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Can we build a multisensory shark repellant?

Paper: Ryan, LA, Chapuis, L, Hemmi, JM, Collin, SP, McCauley, RD, Yopak, KE, Gennari, E, Huveneers, C, Kempster, RM, Kerr, CC, Schmidt, C, Egeberg, CA, Hart, NS, (2018).  Effects of auditory and visual stimuli on shark feeding behaviour: the disco effect.  Marine Biology.  165:11.  doi: 10.1007/s00227-017-3256-0


The need for shark repellant

Left: A finetooth shark caught in net (Credit: NOAA via Wikipedia Commons, Public Domain). Right: A shark sighted sign at a beach (Credit: Christine Cabalo, U.S. Marine Corps via Wikipedia Commons, Public Domain).


When human meets shark or shark meets human, the outcomes are not always positive.  We hear about shark attacks, the fear of which can trigger controversial shark culls, and we also hear how several species of sharks are endangered.  Many factors contribute to declining shark populations, but one is unintended catch in fisheries that are not only unwanted by conservationists, but also unwanted by fishermen.  So how do we prevent negative human-shark interactions?  How about using a shark repellant to keep both humans and sharks safe?  Some shark deterrent devices do exist.  The devices available focus largely on targeting their sense of smell, taste and their unique ability to detect electric currents.  However, these devices are not equally effective across all shark species and some only have a deterring effect within a short radius.  The lead co-authors of a new study–Ryan, Chapuis et al., from the University of Western Australia–wanted to investigate the response of sharks to visual, acoustic and multisensory cues to inform a possible alternative shark repellant by targeting different senses.


How to spook a shark?

Blue shark (Prionace glauca) swimming away from a disco from hell. Blue sharks are one species of shark where unintended catch in fisheries has caused a decline in their population.  (Shark image credit: Mark Conlin, SWFSC Large Pelagics Program. Fire, lasers and explosion graphics – CC0)

How would you spook a shark with light and/or sound?  How about intensely bright strobe lights or erratic, unnatural noises?  Put them together and what do you have?  A disco from hell.  I know what you’re thinking, would it be ethical to subject unwitting sharks to hell discos?  Well, there are guidelines and committees for that!  Sharks come in all different shapes and sizes and each has their own ecological role and range of visual and hearing abilities.  This study tested the effectiveness of light and sound in repelling three species of sharks in the lab and the field.  The lab study was approved by the University of Western Australia Animal Ethics Committee, and research in the field was approved by the South African Department of Environmental Affairs.  


To the lab!

Two species of bottom-dwelling sharks involved in the lab studies. Left: Port Jackson shark (credit: Olivier Cochard-Labbé via Wikipedia Commons CC-BY-SA. Right: Epaulette shark (credit: opencageinfo) CC-SA

In the lab, six wild-caught Port Jackson sharks (Heterodontus portusjacksoni) and five epaulette sharks (Heterodontus ocellatum) were placed in a tank with bait on one end, then exposed to several different sensory treatments.  These two species were selected for the study since they are popular in aquariums and their care requirements are well known.  Treatments included: strobe lights at two different speeds (5 Hz and 10 Hz–click for example but don’t click if you’re susceptible to seizures), sound by itself, and a combination of strobe lights and sound.  Two control treatments involved: sound played out of these sharks’ hearing ranges to control for the electrical fields generated by equipment and no stimulus.  The order of treatments was randomized and every individual was tested six times and video footage was analyzed for bait interaction and delay in bait taking.  


To the field!

A great white shark (Credit: Elias Levy via Wikipedia Commons, CC-BY-SA)

Out in the field, specifically near South Africa’s Seal Island, a tuna head was used to attract great white sharks to anchored boats rigged with bait (a canister of crushed sardines), GoPro cameras and sensory treatment equipment.  Four different treatments were tested: a control treatment with no stimuli, 10 Hz strobe light (there was no difference between 5 Hz & 10 Hz treatments in the lab), audible artificial sound, and the disco from hell (strobe lights and sound).  Video footage was then reviewed to identify individual sharks based on unique markings and assess behaviour change.  Did the shark bite the sardine canister at the same rate across all treatments?  Or was there a delay in the shark approaching the bait between treatments?  



In the lab, both Port Jackson sharks and epaulette sharks had similar results.  The erratic sound had no effect on bait biting.  The strobe light at both frequencies did lower the sharks’ bait interaction, but the strobe light and sound combination significantly decreased bait taking by both species.  The behavioural response was stronger for the Port Jacksons which went for the bait almost 50% less, compared to the control treatment.  However, there was no delay in bait taking with the Port Jacksons while there was for the epaulette sharks.  

As for the great white sharks, 25 great whites interacted with the setup, a total of 242 times.  There was a statistically significant difference in time the sharks spent within the field of view of the cameras that the bait was attached.  However, between the control treatment vs the strobe and sound combo, there was only a matter of 0.8 seconds difference.  In any case, the sharks still went for the bait across all treatments.  


There is no ‘one size’ fits shark

Overall, a multisensory approach seems like the best option to continue researching as it had a slight delaying effect on great whites. Individual sharks within the same species may also react to stimulus differently as observed in trials, further complicating the matter. Again, sharks have different visual systems and auditory ranges that serves them well in their habitat so they will respond to sensory stimulus differently.  For example, while both Port Jackson and epaulette sharks are mostly bottom-dwelling and nocturnal species, epaulette sharks live in the tropics and therefore may be less susceptible by the strobe lights.  They are also not top predators so are prone to being more skittish.  On the other hand, great whites are top predators which could explain some difference in response to treatments.  But also, bright natural sunshine in the wild vs comparatively low lighting of the lab may have decreased sensitivity to light treatments.  The authors suggest testing stronger strobe lights at different frequencies or more powerful sounds which are a challenge at frequencies low enough to be heard by sharks.  

Bottom line, it will take more than this first prototype ‘disco’ to repel great white sharks, although strobe lights, as designed in the study, may help prevent unwanted catch of some shark species.  In the end, more basic research about the extent and ability of sharks senses is needed if we want to build better shark repellents.  The applications of this research would help keep both humans and sharks safe and who wouldn’t want that?


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