Comparing distributions of white, bull, and tiger sharks near and away from the surf break using three tech-based methods

Abstract

Unprovoked shark bites are one of the most recognised human-wildlife conflicts in the marine environment. Historically, management of this threat to public safety largely involved the implementation of lethal strategies. However, there is increasing environmental necessity and social pressure to adopt alternative strategies that minimise harm to sharks and other marine life. While different approaches have been developed that aim to reduce the risk of shark interactions with people, it is difficult to quantify their efficacy due to the rarity of shark bites and a lack of understanding of localised distributions of white (Carcharodon carcharias), bull (Carcharhinus leucas), and tiger (Galeocerdo cuvier) sharks, particularly close to surf breaks. Here, we used a rare opportunity to compare observations of these three ‘target’ shark species nearshore from drone-based surveillance to those caught on SMART (Shark-Management-Alert-in-Real-Time) drumlines and detected from VR4G acoustic receivers, with the latter two both located approximately 500 m offshore. Across 243 operational days, where all three methods were in use off three separate beaches on the east coast of Australia, we found no evidence for nearshore drone-based observations of target shark species to correlate with catches from SMART drumlines or detections from the VR4G acoustic receivers. The absence of correlation was still evident when 2-day, and 3-day moving averages of catches from SMART drumlines and detections from acoustic receivers were considered. While drone-based surveillance can incur sightability errors, it was evident that SMART drumlines do not prevent sharks from entering near the surf zone, and this does not coincide with the portion of sharks detected by the VR4G acoustic receivers. We contend that there is a need for a greater understanding of behavioural processes of large predatory sharks that may pose a threat to ocean users, particularly when considering non-destructive approaches to shark mitigation.