One of the world’s deadliest sharks is developing a taste for Sydney

Sydney’s bull shark season has expanded due to ocean waters warming under climate change in a trend that could bring one of the world’s most dangerous sharks into permanent proximity with swimmers and surfers off Australia’s biggest city.

New analysis has proved the sharks are delaying their northward migrations and, compared to 2009, now spend an extra 15 days in and around Sydney Harbour each year.

Co-author of the study Dr Amy Smoothey from DPI tags a bull shark in Sydney Harbour.Credit: Ben Rushton

The sharks leave Sydney in April or May to spend their winters in warmer seas off Queensland, said lead author of the study Dr Nicolas Lubitz from James Cook University. They return around November.

“When temperatures start dropping just below 20 degrees, that’s the cue for the bullies to start migrating back to northern NSW and Queensland, sometimes all the way up to Cape York,” Lubitz said.

“Our analysis shows those temperatures are occurring less now during the time the bull sharks are present in the Sydney area. They just don’t get that cue to leave, basically.”

Over the past 40 years, average ocean temperatures in Sydney have increased by 0.67 degrees during bull shark season, the analysis found.

That’s more than enough to shepherd bull sharks further south and warp their migration regimes, Lubitz said.

“The central to southern coast of NSW has been deemed a climate change hotspot. It has one of the fastest warming rates out of any marine region in the world.”

The scientists used data from 92 tagged sharks over 15 years for their analysis, which revealed bull shark season had extended by one day per year.

If that trend continues, Lubitz said, “it could be that a few decades from now, maybe bull sharks are present year-round in waters off Sydney”.

Bull shark activity peaks with temperature in Sydney Harbour between January and February. Large females swim up estuaries and give birth to their young in waterways including the Hawkesbury and Parramatta rivers.

Three adults were filmed swimming near Spit West Reserve in Manly in February. Last year a bull shark bit 29-year-old kayaker Lauren O’Neill on the leg while she swam one evening in Elizabeth Bay.

Lubitz said bites from bull sharks and any other species are incredibly rare and people should stay alert at any time given the presence of white sharks year-round.

“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,” he said. “The main message is to keep following SharkSmart advice.”

Drone Shark App operator Jason Iggleden warned swimmers off Bondi this week to proceed with caution due to a huge school of salmon, which can attract large predators such as two bronze whalers spotted on Thursday.

An increasing number of Sydney councils have voted to ditch shark nets and opt for other methods including drone surveillance and SMART drumlines as the NSW government reviews its shark mitigation program.

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