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.”
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