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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

Synchronised video screenshots from the left and right cameras of a ReMoRA, which show C. carcharias interacting with the bait container during a control trial.The screenshots shown are three-dimensional, so for reference to size see Fig 2B. Using Event Measure software, the distance between a shark’s head (a) and the centre of the Shark Shield™ electrode (b) is calculated by comparing the length and angle of lines drawn between these two points on the left and right synchronised/calibrated video clips. The proximity of the shark in the screenshots displayed is 40 cm.
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pone.0157717.g003: Synchronised video screenshots from the left and right cameras of a ReMoRA, which show C. carcharias interacting with the bait container during a control trial.The screenshots shown are three-dimensional, so for reference to size see Fig 2B. Using Event Measure software, the distance between a shark’s head (a) and the centre of the Shark Shield™ electrode (b) is calculated by comparing the length and angle of lines drawn between these two points on the left and right synchronised/calibrated video clips. The proximity of the shark in the screenshots displayed is 40 cm.

Mentions: EventMeasure™ was used to identify and count the number of individuals, estimate individual lengths (where possible), measure time spent in the area, and quantify minimum distance (proximity) to the deterrent during encounters. The software allowed the synchronisation of calibrated stereo-video footage to facilitate the accurate recording of distance and size measurements in three-dimensional space. Time spent in the area was measured between a shark’s first and last appearance within the field-of-view of the cameras. Proximity measurements were obtained using standardised methods for recording fish lengths, as described in the literature [47–49,52–56], but rather than measuring total length, the distance between the closest part of a shark’s head and the center of the Shark Shield’s™ visible electrode was recorded instead (Figs 2 and 3). If poor visibility restricted the observer’s ability to accurately place proximity markers in a specific area of the shark’s head then a more distinct landmark was used instead, such as the eye, nostril or mouth. To ensure consistency, and remove individual bias, a single observer recorded all measurements, which were checked and corroborated by a second observer. A single proximity measurement was calculated for each encounter and defined as the closest observable distance a shark approached during an encounter with the control and active treatment, regardless of whether they interacted with the treatment or not. Therefore, even when a shark interacted by biting a bait canister, their closest proximity to the center of the electrode was still calculated. This allowed for the calculation of the highest electric field strength that a shark experienced during each encounter with an active Shark Shield™ (Figs 2 and 3).


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)

Synchronised video screenshots from the left and right cameras of a ReMoRA, which show C. carcharias interacting with the bait container during a control trial.The screenshots shown are three-dimensional, so for reference to size see Fig 2B. Using Event Measure software, the distance between a shark’s head (a) and the centre of the Shark Shield™ electrode (b) is calculated by comparing the length and angle of lines drawn between these two points on the left and right synchronised/calibrated video clips. The proximity of the shark in the screenshots displayed is 40 cm.
© Copyright Policy
Related In: Results  -  Collection

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

pone.0157717.g003: Synchronised video screenshots from the left and right cameras of a ReMoRA, which show C. carcharias interacting with the bait container during a control trial.The screenshots shown are three-dimensional, so for reference to size see Fig 2B. Using Event Measure software, the distance between a shark’s head (a) and the centre of the Shark Shield™ electrode (b) is calculated by comparing the length and angle of lines drawn between these two points on the left and right synchronised/calibrated video clips. The proximity of the shark in the screenshots displayed is 40 cm.
Mentions: EventMeasure™ was used to identify and count the number of individuals, estimate individual lengths (where possible), measure time spent in the area, and quantify minimum distance (proximity) to the deterrent during encounters. The software allowed the synchronisation of calibrated stereo-video footage to facilitate the accurate recording of distance and size measurements in three-dimensional space. Time spent in the area was measured between a shark’s first and last appearance within the field-of-view of the cameras. Proximity measurements were obtained using standardised methods for recording fish lengths, as described in the literature [47–49,52–56], but rather than measuring total length, the distance between the closest part of a shark’s head and the center of the Shark Shield’s™ visible electrode was recorded instead (Figs 2 and 3). If poor visibility restricted the observer’s ability to accurately place proximity markers in a specific area of the shark’s head then a more distinct landmark was used instead, such as the eye, nostril or mouth. To ensure consistency, and remove individual bias, a single observer recorded all measurements, which were checked and corroborated by a second observer. A single proximity measurement was calculated for each encounter and defined as the closest observable distance a shark approached during an encounter with the control and active treatment, regardless of whether they interacted with the treatment or not. Therefore, even when a shark interacted by biting a bait canister, their closest proximity to the center of the electrode was still calculated. This allowed for the calculation of the highest electric field strength that a shark experienced during each encounter with an active Shark Shield™ (Figs 2 and 3).

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