory to northwestern Western Australia.
The research indicates that there are about 750 adults in the eastern Australasian white shark population (with a range from 470 to 1030), and about double that number in the southern-western population, which includes South Australian waters. Parents that had survived 20 or more years between the births of their offspring were also discovered.
Shark tour operator and researcher Andrew Fox, whose team helped to biopsy sharks off Neptune Islands for the research, said the numbers supported what charters were seeing in local waters.
"For 10 or more years we've been taking careful photo IDs and have recorded over 1000 individual sharks off the Neptune Islands, alone,'' he said.
"There's nothing surprising to us that the genetic analyses also shows numbers in the lower thousands."
Adult populations were estimated to be stable since the onset of white shark protection at the end of the 1990s.
However Mr Fox said the shark population would be expected to grow as more juveniles, which previously would have been fished, reached sexual maturity at the age of 15-20.
"Humpback whale numbers are increasing by 10 per cent a year,'' he said. "You could expect shark numbers to increase along with that."
Researchers hope accurate great white shark population numbers will enable the development of effective policy outcomes balancing conservation and human-shark interaction risk management. While shark numbers remain steady, there's no guarantee you'll see one while on charter at the moment. So far this month, no great whites have been spotted off the Neptune Islands.
Mr Fox said great white shark numbers around the islands in the month of February dropped off three years ago when killer whales were seen in the area. He said it highlighted that there was still so much to learn about the elusive animal.
"It's fantastic that there is still mystery about them, it makes them magical, that such a huge animal is so mysterious still. They are a rare and elusive species," he said.