A surf, swim or splash in the sea can be welcome relief from the sweltering summer sun. But the fear of shark attacks is pervasive (even though by the numbers it’s a small problem – you’re more likely to by killed by the heat).
It’s not surprising that a suite of shark attack mitigation technologies has swept onto the scene, some decades old while others are brand new. Let’s look at them, in turn, to see how effective they really are.
Hook, drum line and sinker
It’s often assumed that nets form a physical barrier that stops sharks approaching beaches. But they tend to do this permanently – most are gill nets, which tangle sharks in the mesh so they drown. Drum lines do the same, but with a large baited hook hanging from a floating buoy.
And they do catch quite a lot of sharks. Queensland nets and lines, for instance, snare an average of 787 sharks each year. In the 2014-15 meshing season in New South Wales, 44 target – or dangerous – sharks were caught.
But these methods aren’t picky – any animal will do. In that same 2014-2015 meshing season in New South Wales, some 77% of animals caught in nets were by-catch – non-dangerous sharks, dolphins and turtles – and many were threatened species.
New, “smart” drum lines aim to lower this collateral damage by alerting rangers when an animal is caught, but it can be difficult to release them in time.
Besides, there aren’t any conclusive studies showing that these technologies reduce shark attacks, says Leah Gibbs from the University of Wollongong in Australia – and interpreting the data can be fraught.
“When we say ‘are they effective?’ it’s a really difficult research question. Because what are you actually trying to determine? Are they effective at catching sharks? Or are they effective at reducing the number of bites on people?”
With relatively few shark attacks anyway, untangling fluctuations due to random chance versus underlying patterns is tricky.
For instance, since the introduction of the Shark Meshing Program in 1937, there’s been only one fatal shark attack at a meshed beach in New South Wales.
But for this to be relevant, we need to know how many people died from shark attacks at these beaches before the nets and deduce if a drop in deaths was because of fewer attacks or something else, such as better first aid.
Attack rates at meshed beaches have, indeed, decreased a little – prior to net installation at now-meshed beaches, there were 37 attacks. Afterwards, there have been 23.
But overall, because of more attacks in non-meshed areas, the Department of Fisheries report into the New South Wales Shark Meshing Program in 2009 suggested that the total annual rate of attack was the same before and after meshing began in 1939.
Sharks detect electrical currents to navigate, find prey and avoid danger through a sensory organ on their head called the ampullae of Lorenzini.
So the idea behind electrical deterrents is to send out a strong pulse to blast surrounding sharks – a bit like an electrical warning that the area should be avoided.
Although shark biology suggests it should work, only one type of this technology has undergone independent testing.
Ryan Kempster, an ecologist at the University of Western Australia, and colleagues tested whether a device called Shark Shield would deter great white sharks (Carcharodon carcharias) in South Africa.
Underwater cameras monitored an electrical deterrent attached to bait. All sharks were repelled by Shark Shield.
Even though they did build a tolerance to the electrical signal over time, and gradually came closer to the bait – to around two metres – no shark actually took it.
A common fear is that sharks might be drawn to an electrical stimulus, not repelled. But there’s no evidence that the device attracts sharks.
Different shark species can also have different reactions to electrical stimuli, so the researchers can’t say for sure Shark Shield will work on all sharks. But great whites are involved in most fatalities, so an adequate deterrent for this species would be a good start.
Now you see me, now you don’t
Some research shows that in the final stage of an attack, a shark’s vision is its primary sense. Could disrupting how it sees divert an attack, or delay it long enough to allow a safe escape?
Despite past beliefs that yellow is attractive to sharks – which is why it’s been called “yum yum yellow” – colour itself doesn’t influence shark attacks. Rather, it’s particularly bright and contrasting shades of grey that seem to stand out.
Two wetsuit designs exploit shark vision. One mimics the contrasting stripes of a sea snake, which is poisonous to sharks, and the other uses blue-green colouration to blend the wearer into the water.
Comprehensive, independent studies have not explored how well visual shark deterrents we, but one identified problem is that a shark approaching from below is likely to see a person as a dark silhouette against the bright sky – no matter what colour wetsuit they are wearing.
With this in mind, plans are underway to reduce this contrast. LED lights placed on the underside of a surfboard provide counter-illumination, a technique used by some aquatic animals, such as the firefly squid, to avoid predators.
In reality, the best technology we have is probably first aid. Modern skills and medicine mean that the chances of shark attack survival are around 80%.