Identification of the southernmost aggregation of scalloped hammerhead sharks (Sphyrna lewini) in Australia
Naima Andrea López – 18 February 2022
New publication
HIGHLIGHTS
· The presence of a recurrent aggregation of scalloped hammerheads (Sphyrna lewini) within Perth metropolitan waters has conservation implications for this Critically Endangered species.
· We conducted weekly aerial drone surveys over two Austral summers at the Shoalwater Islands Marine Park to document the status of the aggregation.
· The aggregation was reliably seen during the months of January and February, and the average length of the sharks was approximately 1.5 meters, making them sexually immature juveniles.
· The presence of this aggregation so far south appears to be a relatively recent phenomenon that may indicate a poleward shift in the distribution of the species as a result of warming oceans and expose these animals to greater fishing pressure.
· The species is currently listed as Conservation Dependent in Australia under the Environment Protection and Biodiversity Conservation Act (EPBC Act), although its conservation status is currently under review.
· The current catch of scalloped hammerheads in Western Australia is unknown and relying on historical catch composition data to understand current catches may be invalid. Until the contemporary composition of commercial and recreational hammerhead catches can be verified, both the State and Federal governments should strengthen protection of the species, especially at their aggregation sites.
ABSTRACT
Scalloped hammerheads (Sphyrna lewini) occur in tropical to subtropical waters across all ocean basins and are globally assessed as Critically Endangered by the International Union for the Conservation of Nature. In Australia, scalloped hammerheads range from Sydney, New South Wales (34°S; 151°E), around northern Australia, down to Geographe Bay, Western Australia (33°S; 115°E). However, in Western Australia, the species has rarely been recorded south of Jurien Bay (30°S; 115°E). We report a recurrent aggregation of scalloped hammerheads within the Shoalwater Islands Marine Park (32°S; 115°E), 240 km south of Jurien Bay, observed from drone footage collected during the 2019 and 2020 Austral summers. These new records challenge previous understanding of the distributional range of this Critically Endangered species and prompt questions about the adequacy of current protection measures.
Citation: López, N.A., McAuley, R.B. and Meeuwig, J.J. (2022), Identification of the southernmost aggregation of scalloped hammerhead sharks (Sphyrna lewini) in Australia. Austral Ecology. https://doi.org/10.1111/aec.13149
01 Hammerheads distribution
Distribution of scalloped (blue) and smooth hammerheads (purple) within Australian exclusive economic zone (EEZ).The bottom left inset shows the sampling area within the Shoalwater Islands Marine Park in Western Australia
02
Top, aerial drone image of the scalloped hammerhead aggregation.
Top, aerial drone image of the scalloped hammerhead aggregation. Bottom, underwater image of the aggregation highlighting the three main identification features. (a) Prominent rostrum median indentation; (b) larger base of anal fin compared to the second dorsal fin; (c) long free rear tip on second dorsal fin. Underwater image captured at the aggregation site within the Shoalwater Islands Marine Park, coincident with the same drone sampling date during the 2020 Austral summer season. Underwater image credit: Adapted from Brett Monroe.
03
Relative abundance of observed scalloped hammerheads
Per survey by month (a), and total length frequency histogram (b) for the 2019–2020 austral summer season.Mean abundance of individuals per survey (n) was 5 (SE 3.0) in December (n=12, 25% on which scalloped hammerheads were present), 19 (SE 8.3) in January (n=109, 7% of presence) and 22 (SE 4.8) in February (n=77, 15% of presence). No individuals were observed in March (n=34).
FUNDING AND ACKNOWLEDGEMENTS
This research was supported by an Australian Government RTP Scholarship, The Jock Clough Marine Foundation through the Oceans Institute Robson and Robertson Award and The Holsworth Wildlife Research Endowment & The Ecological Society of Australia to NAL. We thank Brett Monroe Garner for his image contribution and the Department of Biodiversity, Conservation and Attractions for providing the sea surface temperature data.