Seasonal variation in abundance and sex ratio of Grey Nurse (Sand Tiger) Sharks Carcharias taurus in northern New South Wales, Australia: a survey based on observations of recreational scuba divers

Abstract

The Grey Nurse or Sand Tiger Shark Carcharias taurus is a protected species in Australian waters. In order to gain an insight into this shark's migratory habits and relative abundance at popular recreational diving sites, a survey was conducted using the observations of recreational divers' in northern New South Wales coastal waters over 15 months from August 1996 to October 1997. The bulk of shark sightings were reported during seasons of low diver activity and when sea surface temperatures were around 20?21?C. The number of reported sightings in each month was adjusted for variations in diver activity (i.e., sampling effort) to give an index of shark abundance. More southerly sites experienced peak shark abundance from August to November 1996, whereas sharks were most common at more northerly sites from April to June 1997, suggesting either that the sharks were migrating northwards, or that seasonal movement into shallower waters was occurring later at the northern sites. The sex ratio of the population shifted from a majority of females in spring to a majority of males in autumn/winter at the northern sites, indicating that the movements of the sexes may differ. Management strategies for this species, such as providing adequate protection of habitat at critical localities and times, require more detailed knowledge of this shark's migratory pattern, and the timing of reproductive events.