First aggregation of grey nurse sharks (Carcharias taurus) confirmed in Western Australia

Abstract

Background: The population of grey nurse sharks (Carcharias taurus) found along the coast of Western Australia (WA) is listed as Near Threatened under the International Union for Conservation of Nature and yet has been the subject of minimal targeted research in WA. In contrast, the eastern Australian and southwest Atlantic subpopulations of C. taurus are afforded Critically Endangered status following a much greater research effort. Aggregation sites are important in the life cycle of C. taurus for mating and pupping, and their identification underpins conservation efforts. The present study set out to complete a detailed, multi-year assessment of C. taurus at the Point Murat Navy Pier in Exmouth, WA, where recreational divers have reported occasional sightings over many years. Results: Between 2007 and 2012 a permanent subsurface video camera was deployed, with the subsequent analysis of over 1000 days of underwater footage revealing sixteen C. taurus individuals, which were positively identified using spot patterns. Ten sharks returned to the site over multiple years, with presence/absence data displaying a strong negative correlation with water temperature. Sharks were never recorded above a mean daily sea temperature of 25.5 °C, indicating a possible upper threshold for aggregations of this population. Conclusions: While the study revealed a comparatively small gathering of C. taurus, the authors maintain that the systematic nature of visitations by individual sharks over a number of years qualifies the location as a noteworthy aggregation site, the first ever confirmed in Western Australia, and the northernmost documented for C. taurus in Australia to date.