World first genetic analysis of the white shark population in Australasian waters shows the numbers of the threatened species are lower than scientists thought.
White sharks are also widely known in New Zealand and Australia as white pointers or great white sharks.
Research published in scientific journal Nature Scientific Reports, and updated with new samples and analyses, estimates the total number of adult white sharks across the Australasian region is only around 2,210.
Department of Conservation marine technical adviser Clinton Duffy says the research is significant because it's the first time it has been possible to estimate the total number of adults in the New Zealand white shark population.
The total number of adults in the 'Eastern' population, which includes New Zealand, is estimated to be about 750. Adding juveniles to the numbers results in an estimated total population size of 5,460.
This new information shows how vulnerable the species is - Clinton Duffy, Department of Conservation marine technical adviser
"We had assumed the population was low because of the slow breeding and growth rate of white sharks, but the numbers are a bit lower than we thought," Mr Duffy said.
White sharks migrate seasonally between New Zealand, Australia and the islands of the south-west Pacific. Their threat classification status in New Zealand waters was assessed in 2005 as 'Declining'.
"This new information shows how vulnerable the species is," Mr Duffy said.
The main threats to white sharks in New Zealand waters are through accidental by-catch in fisheries, particularly for small juvenile sharks on long lines and adults in set nets, he said.
The research was led by scientists from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Hobart.
Scientists from DOC and National Institute of Water and Atmospheric Research (NIWA) contributed to the study. They provided genetic samples from white sharks tagged near Stewart Island, Chatham Islands and the New Zealand mainland, and from sharks accidentally caught by fishers.
NIWA principal scientist Malcolm Francis says estimating the abundance of large sharks is difficult because the tools normally used by fisheries scientists, like trawl surveys, acoustic surveys or tag-recapture experiments, don't work well for them.
"This new genetic technique offers strong promise for monitoring rare and threatened species," he said.
Dr Francis says NIWA and DOC have been jointly studying white sharks in New Zealand for more than 10 years, and the opportunity to collaborate with their Australian colleagues in this study is an important breakthrough in understanding the status of the Australasian white shark population.
White sharks are absolutely protected in New Zealand waters and if people accidentally catch one they must release it immediately alive and unharmed, and are required to notify DOC.