Spatial and temporal characterization of a recurrent scalloped hammerhead shark Sphyrna lewini aggregation using drones

Abstract

Scalloped hammerhead sharks Sphyrna lewini are threatened throughout their distribution. The species forms aggregations, which can potentially increase its vulnerability to human impacts. Identification and characterization of aggregation sites are essential to inform management and conservation. We used drones to characterize the spatial and temporal distribution of a recently documented scalloped hammerhead aggregation within the Shoalwater Islands Marine Park, in southwest Western Australia. A total of 267 drone transects were conducted across the Austral summers of 2019-2020 and 2020-2021. The presence or absence of the sharks was examined in relation to fine scale patterns in habitat type, environmental conditions, and the occurrence of human activities. Scalloped hammerhead sharks had a small spatial and temporal distribution within the study area. Their presence in the study area was higher on days associated with higher lunar illumination phases, which could indicate that the aggregation site is potentially acting as a refuge for resting and/or predatory avoidance. The extent to which human interactions affect scalloped hammerhead sharks at the aggregation site remains unclear and raises questions about the adequacy of management measures at Shoalwater Islands Marine Park. Recommendations are proposed to limit potential impacts, such as establishing spatiotemporal fishing restrictions and regulating public interactions.