Counting sharks: CSIRO has great white number

THE number of great white sharks in the waters off the Australian coastline has finally been estimated, following a breakthrough in genetic and statistical methods by the CSIRO.

Australia has two white shark populations – an eastern population ranging east of Wilson’s Promontory in Victoria to central Queensland and across to New Zealand, and a southern-western population ranging west of Wilson’s Promontory to the north-west of Western Australia.

According to CSIRO’s research paper, 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.

This research has also provided important details on adult survival rates, which were estimated to be above 90 per cent in both populations.

Until now, information about adult white sharks has been elusive as adults are very difficult to sample, particularly on the east coast.

However, scientists can now estimate adult shark numbers without having to catch or even see them by locating the tell-tale marks of the parents in the DNA collected from juveniles.

Two juvenile sharks sharing a parent has a statistical relationship to the total size of the adult shark population.

“In a small population, more juveniles share a parent than in a large population, and vice versa,” the paper’s lead author Dr Richard Hillary said.

“And as more juveniles are sampled over time, the parental marks we detect also reveal patterns of adult survival, which we determined to be greater than 90 per cent in the east.”

Adult populations for both the eastern and southern-western populations were estimated to have been stable since the onset of white shark protection at the end of the 1990s.

This is consistent with the long time it would take for the effects of the various control programs and levels of fishing that existed pre-protection (which focused mostly on juveniles) to flow through to the adult population.

Sharks take 12-15 years to become mature adults, so CSIRO does not expect to see the effect on the adult population of that reduction in juvenile shark mortality until the next few years.