Do shark deterrents actually work? We look at the science

From magnets to smelly wax, eye-stickers to electric fields, shark deterrents have hit the market in a big way.

Their promise is tantalising: taking to the surf without fearing “the one you don’t see”; anglers reeling in whole fish instead of paying half to “the taxman”.

But as a researcher with an interest in shark attack mitigation, Daryl McPhee from Bond University says they’re about the only safety device in Australia he can think of that doesn’t have to meet any national standards.

“If you or I were to use a seatbelt, or a life-jacket, we don’t think about how they work — we recognise they have some sort of Australian standards, so we don’t have to think about that,” Dr McPhee says.

“That’s the missing piece in the shark deterrent landscape. In effect anybody can market a safety device and make wild claims [about its efficacy] and that’s allowed.”

While there is no silver bullet, the evidence shows some deterrents do actually work, while there’s very little scientific data to back up some of the claims made about others.

So here’s how they compare.

Electric-field-generating devices

Shark researcher Charlie Huveneers has tested electric, electro-magnetic, permanent magnetic and olfactory approaches attached or applied to replica surfboards and compared their effectiveness on great white sharks off the coast of South Australia.

Great whites cause around a third of shark fatalities in Australia, and are responsible for around 20 per cent of unprovoked bites.

After chumming the water with fish oil and minced tuna to attract the sharks, Professor Huveneers’ team from Flinders University floated out the surfboards with a piece of tuna dangling about 30 centimetres below — or about the depth of a human foot when a surfer sits on their board.

When there were no deterrent devices attached to the boards, great white sharks took the bait 96 per cent of the time. That’s out of nearly 300 tests.

But when an electrical-field-emitting device was attached, that dropped by more than half — down to 40 per cent.

Some small deterrent effect was observed in that trial for the other approaches, but was not statistically significant.

“The best device with the correct configuration can reduce the risk of shark bite by about 60 per cent,” Professor Huveneers says.

“People assume that when the electric deterrent did not work, it was because the shark was approaching at full speed in a ‘predatory mode’. But even in that situation, we often saw the deterrent stop the shark and preventing it biting.”

Chumming the water and presenting a bait may have intensified the sharks’ feeding behaviour, and it’s possible that the electric deterrent may be more effective for a surfer or swimmer in a real-life scenario without the attractants.

Other research found bull sharks were more than 40 per cent less likely to take a bait when an electric deterrent was present, and an electromagnetic device also had a significant deterrent effect.

How, then, does an electrical or electromagnetic current discourage sharks from snapping up tasty bait?

A sharks’ snout contains electroreceptors called ampullae of Lorenzini — tube-like structures full of thick fluid — that can pick up tiny electrical and magnetic fields. The theory is electric and electromagnetic deterrent devices overwhelm or confuse the shark.

So if they’re effective, why can’t we use them instead of shark nets or drumlines, which can be lethal for sharks and other marine life?

The strongest personal electric devices on the market right now come with a warning from at least one manufacturer that incorrect installation on a surfboard may cause “the electrical field [to] be felt on your fingertips when duck diving”.

Yet even at that power, their effective range is just a little over a metre.

That’s because electricity and electromagnetism dissipate exponentially in seawater. 

To replace shark nets, even very large devices would need to be spaced at very small intervals to create a consistent barrier.

Visual deterrents

Anyone living in range of a magpie during breeding season is probably familiar with this concept.

Cyclists paint or stick shapes to the back of helmets in an attempt to ward off the dive-bombing birds.

In nature, some species have evolved patterns that look like eyes. The fact those patterns have been evolutionarily selected for indicates they convey some sort of advantage.

Some species, such as big cats, are ambush predators, and are less likely to attack if they think they’re being watched. So the theory is, big eyes on a surfboard means sharks might stay away.

But in other cases, animals have evolved eye-like patterns to draw a predator to a less vulnerable body part. Rather than deterring a shark, putting eyes on a surfboard may in fact create a target, Professor Huveneers says.

“Many fish species have a black spot towards their back, referred to as a ‘false eye’. But in this case, it draw a predator’s attention away from the prey’s most vital body parts.

“A predator will target that big ‘eye’ thinking it’s the head, but because it’s not a vital area, the fish is still able to swim away and survive, even though it may be injured.”

Dr McPhee says he also “has concerns” about this approach to shark deterrence.

“It might actually heighten the risk.

“It is potentially making you look more like prey. It would certainly give sharks something to orientate where’s the front and where’s the back.”

One manufacturer of a visual deterrent says many professionals who make their living on the water, including some surfers, abalone divers, and marine scientists, agree that sharks change their behaviour once they know they’ve been spotted.

It’s also claimed that visual deterrents may work up to 15 metres away, giving them an advantage over sensory devices.

However, the manufacturers don’t claim their approach, nor any others, are 100 per cent effective.

There is still room for more independent research to establish the effectiveness or lack thereof of this technology, and whether different species respond differently.

But both Professor Huveneers and Dr McPhee say there’s limited independent scientific evidence at this point to suggest the visual deterrent approach will work.  

Part of the problem, Dr McPhee says, is that it’s very difficult to study these things without endangering lives. And anecdotal evidence has to be taken with a grain of salt.

“I met a surfer from Reunion Island and his friend painted broccoli on the bottom of his board because sharks don’t eat broccoli.”

Although it was obviously a joke, Dr McPhee says people using marketed deterrents can fall into this logical trap.

“I have broccoli painted on my board and I haven’t been eaten, therefore it works.”

Smelly repellents and permanent magnets

Another approach is using chemical and olfactory deterrents (smell) to repel attacks.

Back in World War II, the US Navy developed a mix of copper acetate and black dye that they hoped would mimic the smell of dead shark to repel living ones.

Following the sinking of the USS Indianapolis in 1945, they released the product to try to counter horror stories of sharks picking off sailors as they floated for days waiting to be rescued.

“They basically knew it was a placebo,” Dr McPhee says.

“That was sort of the first ever ‘deterrent’. They guaranteed it worked but later on admitted that it didn’t.”

But the real smell of dead shark has been shown to repel some species.

Necromones — chemicals sourced from putrefying shark tissue — were experimentally shown to halt feeding in Caribbean reef sharks and blacknose sharks.

And there are some olfactory deterrents on the market that use these necromones in an underwater spray or slow release mechanism.

But Dr McPhee says that doesn’t mean it will work on all species.

“Things like tiger sharks and white sharks eat other sharks.

“That’s not a criticism of that particular approach. It just means there’s no magic bullet.”

One manufacturer has loaded surfboard wax with compounds like cayenne pepper and essential oils.

This product was included in Professor Huveneers’ study, although it had no significant repellent effect on great whites.

It’s important to point out that was the result of one study, and is not conclusive proof that particular olfactory deterrent doesn’t work.

The creator of that product questioned the results of Professor Huveneers’ study in a social media post, arguing that the wax was never intended to be used in circumstances where sharks were “agitated into feed mode” by the chum.

And then there are magnets. Whereas electromagnetic devices use power to amplify a magnetic field, some rely on unpowered or permanent magnetism.

“There is clearly some evidence that sharks can detect a magnetic field,” Dr McPhee says.

A 2018 study found that magnets attached to snapper fishing nets reduced the catch of unwanted elasmobranchs (sharks, rays, skates) by a third.

But like electricity, magnetic fields dissipate quickly over distance, and as a general rule, the smaller the magnet, the smaller the force.

Dr McPhee says permanent magnets will only have an impact at very close range.

“The magnetic field created by a wearable magnet is so small it can be measured in centimetres.”

One product claims to have an effective range of 1–2 metres, and that the movement of the wearer increases the strength of the field.

A video on the website of one of these magnetic repellent manufacturers shows sharks appearing to avoid a dummy wearing the device. Sharks later attack a dummy that is not fitted with the magnets.

An independent undergraduate researcher, according to the website, says they verified the observations made in the video and confirmed that bull sharks were less likely to attack the dummy wearing the magnetic repellents.

Orcas calling for shark-free fishing?

Sharks aren’t just an issue for swimmers. Commercial and recreational fishers are reporting more and more depredation — where sharks take hooked fish.

Similar technologies that are being used for personal protection are now being trialled for fishing, and many companies are claiming promising results.

Gary Jackson from Western Australia’s Department of Primary Industries and Regional Development (DPIRD) fisheries team looked at three deterrent approaches — electric, magnetic, and acoustic.

When they pooled results from all three approaches, they found a significant reduction in fish being taken by sharks.

“You were able to retain 60 per cent more of your catch in a fishing session with a device in the water than without,” Dr Jackson says.

“We were quite surprised. That’s a very good result.”

He’s since done a follow up study to compare each device individually but it’s too early to release the results, other than to say one is a fairly clear leader, and one is not as effective as the other two.

The acoustic approach involved an American product which, through an underwater speaker, played the sound of a familiar shark predator: orcas.

A similar device is being trialled here, though it’s in the very early stages of development, Dr Jackson says.

“[The developer] gave a presentation and … he plays the sound of orcas and it seems to scare the bejeebers out of everything — dolphins, whales, the whole shebang.

“[That’s] a whole other set of environmental issues to think about.”

Shark deterrent research in general is a “very active space”, Dr Jackson says, and potentially there will be some big breakthroughs.

“Potentially someone is going to crack it and make a lot of money.”

But it’s also, for the time being, a space at the mercy of confirmation bias and wishful thinking, Dr McPhee says.

“No-one can legally make a seat belt in their garage and sell it as an effective safety device. The same should apply to shark deterrents.”