It is a spectacle ferry drivers say they catch sight of up to four times a week. Craig Wilkins, who has spent years on the river as a City Cat master and at the helm of the Kookaburra Queen, says the wide stretch of river at Bulimba is a common spot for the behaviour. “The warmer the water, the more times you will see them jumping,” he said. As the sun gets up, that's when you will see them jumping. “About two years ago the river was a little bit cleaner than usual and everybody was seeing them. You ask any City Cat master, they'll say the same thing. They see them jumping all the time.” As well, their aerials show the sharks are also more active underwater as the river at this time of year.

Sharks found upstream are normally less than 1.5m long and pose little threat to people. However, dog owners have been warned to keep their pets out of the river at dusk and dawn, when sharks are most active. There have been numerous reports of dogs being taken from the river's edge and even instances of more ambitious sharks taking on larger prey. My belief is that they are getting rid of parasites and that's why they spin around so fast In 2005, Ipswich locals were shocked after a bull shark attacked a race horse being put through its paces in the Brisbane River at Kholo.

The attack came just weeks after a teenager was bitten on the head and finger at Karalee. One ferry driver recently told of seeing a Chihuahua snapped up in shallow water at the edge of the river and Professor Franklin said small dogs would be attractive to sharks. “Small snacks like [Chihuahuas] could be within the size range of prey items that a bull shark would take,” he said. “You wouldn't want to let your dog go for a swim around dusk and dawn. That's when they are most likely to be feeding.” A “cosmopolitan” creature found in tropical and sub-tropical waterways around the world, the bull shark has long baffled scientists with its behaviour in Indonesia, Thailand, Fiji and Florida.

In some cases they have been observed leaping for bats, but by and large their motive is a mystery, according to Professor Franklin. “I've got no idea to tell you the truth,” he said. “Whether it's to scare prey out, I've got no idea why they do that. “I've seen it and it tends to be the small ones.” City Cat and ferry drivers say they often see it during the long hours they spend on the waterway. They have even ventured theories on why it happens.

Mr Wilkins said the manoeuvre could be a type of housekeeping. “My belief is, and I don't know if it is accurate, but my belief is that they are getting rid of parasites and that's why they spin around so fast,” he said. “That's what we see as we are driving along occasionally is them jumping up and doing a bit of a spin and hitting the water again. They're usually about four foot long and the biggest I've ever seen was about six foot, I guess.” His theory was echoed by another ferry driver who spoke to brisbanetimes.com.au, but Professor Franklin wasn't sold on the idea. “I doubt that,” he said. “There are external parasites that they can have, but I've never heard that it's a successful strategy to remove parasites.”

Professor Franklin said it was possible bull sharks were among those filmed feeding on a school of bait fish off Teewah Beach north of Noosa on Friday and were often found in the open ocean. But he said what set them apart from other species was their ability to survive in fresh water thanks to being able to retain salt in their bodies. He said the sharks give birth near the mouth of the river, with the 50cm pups heading upstream, where they would live for up to four years until they venture into the ocean in search of larger prey. “I think to be in a state capital city and to have bull sharks in our waterways is a wonderful thing. They are undoubtedly an incredible animal to have in our waterways,” he said. Around the world, more fatalities are attributed to bull sharks than any other species.

In the past decade bull sharks have been blamed for deaths in the Gold Coast canal system, and an attack which killed a teenage girl at North Stradbroke Island in 2006. “You could almost say they are a cosmopolitan shark in that most capital cities are built around river systems, they are circum-global, so right around the globe, so the likelihood of people being injured or killed is higher,” said Professor Franklin. “But they don't target humans. People need to realise that. They are opportunistic feeders. All we have to do is modify our behaviour and be careful where we swim and the time at which we swim. “You look at the unfortunate incidents on the Gold Coast in the canals where people were taken and they were swimming around at night time or dusk or dawn.”