Sharks inspire fear despite their rarity, but are drum lines the best solution? Photo: Alex Steyn // Unsplash
No ocean creature has a greater ability to strike fear into the hearts of surfers or swimmers than the shark. Though, by all accounts, they do not actively to hunt humans, sharks and people still inevitably collide. Whenever these incidents do happen, shark mitigation measures, specifically drum lines, come back to into popular discussion. However, alongside ethical considerations, the most vexing issue with drum lines is the fact that it’s surprisingly difficult to definitively know if they even work or not. We spoke to an expert in human/shark interactions to see why it is that these measures are so difficult to study, and what problems present themselves in researching them.
Drum lines, in their simplest form, are baited hooks attached to large buoys, like a massive fishing bobber. Depending on the type of drum line and the policy of those deploying it, sharks caught on the line are either killed on the spot or taken farther offshore and released. The aim of the devices is to reduce the incident of shark bites by lowering the amount of sharks in the areas where people use the ocean.
The places to most prominently deploy shark control measures are Australia, South Africa, and the French overseas department of Reunion Island. Queensland, Australia has used lethal drum lines along its coast since 1962. More recently, New South Wales, Queensland and Western Australia have all had trial runs for smart drum lines that allow for caught sharks to be released away from ocean-users. Kwazulu-Natal, South Africa, has implemented shark nets since 1952, but has begun to replace many of the nets with drum lines since 1999. In Reunion, regular operation of smart drum lines began in August 2015 and they are used in conjunction with bottom long lines.
At first glance, anecdotal evidence seems to point to the notion that mitigation measures such as drum lines and shark nets work. As ABC News reports, “Statistics from the NSW Department of Primary Industries indicate that before nets were introduced in NSW in 1936 there was an average of one fatal shark attack every year, but there has been only one fatal attack on a protected beach since then and that was in 1951.” The Kwazulu-Natal Sharks Board has reported a marked decrease in shark attacks since they first installed shark nets, and maintain that the drum lines are having the same effect. In Reunion, once the shark attack capital of the world, there have been no fatal shark attacks since the drum lines were installed.
On the other hand, detractors of shark mitigation measures also frequently claim the opposite, that these mitigation measures are proven to not work. Opponents of shark culls often cite a Hawaiian study in which over 4,500 sharks were killed, with no measurable effect on the rate of human/shark encounters. Others point to the 2019 trial in Western Australia that only managed to catch two white sharks over two years.
For insight into why it’s so hard to answer the seemingly simple question of whether drum lines work, I spoke to Leah Gibbs, an Associate Professor in the School of Geography and Sustainable Communities at the University of Wollongong. She is a social scientist who works in the discipline of human geography, with a broad interest in the relationships between people and nature. She approaches her work through a cultural and political perspective, but also has a background in the biophysical sciences, and has done research with marine biologists and coastal geomorphologists. She started working on sharks through a broader interest in our relationships with nature and how we interact with the non-human world, and has authored several publications on human/shark interaction.
The first problem is how little we know about sharks. “There’s not very many of them, so it’s very hard to design studies around sharks in general,” Gibbs told me. “It’s surprising how little we know about lots of species, regardless of the fact that great white sharks are in the popular imagination and in the media and in the movies.” As a result of the low numbers of sharks, human/shark encounters are also statistically exceedingly rare, even in a place like Reunion, which had a highest rate of fatal shark attacks per capita during the “Shark Crisis” from 2011 to 2017.
Then, there’s the inherent difficulty of designing studies around phenomena that take place in the natural world. “You can’t design a study that sets up ‘what if we did it this way or that way,’” said Gibbs. “There’s never going to be a case where you can say ‘Absolutely, we know for sure that it was this factor,’ because it’s not like a laboratory study situation where you have control groups.”
As a result, even though there may be fewer shark attacks in the places with drum lines, it’s far from definitive proof. “My understanding is that we can’t really say definitively that anecdotal evidence over a short period of time can point to the cause of more or fewer shark bites,” said Gibbs. “It might be quite promising that perhaps it’s helping, but it just doesn’t work to say ‘oh we’ve seen a decline, so therefore it must be the drum lines,’ because it could be other things. If you want to do a sort of scientific study, you can’t just say ‘It looks like it’s that one,’ you do have to determine what the other possibilities are. And the other possibilities could be a whole range of things. It could be that the ocean currents have changed over that period of time, so sharks are not coming in so close to areas where people are using the area for surfing or diving. It could be overfishing. It could be that the pressure on fisheries means sharks are going elsewhere to feed. It could be a whole bunch of different things. So it can point to the possibility of maybe it’s helpful, but it can’t tell us for sure.”
Conversely, those same reasons make it equally difficult to conclusively prove that the drum lines are not working. Without any way of isolating drum lines from the many, many variables at play when it comes to human/shark interactions, there’s just no way to say one way or the other. The lack of evidence that the mitigation measures work also doesn’t constitute proof to the contrary, though opponents sometimes claim so.
However, asking whether drum lines work to protect humans may be a distraction in and of itself. There have been very clear negative consequences observed in the use of drum lines. Killing too many sharks can upset marine ecosystems. In addition, unintended bycatch can kill other, non-target species, some of them threatened. Even “non-lethal” SMART lines can cause longterm harm to sharks, due to the stress and shock of being caught on a line. Focusing on the efficacy of drum lines, in the face of these known negative consequences, creates a false dichotomy of human versus shark. In a sense, it’s asking how well drum lines have protected humans in order to justify the harm we know they do.
“What’s more important, the person or the shark?” is something Gibbs gets asked all the time, but she thinks it’s the wrong question. “We shouldn’t have to devise methods that work for humans but are terrible for sharks, or the other way around. Humans are pretty clever and creative, and I think we should be able to devise methods that protect both people and sharks,” she said. “I think while we fix on the efficacy of a method that we know harms somebody, the sharks or the people, it’s preventing us from thinking more creatively about methods that work for all of us.”
“There’s a whole range of different kinds of approach: protect the person, protect the area, look out for animals. Also just the kind of more fundamental research about better understanding sharks is useful,” said Gibbs. For instance, there are the personal electronic deterrent devices that surfers are already using in Reunion. Researchers have also looked into using artificial kelp forests to deter sharks from certain areas. A PHD student Gibbs had worked with was even developing a blimp that would float over surfing beaches and observe them with a camera. All these solutions are more creative than a giant hook on a bobber. There could be even more possibilities out there, but we’re less likely to find them if we’re going back and forth on whether drum lines are effective enough to justify their use.
