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How Close is too Close? The Effect of a Non-Lethal Electric Shark Deterrent on White Shark Behaviour.

Kempster RM, Egeberg CA, Hart NS, Ryan L, Chapuis L, Kerr CC, Schmidt C, Huveneers C, Gennari E, Yopak KE, Meeuwig JJ, Collin SP - PLoS ONE (2016)

Bottom Line: Therefore, there is a clear need for thorough testing of commercially available shark deterrents to provide the public with recommendations of their effectiveness.With each subsequent encounter, their proximity decreased by an average of 11.6 cm, which corresponded to an increase in tolerance to the electric field by an average of 2.6 (± 0.5) V/m per encounter.The results of this study provide quantitative evidence of the effectiveness of a non-lethal electric shark deterrent, its influence on the behaviour of C. carcharias, and an accurate method for testing other shark deterrent technologies.

View Article: PubMed Central - PubMed

Affiliation: The Oceans Institute and the School of Animal Biology, The University of Western Australia, Crawley, Western Australia, Australia.

ABSTRACT
Sharks play a vital role in the health of marine ecosystems, but the potential threat that sharks pose to humans is a reminder of our vulnerability when entering the ocean. Personal shark deterrents are being marketed as the solution to mitigate the threat that sharks pose. However, the effectiveness claims of many personal deterrents are based on our knowledge of shark sensory biology rather than robust testing of the devices themselves, as most have not been subjected to independent scientific studies. Therefore, there is a clear need for thorough testing of commercially available shark deterrents to provide the public with recommendations of their effectiveness. Using a modified stereo-camera system, we quantified behavioural interactions between white sharks (Carcharodon carcharias) and a baited target in the presence of a commercially available, personal electric shark deterrent (Shark Shield Freedom7™). The stereo-camera system enabled an accurate assessment of the behavioural responses of C. carcharias when encountering a non-lethal electric field many times stronger than what they would naturally experience. Upon their first observed encounter, all C. carcharias were repelled at a mean (± std. error) proximity of 131 (± 10.3) cm, which corresponded to a mean voltage gradient of 9.7 (± 0.9) V/m. With each subsequent encounter, their proximity decreased by an average of 11.6 cm, which corresponded to an increase in tolerance to the electric field by an average of 2.6 (± 0.5) V/m per encounter. Despite the increase in tolerance, sharks continued to be deterred from interacting for the duration of each trial when in the presence of an active Shark Shield™. Furthermore, the findings provide no support to the theory that electric deterrents attract sharks. The results of this study provide quantitative evidence of the effectiveness of a non-lethal electric shark deterrent, its influence on the behaviour of C. carcharias, and an accurate method for testing other shark deterrent technologies.

No MeSH data available.


Related in: MedlinePlus

Schematic of the equipment used to measure the voltage gradient of the Shark Shield™.For clarity, the electrodes of the Shark Shield™ are displayed in white to highlight their position.
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pone.0157717.g004: Schematic of the equipment used to measure the voltage gradient of the Shark Shield™.For clarity, the electrodes of the Shark Shield™ are displayed in white to highlight their position.

Mentions: To estimate the electric field gradient that a shark experiences during each encounter, a voltage gradient probe was constructed and connected to an oscilloscope to record the electric field gradient at set distances, and angles, relative to an active Shark Shield™ (Fig 4). The probe consisted of two electrodes separated by 10 cm (a separation distance of 10 cm was necessary to detect a change in voltage gradient over the background noise while maintaining a high enough resolution to accurately determine changes in the electric field gradient with increasing distance). Measurements were recorded at 50 cm intervals proximal to an active electrode in both perpendicular and parallel planes, to determine the effect of the probe’s angel (relative to the electrode) on the field strength recorded. These measurements were then used to plot a curve to estimate the voltage gradient decline with increasing distance. Measurements were recorded in a sheltered bay with a bottom depth of 4 m, at a temperature and salinity consistent with Mossel Bay (15°C; 37 ppt). Due to the shallow depth, the Shark Shield™ was positioned horizontally to allow the probe to be positioned at distances greater than the vertical depth would have allowed (Fig 4). The shallow depth was also necessary to allow the probe to be accurately positioned by an experimenter and to minimise wave disturbance. However, the proximity of the Shark Shield™ to the seabed and the surface is likely to have an effect on the distribution of the electric field. Furthermore, for logistical reasons, the voltage gradient of the Shark Shield™ was measured by itself, without being attached to a ReMoRA. Therefore, electric field measurements presented in this study should only be used as an estimate and not absolute, as they are likely to vary depending on the conditions in which the device is used. Finally, an inactive Shark Shield™ was also measured, to confirm that no voltage gradient (above background noise) was produced when the device was turned off.


How Close is too Close? The Effect of a Non-Lethal Electric Shark Deterrent on White Shark Behaviour.

Kempster RM, Egeberg CA, Hart NS, Ryan L, Chapuis L, Kerr CC, Schmidt C, Huveneers C, Gennari E, Yopak KE, Meeuwig JJ, Collin SP - PLoS ONE (2016)

Schematic of the equipment used to measure the voltage gradient of the Shark Shield™.For clarity, the electrodes of the Shark Shield™ are displayed in white to highlight their position.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4930202&req=5

pone.0157717.g004: Schematic of the equipment used to measure the voltage gradient of the Shark Shield™.For clarity, the electrodes of the Shark Shield™ are displayed in white to highlight their position.
Mentions: To estimate the electric field gradient that a shark experiences during each encounter, a voltage gradient probe was constructed and connected to an oscilloscope to record the electric field gradient at set distances, and angles, relative to an active Shark Shield™ (Fig 4). The probe consisted of two electrodes separated by 10 cm (a separation distance of 10 cm was necessary to detect a change in voltage gradient over the background noise while maintaining a high enough resolution to accurately determine changes in the electric field gradient with increasing distance). Measurements were recorded at 50 cm intervals proximal to an active electrode in both perpendicular and parallel planes, to determine the effect of the probe’s angel (relative to the electrode) on the field strength recorded. These measurements were then used to plot a curve to estimate the voltage gradient decline with increasing distance. Measurements were recorded in a sheltered bay with a bottom depth of 4 m, at a temperature and salinity consistent with Mossel Bay (15°C; 37 ppt). Due to the shallow depth, the Shark Shield™ was positioned horizontally to allow the probe to be positioned at distances greater than the vertical depth would have allowed (Fig 4). The shallow depth was also necessary to allow the probe to be accurately positioned by an experimenter and to minimise wave disturbance. However, the proximity of the Shark Shield™ to the seabed and the surface is likely to have an effect on the distribution of the electric field. Furthermore, for logistical reasons, the voltage gradient of the Shark Shield™ was measured by itself, without being attached to a ReMoRA. Therefore, electric field measurements presented in this study should only be used as an estimate and not absolute, as they are likely to vary depending on the conditions in which the device is used. Finally, an inactive Shark Shield™ was also measured, to confirm that no voltage gradient (above background noise) was produced when the device was turned off.

Bottom Line: Therefore, there is a clear need for thorough testing of commercially available shark deterrents to provide the public with recommendations of their effectiveness.With each subsequent encounter, their proximity decreased by an average of 11.6 cm, which corresponded to an increase in tolerance to the electric field by an average of 2.6 (± 0.5) V/m per encounter.The results of this study provide quantitative evidence of the effectiveness of a non-lethal electric shark deterrent, its influence on the behaviour of C. carcharias, and an accurate method for testing other shark deterrent technologies.

View Article: PubMed Central - PubMed

Affiliation: The Oceans Institute and the School of Animal Biology, The University of Western Australia, Crawley, Western Australia, Australia.

ABSTRACT
Sharks play a vital role in the health of marine ecosystems, but the potential threat that sharks pose to humans is a reminder of our vulnerability when entering the ocean. Personal shark deterrents are being marketed as the solution to mitigate the threat that sharks pose. However, the effectiveness claims of many personal deterrents are based on our knowledge of shark sensory biology rather than robust testing of the devices themselves, as most have not been subjected to independent scientific studies. Therefore, there is a clear need for thorough testing of commercially available shark deterrents to provide the public with recommendations of their effectiveness. Using a modified stereo-camera system, we quantified behavioural interactions between white sharks (Carcharodon carcharias) and a baited target in the presence of a commercially available, personal electric shark deterrent (Shark Shield Freedom7™). The stereo-camera system enabled an accurate assessment of the behavioural responses of C. carcharias when encountering a non-lethal electric field many times stronger than what they would naturally experience. Upon their first observed encounter, all C. carcharias were repelled at a mean (± std. error) proximity of 131 (± 10.3) cm, which corresponded to a mean voltage gradient of 9.7 (± 0.9) V/m. With each subsequent encounter, their proximity decreased by an average of 11.6 cm, which corresponded to an increase in tolerance to the electric field by an average of 2.6 (± 0.5) V/m per encounter. Despite the increase in tolerance, sharks continued to be deterred from interacting for the duration of each trial when in the presence of an active Shark Shield™. Furthermore, the findings provide no support to the theory that electric deterrents attract sharks. The results of this study provide quantitative evidence of the effectiveness of a non-lethal electric shark deterrent, its influence on the behaviour of C. carcharias, and an accurate method for testing other shark deterrent technologies.

No MeSH data available.


Related in: MedlinePlus