Climate shifts extending Sydney’s bull shark season

As warming seas cause bull sharks to spend more time off Sydney’s coast, a James Cook University expert says that this may increase overlap between sharks and people along the coast of Australia’s most populated city.

The research reveals that migratory bull sharks, which spend their winters in Queensland are now on average staying 15 days longer off Sydney's coast during the summer than they did 15 years ago.

This is due to rising water temperatures, extending the potential for human-shark encounters while possibly also increasing predation pressure on local prey populations.

JCU Postdoctoral Research Fellow Dr Nicolas Lubitz said continued climate change could transform seasonal shark patterns, potentially leading to a year-round bull shark presence in southern parts of New South Wales in the future.

“We did a climate analysis of water temperatures of the coast around Sydney and found that average temperatures during the period from October-May each year have been increasing over the past 40 years” he said.

“We've been tracking migratory bull sharks moving seasonally between Queensland and New South Wales, specifically the area off Sydney for 15 years, and now on average they're staying 15 days longer than they used to back in 2009.

“Sydney is Australia's most populated city, where bull sharks disappear during the wintertime and migrate back to Queensland during the cooler months as bull sharks avoid long-term temperatures below 19C.

“But if they're staying longer, it means that people and prey animals have a longer window of overlap with them.”

Dr Lubitz said the research found clear evidence of a warming of the water in the past 40 years, with the number of days that bull sharks are present increasing at the same time.

“If this trend persists, which it likely will, it just means that these animals are going to spend more and more time towards their seasonal distributional limit, which currently is southern and central New South Wales,” he said.

“So it could be that a few decades from now, maybe bull sharks are present year-round in waters off Sydney.

“While the chances of a shark bite, and shark bites in Australia in general, remain low, it just means that people have to be more aware of an increased window of bull shark presence in coastal waters off Sydney.”

Climate change could also alter bull shark breeding patterns, with early evidence indicating juvenile sharks are already appearing in rivers further south than was historically the case.

This could lead to these apex predators establishing new breeding grounds further south, challenging existing ecological boundaries and transforming marine ecosystems along Australia's east coast.

“And if that trend persists there's a fair chance that a lot more tropical species are going to be pretty much year-round in the Sydney area, which obviously changes the whole dynamic,” Dr Lubitz said.

“There's also some evidence in recent studies that great white shark summer habitats are actually decreasing in Northern NSW and QLD, as they're more of a cold water species.

"It remains to be seen what that means for a potential increased overlap of white sharks and bull sharks and resource competition between the two species."