AUSTRALIA Day, 2011: A glorious, sunny day when tens of thousands of people splashed about in Sydney Harbour and, it can now be revealed, shared the water with seven mature bull sharks.

Government scientists have released an amazing map of shark movements in the Harbour on its busiest day of the year.



The sharks' presence alongside so many people shows they are not mindless killing machines, Dr Amy Smoothey said.

Each of the bull sharks was fitted with transmitters which alert a series of 370 acoustic listening stations around the Harbour and along the coast.

All of the sharks in the Harbour on Australia Day were males, ranging from 2.2m to 2.96m long.

They swam everywhere from the upper reaches of the Parramatta River to North and Middle Harbour, including the main harbour waters where the Ferrython was run.

A 2.75m-long shark swam 37km on the day, moving from Point Piper, past Mosman to Manly and the Spit Bridge and near Balmoral Beach where hundreds were enjoying a holiday swim.

NSW Primary Industries department shark researcher Dr Smoothey said knowing which parts of Sydney Harbour the sharks preferred would help minimise the risk of an attack on humans.

"The project is determining if there are any areas within Sydney Harbour that are favoured by the species, what times of the year the sharks visit the Harbour and whether factors such as water temperature and presence of bait fish influence their movements," she said.

Given the number of sharks and people in the Harbour on the same day, and the fact there were no incidents nor any reported sightings, "one could argue bull sharks may not be the voracious predator we once thought", she said.

Sixteen bull sharks - six females and 10 males - have been fitted with the tags since navy diver Paul de Gelder was mauled by a bull shark and lost a hand and a leg during a training exercise at Woolloomooloo Bay in February 2009.

Dr Smoothey's research shows at least two bull sharks were in the same general area on Australia Day this year.

Her research has so far found that bull sharks on average spent up to 17 days at any one time in the Harbour, and travel similar distances during the day and night.

They swim to Sydney in late November or early December and head back north by late April or early May.

Several tagged bull sharks have also spent time around Port Stephens, while a 2.93m male shark tagged in Sydney Harbour headed north to Yamba - more than 550km - before heading south to Sydney Harbour nine days later.

Another has recently been detected meandering north and was last spotted off the coast of Townsville.