Rob Harcourt is heading back from a “beautiful surf” at Bondi on a warm and sunny winter’s morning in Sydney.

But for him and many of his surfing mates, the compelling pull of the city’s world famous surf breaks has been neutered by tragedy, fear and uncertainty.

“A lot of my surfer friends are not going in,” says Harcourt, who, at 65, mixes his retirement and daily swims and surfs with ongoing research as an emeritus professor and the leader of Macquarie University’s marine predator research group. “A lot of people are very nervous – they’re traumatised.”

Sydney’s beaches have been rocked by a spate of shark bites and deaths.

The latest was last Saturday, when 34-year-old Leah Stewart suffered shocking injuries from a suspected great white shark bite. Her arm has been amputated and her family says she remains critical in hospital intensive care.

Conditions should have been perfect for a safe swim. Stewart was between the flags on a patrolled beach in clear water in the middle of the day – all things experts like Harcourt say reduce a person’s chances of being bitten.

While the recent spate of bites has many Sydney ocean lovers on edge, they are part of a sharply rising trend of shark bites on Australia’s coastlines.

“People just want to know why,” says Harcourt. “We don’t have a definitive answer. But we do know some things.”

Globally, Australia is second only to the US for shark bites on humans. But where global trends are mostly flat, the number of bites in Australia is rising.

Australia’s population centres overlap the homes of the three sharks most often responsible for the worst bites: great whites, tigers and bulls.

The Australian Shark Incident file shows in the 1950s there was an average of 3.1 unprovoked “incidents” each year across the country – that includes bites and attempted bites, but doesn’t include incidents where the shark may have been provoked, or if a person was spear fishing at the time.

The number of bites rose slowly until the 2000s, when it went from 12 incidents per year to the current rate this decade of 21.

The number of deaths from shark bites has gone up from an average of 1.7 per year in the 50s to 3.8 in this decade so far (better response times and the availability of tourniquet kits at surf life-saving clubs have likely saved lives).

The bite figures don’t account for population growth but, even if they did, they would not account for how many of those people actually go in the water or whether they are swimming or surfing in places that overlap with sharks.

One review of shark bites found 40 different factors had been suggested as contributors to the risk of a shark bite – from an increase in the popularity of board sports to the proximity of popular beaches to river mouths. But there was little research into most of them.

Harcourt says despite all this uncertainty, some changes are known.

Warming ocean temperatures mean that bull sharks are spending more time in the Sydney area. Tiger sharks also prefer warmer water.

The numbers of seals and whales – food for larger sharks – have also been recovering since hunting stopped, he says.

“Swimming next to a seal colony probably puts you in greater risk,” says Harcourt, because sometimes sharks bite a human to see if it’s food, like a seal.

But could the rise in bites be because there are more sharks in the water, as former prime minister Tony Abbott claimed this week?

The state government keeps data on the numbers of marine animals caught in shark nets. Harcourt says if there was an explosion in shark numbers then a lot more would also be caught in those nets. But he says the data shows no significant changes.

Dr Daryl McPhee researches shark bite trends at Bond University on the surf-crazy Gold Coast in Queensland.

In the last five years, there has been an average of four deaths a year in Australia from shark bites – this year, he says there have already been four.

So he says the rising trend of bites is “consistent with what people are feeling” in places such as Sydney.

But the risk of a bite, he says, can vary from beach-to-beach and day-to-day and species-to-species. The location of a shark’s food, such as seals or large schools of fish at any given time, is another big factor.

Despite calls for culls of sharks in social media and from some high profile people, experts say it’s unlikely to work in Australia because sharks are migratory.

“It’s an old colonial view that we can bend nature to our will,” McPhee says.

Fearing things we can’t control

When shark bites occur, the public will often be told by experts that they are very rare events and they are more likely to die, to use a few examples, from being hit by a coconut, struck by lightning or, perhaps a more relevant comparison, from drowning (there were 82 deaths from drowning at Australian beaches last year).

Dr Brianna Le Busque, who researches public perceptions of sharks at Adelaide University, says these comparisons do little to assuage our fear of sharks.

“We know it’s not helping,” she says. “We talk about how rare bites are and that almost makes it feel even more random and that we have even less control.”

Le Busque says humans fear things we think we can’t control.

“A lot of the time bites occur without a reasonable explanation for what’s happened at that time and in that place and so we feel out of control,” she says.

Le Busque surveyed surfers from around the world – mostly in the US – and found they feared sharks less than the general public, even though they were more likely to encounter sharks, or hear about encounters.

“A lot of them said these encounters were non-events. Maybe because these encounters are not negative, that gives them an anchor point [for their response].”

How to keep safe

Public discussion about shark safety are further muddied because of people’s differing values. Killing sharks on the pretence of protecting humans may be fine for some people, but anathema to others.

Australia first introduced shark nets at beaches more than 80 years ago.

In recent years, governments have introduced other measures, including baited hooks to catch sharks (controversial because of the high numbers of non-target species caught), improved safety information, drone monitoring and “listening stations” to alert beachgoers when a tagged shark is close.

Prof Corey Bradshaw, an ecologist at Flinders University, has been part of studies into the various measures, including those deployed on beaches. He says when done well, steps like public education, drones, and some personal protection, can cut the risk.

But on shark nets, he is clear.

“I think they are bullshit,” he says.

“They’re an environmental catastrophe and there is no evidence that they reduce the incidence of shark bites. They should have been pulled out of the water 50 years ago.

“As humans we have this innate evolutionary response to predators, and so we inflate the risk in our brains, even though the risk is extremely small.

“Then we do other risky behaviours because they’re familiar and we do them all the time. Like driving to the beach.”