Kelly Slater is a surfing legend, he’s also a keen supporter of environmental issues, which is why his recent comments about a shark cull for Reunion Island are so off the mark. However, he has called for evidence to counter his claim. As a surfer, marine biologist and shark enthusiast in general, this seems like a good reason put together a scientifically defensible reason as to why shark culls won’t work, and maybe even make the problem worse.
Before starting, I would like to clarify that Kelly’s calls for evidence to counteract calls for culling are a responsible thing to do (although arguably, calling for a cull before researching this may not be). He has received a huge amount of abuse on social media, and very little in the way of constructive arguments against culling. I do think the calls for a cull were a genuine response of concern for victims of the attacks. However, I hope to address here that 1) there is no evidence that shark attacks are out of control in Reunion and they pose a low risk, 2) there is a likely reason for a ‘slight’ increase in the number of attacks over time, 3) culling may make the problem of attacks worse. The links all point to peer reviewed journal articles – while these are official links, you may find you can obtain access to them without paying by copying and pasting the article title into Google or Google Scholar.
The Reunion problem in numbers
The headlines are 20 shark attacks since 2011 (eight of which have been fatal). However, 2011 itself saw five attacks, meaning there have been less than three per year on average since 2012. Scientific studies have shown that the rate is slowly increasing (see below for why), and on long-term data collected since 1980, typically there would now be about two attacks per year. So, with the exception of 2011, nothing major has changed, there hasn’t been a recent surge of attacks that can be statistically validated as different from long-term trends.
The risk is also relatively low. The population of Reunion is around 850,000 with another 405,000 visitors each year. As a quick sweeping assumption, if half of these people go in the sea, then that means going in the sea in a typical recent year means an approximately 1 in 200,000 chance of being attacked by a shark. The odds of dying in a transportation accident in 2013 were approximately 1 in 48,000, over four times higher than even being bitten by a shark in Reunion.
Reasons for increasing attacks
Since 1980, there has been a steady but small increase in the risk of a shark attack on Reunion Island. To some extent, this can be attributed to more tourism and more water related activities. Simply put, if sharks bite humans randomly (albeit with a very low probability of doing so), then more humans in the water will mean more bites. However, there are also other issues effecting likely shark bite rates in Reunion. While many theories exist, including over fishing of prey species, a recent study suggests that for Reunion, the main issue is a rise in poorly regulated agriculture. The study states: “Agriculture […] represents an important component of the island’s economy. However, run-off and waste-water are poorly contained.” Agricultural run-off affects water clarity, and therefore sharks’ vision. It is well known that areas of poor visibility are the site of higher numbers of shark attacks. Indeed, the attack on a bodyboarder last week was in a river mouth (which had been closed to all water users by the authorities, due to increased risk of shark attacks).
The problems with culling
Bull sharks (the most likely species thought to have caused the majority of attacks) are seasonal visitors to Reunion. While studies have shown good site fidelity for bull sharks (many return to the same site each year), some do not, and other new individuals arrive. Culling may reduce the population in one season, or even for a few seasons, but it will have little mid- to long-term effect on numbers and can not eliminate the risk of an attack.
Attracting sharks (to catch them or cage dive) usually involves the use of chum (dead fish, blood and oil thrown into the water). Ultimately the use of chum may lead to further attacks on humans, and could attract them into shallower water. The evidence is far from certain, but a recent review of scientific literature says: “We are not aware of any published studies that have examined [whether chumming increasing shark attacks] and therefore, this is certainly an area requiring further research. However, research on a variety of other species has shown that some increased risk of aggression toward humans is possible in provisioned animals.”
Sharks are apex predators which control marine ecosystems and have even been shown to help alleviate climate change by storing or regulating carbon dioxide release. The risk of a shark attack at Reunion is very low, and culling is unlikely to be effective and may even be counterproductive. The solution to preventing attacks is to respect sharks, learn about their behaviour, and avoid more dangerous areas. Along with that, we need to protect the ocean from over fishing (so sharks have food) and pollution (so they can identify humans from their main prey)– as protecting the ocean also protects us from the risk of attack.