Is it dangerous to swim in Sydney?

The only slightly unusual factor behind the attack, if it was a bull shark, was the time of day.

Bull sharks are crepuscular, meaning they usually hunt in the low light of dawn and dusk.

Are shark populations off NSW increasing?

There is no evidence that populations of the three most dangerous sharks to humans – bulls, tigers and whites – are on the rise.

Tiger shark numbers are declining, while DNA estimates there are fewer than 500 adult breeding great white sharks along the entire Australian coastline.

For bull sharks, however, there may not be more of them but the predators are spending more time in Sydney.

The sharks migrate northward towards Queensland each year when water temperatures drop below 20 degrees, usually between April or May.

Bull sharks spend an extra 15 days in and around Sydney compared to 2009 due to warming waters, a study published last year concluded.

“It’s just something to be aware of – that one of the top three shark species that are considered potentially dangerous for people is spending more time in this area,” warned lead author of the study Dr Nicolas Lubitz from James Cook University.

Unlike white and tiger sharks, bull sharks are not a protected species in NSW.

Why are shark bites on the rise?

The four-fold increase in shark bites along the NSW coast since 2000 is mostly driven by white sharks, according to a study led by Flinders University shark expert Professor Charlie Huveneers.

But the factors underpinning the uptick are many and varied. Shark attacks remain so rare that it’s difficult to tease out trends or tie the uptick in attacks to any one factor.

Huveneers’ review identified 40 factors that may be driving the rise in attacks.

“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 are all often proposed to explain the recent increases in shark bites,” he said.

“However, the 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.”

What mitigation factors work?

The NSW government says it has the largest shark management program of its kind in the world.

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The program involves seasonal nets, drone surveillance during school holidays, and more than 300 SMART (Shark Management Alert in Real Time) drumlines.

The SMART system works by catching the three target shark species (white, tiger and bull sharks) using baited hooks, tagging the sharks and re-releasing them a kilometre offshore.

Listening stations near popular beaches can then detect the presence of the shark within 500 metres and trigger alerts.

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Proving which mitigation efforts work the best is difficult because shark attacks themselves are so rare and unpredictable.

But the analysis by Huveneers and his colleagues found no statistical difference between the number of shark bites and netted versus non-netted beaches since 2000.

There were no shark bites at beaches where SMART drumlines or drones were in use, but 19 attacks at the same beaches before their implementation, which “might indicate that SMART drumlines and drones could potentially reduce shark-human interactions” the researchers wrote.

What’s the advice for staying safe?

To reduce the risk of a shark bite, people should swim, dive or surf with other people, stay out of the water with bleeding cuts and avoid swimming in murky water, according to the NSW government’s SharkSmart advice.