A RELIABLE estimate of the number of white sharks in waters off the WA coast is still unknown as researchers only have estimates of adults in the southern-western population.
This is because researchers have not been able to estimate the number of juvenile white sharks off the WA and SA coasts.
Juvenile white sharks are in the 3.5m to 4m range while an adult, which can live for about 50 years, grows up to 6.5m.
The southern-western population ranges from west of Bass Strait around the South Australian and Western Australian coasts as far north as Ningaloo Reef.
To estimate the total population researchers need juvenile survival rates, which are obtained by tagging a relatively high number of juvenile sharks but this information is not yet available.
According to a CSIRO white shark fact sheet dated February 8, this is because sampling in the south west is limited to older animals as there are no identified nursery grounds where samples of juveniles can be reliably collected.
This seems somewhat at odds with comments made by Fisheries Minister Dave Kelly on Friday, February 9.
In a statement Mr Kelly said a CSIRO report released this week had found there had been no increase in the southern-western population of white sharks and that there had been a downward trend in adult numbers since the early 2000s.
But in Looking for brothers and sisters among juvenile white sharks has provided the final pieces of information needed to estimate the size of populations in Australian waters it says 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,’’ lead author of the report Dr Richard Hillary said.
“Sharks take 12–15 years to become mature adults, so we wouldn’t expect to see the effect on the adult population of that reduction in juvenile shark mortality until the next few years.’’
“Now that we have a starting point, we can repeat the exercise over time and build a total population trend, to see whether the numbers are going up or down.
“This is crucial to developing effective policy outcomes that balance the sometimes conflicting aims of conservation initiatives and human-shark interaction risk management.”
The report said for the southern-western population, the 2017 estimate was 1460 adult white sharks with a range of 760 to 2250.
Mr Kelly said the report had shut down claims by federal Environment Minister Josh Frydenberg that ‘blind Freddy’ could see white shark numbers had increased off Western Australia.
The CSIRO white shark fact sheet said while there were examples where white sharks were abundant the incidence of attacks was low shark attacks were increasing not only in Australia but worldwide.
Yanchep News Online asked the McGowan Government that given 15 people have been killed by sharks in WA in the past 17 years and that the CSIRO is saying that estimating the total number of white sharks including juveniles off the WA coast could take another five years or more should the WA Govt in the interest of public safety and the state’s tourism industry be assessing whether protecting white sharks off the coast of WA needs to be balanced with public safety.
A McGowan Government spokeswoman said the government had introduced a number of new shark mitigation measures based on new technology and science, including the use of drones and personal shark deterrents.
The spokeswoman said the government would continue to look at any new measures that were proven to actually make beaches safer.
“Since March 2017 the McGowan Government has introduced a world-first shark deterrent subsidy, extended the shark monitoring network to Esperance, provided more than $6 million to Surf Life Saving WA to carry out patrols, allocated funding for beach emergency numbering signs and provided funding for a beach enclosure at Falcon,’’ she said.
“There has never been a fatal shark attack at a patrolled beach in WA in living memory.
“And while shark attacks have not had an impact on visitor numbers, Tourism WA advises tourists to swim between the flags at patrolled beaches.”