Population distribution, aggregation sites and seasonal occurrence of Australia’s western population of the grey nurse shark Carcharias taurus

Abstract

Western Australia (WA) is host to a genetically discrete population of grey nurse sharks Carcharias taurus, listed by the IUCN as Near Threatened based on mean annual catch rates prior to protection within Australian waters in 1997. Only one small aggregation has been documented in WA to date with little data available on population dynamics, or spatial and temporal distribution along the WA coastline. We analysed C. taurus data from 16 years (2006-2021) of diver observations (2347 sightings) and commercial fisheries bycatch records (574 sharks), finding evidence of 4 new aggregation sites. Sightings at Shark Cave, a popular dive site near Perth, were related to water temperature and time of year (non-linear), with sightings per unit effort generally higher during the warmer months of summer and autumn (20-22°C). Evidence of fishing injuries was found on 17% of all C. taurus photo-tagged at Shark Cave. Strong site philopatry was apparent at most sites, and included a male shark that returned for 12 consecutive years to the Exmouth Navy Pier. This study confirms the range of Australia’s western population of C. taurus from near the South Australia/WA border, north to Shark Bay, with distribution extending further north along the coast when sea temperatures are lower (May-December). The beneficial role of citizen science in large observation studies such as this one is supported. However, while this research greatly increases the C. taurus knowledge base in WA, a large information deficit still exists. The identification and assessment of further aggregation sites is recommended, underpinning future management and protection measures for this iconic species.