SMART Drumlines Ineffective in Catching White Sharks in the High Energy Capes Region of Western Australia: Acoustic Detections Confirm That Sharks Are Not Always Amenable to Capture

Abstract

The management of human-shark interactions can benefit from the implementation of effective shark hazard mitigation measures. A Shark-Management-Alert-in-Real-Time (SMART) drumline trial in the Capes region of Western Australia was instigated after several serious incidents involving surfers and white sharks (Carcharodon carcharias). The project aimed to determine whether white sharks (target species), which were relocated after capture, remained offshore using satellite and acoustic tagging. Over a 27-month period, 352 fish were caught, 55% of which comprised tiger sharks (Galeocerdo cuvier). Ninety-one percent of animals were released alive in good condition. Only two white sharks were caught; both were relocated ≥ 1 km offshore before release and moved immediately further offshore after capture, remaining predominately in offshore waters for the duration of their 54-day and 186-day tag deployments. Our results confirm that desirable animal welfare outcomes can be achieved using SMART drumlines when response times are minimised. The low target catches and the detection of 24 other tagged white sharks within the study area supported the decision to cease the trial. Our results reiterate there is no simple remedy for dealing with the complexities of shark hazards and reinforce the importance of trialing mitigation measures under local conditions.

Keywords: bather protection; human–wildlife conflict; power analysis; shark attack.

Figure 1

Map of the Capes region of Western Australia highlighting the 10 fixed locations where SMART drumlines were deployed and the locations for the VR2 and VR4G or Rx LIVE receivers. Left panel displays the location of the drumlines in relation to popular surf breaks and the receivers within the Gracetown array. Right panel displays the primary and secondary receiver arrays and the location of the Capes region in relation to the state capital of Perth.

Figure 2

Relationship between the power of an experiment for varying sample size. This assumes a one-way test of one-sample proportion, with small, medium and large effect sizes specified by 0.2, 0.5 and 0.8, respectively, corresponding to an increase in proportion (from the base level with no mitigation) of approximately 0.09, 0.22 and 0.34 respectively. The effect size for two proportions are calculated using the arcsine transformation of the proportions. Numbers in colour denote the number of white sharks that would be required to be caught and relocated in order to detect an effect size.

Figure 3

Location of VR2 (open dots) and VR4 (open triangles) receivers off Gracetown with major surf breaks (black squares) and SMART drumline locations (filled triangles) indicated. Arrows are inferred straight-line movements for sharks between successive detection locations (solid dots). Relocation paths (dashed line) to release points and detections (solid dots) are presented for: (a) white shark 1; and (b) white shark 2. All acoustically tagged white sharks that were detected within the Gracetown array while SMART drumlines were being actively fished are presented in (c,d) with colours denoting separate movements through the array.

Figure 4

Estimated tracks of: (a) a 460 cm TL white shark caught on 25 April 2019, tag duration of 54 days; and (b) a 330 cm TL white shark caught on 20 August 2019, tag duration of 186 days. Tracks are based on model-estimated daily locations from PAT tags using GPE3.

Figure 4

Estimated tracks of: (a) a 460 cm TL white shark caught on 25 April 2019, tag duration of 54 days; and (b) a 330 cm TL white shark caught on 20 August 2019, tag duration of 186 days. Tracks are based on model-estimated daily locations from PAT tags using GPE3.