Date Published: March 11, 2019
Publisher: Public Library of Science
Author(s): Channing A. Egeberg, Ryan M. Kempster, Nathan S. Hart, Laura Ryan, Lucille Chapuis, Caroline C. Kerr, Carl Schmidt, Enrico Gennari, Kara E. Yopak, Shaun P. Collin, Rui Coelho.
Personal shark deterrents offer the potential of a non-lethal solution to protect individuals from negative interactions with sharks, but the claims of effectiveness of most deterrents are based on theory rather than robust testing of the devices themselves. Therefore, there is a clear need for thorough testing of commercially available shark deterrents to provide the public with information on their effectiveness. Using a modified stereo-camera system, we quantified behavioural interactions between Carcharodon carcharias (white sharks) and a baited target in the presence of a commercially available electric anklet shark deterrent, the Electronic Shark Defense System (ESDS). The stereo-camera system enabled accurate assessment of the behavioural responses of C. carcharias when approaching an ESDS. We found that the ESDS had limited meaningful effect on the behaviour of C. carcharias, with no significant reduction in the proportion of sharks interacting with the bait in the presence of the active device. At close proximity (< 15.5 cm), the active ESDS did show a significant reduction in the number of sharks biting the bait, but this was countered by an increase in other, less aggressive, interactions. The ESDS discharged at a frequency of 7.8 Hz every 5.1 s for 2.5 s, followed by an inactive interval of 2.6 s. As a result, many sharks may have encountered the device in its inactive state, resulting in a reduced behavioural response. Consequently, decreasing the inactive interval between pulses may improve the overall effectiveness of the device, but this would not improve the effective deterrent range of the device, which is primarily a factor of the voltage gradient rather than the stimulus frequency. In conclusion, given the very short effective range of the ESDS and its unreliable deterrent effect, combined with the fact that shark-bite incidents are very rare, it is unlikely that the current device would significantly reduce the risk of a negative interaction with C. carcharias.
As human populations increase, more people continue to enter the ocean for leisure, resulting in an increase in human-shark interactions globally [1, 2]. Although negative interactions between humans and sharks are extremely rare, each incident attracts a high level of interest, as they often result in serious consequences for those involved. Despite the worldwide media attention that shark bite incidents receive, over 80% of them have occurred in just 6 regions: The United States, Australia, South Africa, Brazil, The Bahamas, and Réunion Island . In response, all of these regions (except The Bahamas) have, at some point, instituted some form of government-controlled mitigation strategy in an attempt to reduce the number of shark bite incidents in their waters [3–6]. Unfortunately, most of these strategies have involved the removal of sharks in order to reduce the local population, yet no evidence has been presented to support the effectiveness of such programs in reducing the risk of a negative encounter with a shark [3, 7]. Furthermore, these programs are at odds with the important ecological role that sharks play in ocean ecosystems [8, 9]. Since these control programs do not discriminate by species or size, they place increased pressure on non-target and potentially vulnerable species [10–13], including elasmobranchs and marine mammals, the effects of which could be ecologically and economically damaging [9, 14–18]. There is, therefore, a clear need for alternative non-lethal shark mitigation solutions that will allow humans and sharks to safely co-exist.
A total of 17 control deployments (inactive ESDS) and 17 active deployments (active ESDS) were conducted (totalling 51 hours of video footage), which resulted in 395 encounters (238 control; 157 active) from 44 individual C. carcharias.
Initial observation of C. carcharias interactions with an active ESDS might suggest that the device was having a repellent effect, as significantly fewer individuals were observed biting (Type 2 Interactions) the active device compared with the control (Fig 4). Furthermore, when only considering interactions (not proximity), the observed effect remained constant even after multiple encounters, suggesting that a shark’s behaviour was not changing over time in the presence of the active device. However, when considering proximity, sharks did show evidence of habituation as they would approach closer with each subsequent encounter (Fig 5B). When you also account for sharks bumping the device as well as biting (Type 1 and 2 Interactions), there was no significant difference in the effectiveness of the active ESDS over the inactive control (Fig 4). Thus, any effect that the active ESDS may of been having was at such a short range that the sharks would likely have only experienced it when they were about to bite.