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

Background

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.

Objectives
  1. To provide a scientifically valid, robust estimate for the east coast grey nurse shark population using 2006 - 2008 image capture-recapture data
  2. To create a cost effective, non-invasive, long-term monitoring program
  3. To determine important aspects of the population including – size and sex ratios and segration, site fidelity and occupancy, proportion of the population with fishing tackle (hooking injuries, trailing line, etc.).
  4. To increase public awareness about the plight of the grey nurse shark and to provide correct data for management purposes via an open and transparent mechanism that involves all interested parties.
Outcomes:

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.

 
Figure 1. A male grey nurse shark (Smiley) identified through images taken at Flat Rock in Queensland from 2004 for three consecutive years. The natural ‘spot’ markings, the sex, and the healed wound to the mouth are used to identify this shark.

Figure 2. Lasers provide a means to measure the total length of an individual grey nurse shark (without restraint). When the lasers are aligned parallel with the shark the distance between the two laser marks is 50 cm.