Mark-recapture population estimate and movements of Grey Nurse Sharks
|Mark-recapture population estimate and movements of Grey Nurse Sharks
Non Technical Summary
This project was done to obtain a mark/re-sighting estimate of the total population of Grey Nurse sharks in SE Australianwaters and to provide preliminary information on the animal's small- and large-scale movements. This involved tagging 24 sharksacross five sites in the coastal waters of New South Wales and southern Queensland. The sharks were caught, brought up to the surface and placed in a water-filled stretcher alongside a boat. Numbered tags were then inserted into the two dorsal fins. Finally, the lengths of the sharks were measured and then the animals were released.
Re-sightings of the tagged Grey Nurse sharks were provided by NSW Fisheries and recreational scuba divers, spearfishers andcommercial fishers. These observations showed that the same sharks occupied particular sites along the coast (termed critical habitatsites) at various times throughout the year. The sharks spent differing amounts of time at particular sites. The distancesmoved among these sites also varied and ranged from less than 100 kilometres to greater than 600 kilometres.
A survey was done in winter 2003 to obtain a standard, mark/recapture (re-sighting) estimate of the total number of Grey Nurse sharkscomprising the population along the SE Australian coast. Divers from NSW Fisheries, with assistance from recreational scuba diversand spearfishers, sampled 44 sites in the coastal waters of New South Wales and southern Queensland over a two-week period. Atotal of 313 sharks was observed and the number of male and female Grey Nurse sharks did not differ from a 1:1 sex ratio. Also,the sizes of juveniles and adults of both sexes showed an identical pattern of dispersion along the coast when compared with twoprevious winter surveys in 1999 and 2000. Of the sharks observed, 16 were tagged. The mark/ recapture (re-sighting) estimates fromthe data gathered showed that the total population of Grey Nurse sharks in the coastal waters of SE Australia was between 410and 461 individuals. These estimates indicated that 74 - 89% of adults and 68 - 79% of all individuals were observedduring diver surveys in the coastal waters of SE Australia.
Finally, 7 of the 24 tagged Grey Nurse sharks (i.e. 29.2%) had been hooked within one year of tagging. This independent estimate of the incidence of hooking is greater than the previous estimate obtained via the diver surveys over the period 1998 - 2001.