Do shark culls keep people safe in the ocean? Here’s what the science says

A young mother remains in hospital after being bitten by a shark at Coogee beach in Sydney on Saturday morning. Leah Stewart, 35, was swimming about 30 metres offshore when the shark – believed to be a three to four metre great white shark – struck.

In the wake of this tragic incident, there have been renewed calls for a shark cull to be launched. Federal Liberal Party president Tony Abbott, for example, said :

It’s so wrong that we don’t cull sharks after attacks. It’s so wrong that we don’t have a commercial shark fishery given the explosion of shark numbers, and it’s so wrong that we don’t put people before sharks.

New South Wales Premier Chris Minns has since said his government is actively considering a cull of bull sharks, in response to a surge in bull shark bites over summer.

So is the number of shark bites really increasing and is it because shark numbers are exploding? What does the research say about the effectiveness of culling? And what other measures could keep ocean users safe?

Rising numbers of shark bites

Shark bites have increased in Australia and globally over the past four decades (although they are still very rare).

The exploding number of sharks is often mentioned to explain this trend.

Many species of sharks are threatened globally. But Australia has many protections in place and relatively well-managed fisheries that support the recovery of vulnerable species, such as the great white shark, and which prevent the decline of species which are threatened in other countries, such as the bull shark.

But it’s unlikely that the recovery of the great white shark or reduced fishing pressure can alone explain the rise in shark bites.

Last year colleagues and I published a study on the factors influencing the number of shark bites. We found 40 factors suggested to affect shark-bite risk. These included human population growth, habitat modification and destruction, declining water quality, climate change and anomalous weather patterns, and changes to the distribution and abundance of sharks and their prey.

However, the relatively infrequent occurrence of such events reduces our ability to determine which of these factors explain the increase in shark bites the most. It is likely to be due to a combination of these factors.

Do shark culls work?

There are many ways to reduce the risk of shark bites. Shark culls are the most controversial.

Some research has suggested culling sharks has reduced the rate of interactions with humans in certain locations. However, other studies have highlighted no changes in bite rates after large culling programs were implemented.

The efficacy of culling varies between species and regions, and the number of sharks needed to be culled is unknown but is likely to be high before it starts affecting shark-bite risk.

While you could argue that even culling one shark reduces risk, that shark might never have bitten humans, so culling that shark does not impact the number of shark bites. This problem is best illustrated by the shark control program in Hawaii in the 1960s and 1970s, during which 4,668 tiger sharks were killed but there were no resulting changes in the rate of shark bites .

Several beaches in Sydney were closed after a shark bit a woman at Coogee on Saturday.

Nadir Kinani/AP

What about other measures?

Shark nets aim to catch potentially dangerous sharks close to popular swimming areas. However, this method can also kill non-dangerous sharks and a range of other marine animals such as rays, turtles and dolphins. There is also little evidence to show shark nets keep people safe.

There are a range of other area-based mitigation measures that are designed to minimise impact on the marine ecosystem. These include using drones to observe sharks, SMART drumlines (which intercept sharks close to shore and allow authorities to tag and then safely relocate them), and shark listening stations that detect the presence of a tagged shark.

Many studies show that early-warning systems using drones can detect sharks and quickly enable the evacuation of beaches, while SMART drumlines intercept sharks with the capture leading to sharks leaving the area upon release.

So, what now?

So, what should we use to reduce shark-bite risk? This question should be considered from both an efficacy and ethical perspective.

Culling might theoretically reduce risk. But we have no idea of how many sharks need to be culled to reduce the number of bites. As the experience of Hawaii shows, culling may see thousands of sharks killed without improvements in public safety. It would also have significant impacts on marine ecosystems.

It makes far more sense to invest more in non-lethal measures such as drone surveillance, as the evidence shows these measures reduce shark-bite risk and are preferred by the public .

But all the measures discussed so far are only part of the arsenal available to reduce the risk of sharks bites. They can be complemented with personal deterrents, w…