Australia’s east coast Grey Nurse Shark (Carcharias taurus) population is listed as critically endangered under the Environment Protection and Biodiversity Conservation Act 1999. The population has undergone a severe reduction in size due to activities such as fishing and shark control programs.
Population estimates for the east coast population range from 410─1660 individuals, with the most robust estimate at the upper end (1150─1660). These estimates have been based on photo identification techniques which are prone to bias due to site fidelity if the entire population is not sampled.
A robust estimate of population size and trend is the number one priority of a recovery plan developed by the Department of Environment in 2014.
This project will collate existing tissue samples and collect new samples from Grey Nurse Sharks at aggregation sites in Queensland and New South Wales (in partnership with the University of Queensland and the NSW Department of Primary Industries) and use close kin-mark recapture to estimate adult population size and trend.
Close-kin mark-recapture builds on techniques developed in Hub research for euryhaline elasmobranchs and white sharks. It uses genetic analysis of tissue samples to identify sharks that are related by sharing one parent (half-siblings). The number of half-siblings in a population is directly related to the number of breeding adults. (A smaller adult population will have a larger proportion of related sharks, and vice versa.)
The same genetic samples can be used to identify animals that share one grandparent, and this in turn can be used to estimate the number of adults in the previous generation. Comparing this estimate with the present number of adults shows the generational change in population size, or population trend.
Evidence-based estimates of population size and trend for the east coast Grey Nurse Shark will enable the recovery of the species to be assessed, and provide guidance for long-term monitoring and other actions to assist species recovery.
The project builds on earlier research funded by the Department of the Environment and the Office of the Threatened Species Commissioner.
Grey Nurse Shark. Image: David Harasti, New South Wales Department of Primary Industries