As the state grapples with yet another mauling, this time at a surf break at Bombo Beach in Kiama, researchers have found a unique insight into when one deadly shark species appears more likely to go on the bite.

An exhaustive study, Patterns of Occurrence of Sharks in Sydney Harbour, has found that the appetite of bull sharks (Carharhinus leucas), a species known to attack humans, appears to coincide with a rise in water temperature.

The researchers' two-year study, published in scientific journal PLOS ONE, found that, when surface water temperature rose above 23.2 degrees, bull shark catches went up, leading to speculation that "temperature may therefore directly or indirectly influence the occurrence" of the sharks.

They also found that the occurrence of the sharks might be linked to the movements of the warm waters of the East Australian Current, according to the research team who were employed by the NSW Primary Industries Department and included Vic Peddemors, who is at the forefront of the NSW government's project to try to minimise shark attacks in the state

The sharks were thought to be using the current to migrate southwards during summer and to return northwards during winter when the current was weakest and inshore waters cooler, they noted.

The findings were supported by unpublished data that noted free-swimming bull sharks only being sighted in Sydney Harbour in the warmer months.

The project had involved researchers fishing on 85 days using a series of long lines set in the depths of the harbour, garnished with a burley of chicken layer pellets and tuna oil and baited with frozen sea mullet.

Their 340 lines snared 45 sharks of several species but of key interest were the 12 bull sharks, which ranged in size from about 2.15 to 3.12 metres.

The catch results revealed that, when harbour water temperatures dropped to below 19 degrees, bull sharks were "very unlikely" to be caught.

But the scientists cautioned that, because sharks were poikilothermic or had a body temperature that changes regularly, temperature was not thought to be regulating the animals' movements.

"Movement in apex predators is generally considered to be driven by food or reproductive requirements," they said.

They speculated that the sharks might be following the movements of prey such as yellowtail kingfish, Australian bonito and frigate mackerel, which schooled during summer and autumn - also the key catch times for the sharks.

Another possibility was that the harbour was a mating ground. But the scientists noted that mating behaviour by sharks was poorly documented.

Of the total bull shark catch, four were caught in the entrance region, six in the central region and two in the upper regions of the harbour.

All the bull sharks were tagged with acoustic transmitters to provide further information on their movements.

Three other species of shark were caught in the survey including Port Jackson sharks, wobbegongs and dusky sharks.

All bull sharks were caught only between January and April.

The research paper was authored by Amy Smoothey, Charles Gray, Steve Kennelly, Oliver Masens, Wayne A Robinson and Dr Peddemors.

The study was supported by funding from the NSW DPI and Independent Consulting and Wildfish Research.

A DPI spokeswoman confirmed the department was still in the process of liaising with the Kiama victim, Brett Connellan, to try to identify the shark.

smh.com.au