Determining the Australian east coast grey nurse population size and structure using archival and contemporary photographic images and visual surveys
A project undertaken at the Centre for Biomedical Sciences, Univeristy of Queensland, and supervised by C.Bansemer and M.Bennett
The grey nurse shark, Carcharias taurus is listed globally under the IUCN Red List as vulnerable, with the Australian east coast population listed as critically endangered. A priority action in the 'Commonwealth Recovery Plan for the Grey Nurse Shark' (Recovery Plan) is to establish long-term monitoring of the east coast’s population status. This key action has not yet been fulfilled.
In 2002, NSW Fisheries intiated a fin-tagging program of grey nurse sharks (GNS) to facilitate population estimates and achieve the Recovery Plan’s priority action. Unfortunately, the physical tagging of grey nurse sharks was found to have undesirable side-effects due to chronic fouling of tags that lead to abrasion injuries and secondary infections and consequently was stopped.
We conducted a two year study of the patterning of grey nurse shark skin, on animals held at Underwater World Mooloolaba (Qld aquarium). This study has shown that spot patterns are stable through time, providing a means to reliably indentify sharks from photographic images. A collecton of images of wild grey nurse sharks has been amassed by encouraging the diving public, through free workshops and random monthly prizes, to supply images of grey nurse sharks from known locations on known dates. To date, over 300 individual grey nurse sharks have been identified by their skin markings. Many of these individuals have been photographed on numerous occasions at different sites and on different dates with the longest image 'capture-recapture' period spanning 14 years between photographs. This demonstrates the viability of this technique for long-term monitoring.
This project will refine the image capture-recapture technique and allow a rigorous, scientifically valid estimate of current grey nurse shark numbers ('minimum population size') to be calculated achieving a priority action of the Commonwealth Recovery Plan for the critically endangered east coast population of grey nurse sharks. Additionally, the methodology used in this project lends itself to the involvement of divers, dive operators and the media. Increased public participation, awareness and compassion will all help to further protect this iconic species.