Journal du Club des Cordeliers - Bull sharks linger in warming Sydney waters

The predators are migratory, swimming north in winter when Sydney's long-term ocean temperatures dip below 19 degrees Celsius (66 degrees Fahrenheit) to bask in the balmier waters off Queensland.

A team of scientists looked at 15 years of acoustic tracking of 92 tagged migratory sharks in an area including Bondi Beach and Sydney Harbour.

Records show the sharks now spend an average of 15 days longer off Sydney's coast in summer than they did in 2009, said James Cook University researcher Nicolas Lubitz.

"If they're staying longer, it means that people and prey animals have a longer window of overlap with them."

Shark attacks are rare in ocean-loving Australia, and most serious bites are from three species: bull sharks, great whites, and tiger sharks, according to a national database.

There have been more than 1,200 shark incidents around Australia since 1791, of which over 250 resulted in death.

Researchers found an average warming of 0.57C in Bondi for the October-May period between 2006 and 2024, said the study published in the peer-reviewed journal Science of The Total Environment.

Over a longer period, remotely sensed summer sea-surface temperatures in the area rose an average 0.67C between 1982 and 2024, they said.

- Bull sharks 'year-round' -

"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," Lubitz said.

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

"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 change breeding patterns, Lubitz said, with early evidence indicating juvenile sharks were appearing in rivers further south.

There was some evidence as well that summer habitats for great whites, which prefer colder waters, were decreasing in northern New South Wales and Queensland, he said.

Tagged sharks trigger an alarm when they swim within range of a network of receivers dotted around parts of the Australian coast, giving people real-time warnings on a mobile app of their presence at key locations.

C.Caron--JdCdC