Australia Swan River shark attack a rare event, but experts say bull sharks exist in most of our rivers

02:15  15 january  2021
02:15  15 january  2021 Source:   abc.net.au

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It's been 50 years since the last shark attack in Perth's river system, but yesterday a man in his 50s was bitten by a bull shark, suffering critical injuries. 

a fish swimming under water: Marine ecologists say bull sharks are thought to be slightly more aggressive than others. (Supplied: Jayne Jenkins) © Provided by ABC NEWS Marine ecologists say bull sharks are thought to be slightly more aggressive than others. (Supplied: Jayne Jenkins)

But bull sharks are actually incredibly common in waterways across the country.

Are bull sharks in most rivers?

Essentially, yes.

While we've all come to accept that sharks are common at beaches and in coastal waters around the country, bull sharks can be found in most rivers around Australia, according to marine ecologist Johan Gustafson from Griffith University.

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"Except the southern end where it's a bit too cold," Dr Gustafson says.

"It'll be the same from Perth all the way over to Sydney".

Dr Gustafson says bull sharks live in near-coastal and estuarine waters, while larger sharks are the ones hang out along the coast.

"You won't actually find a massive, huge white shark up in a river. There's not enough food for it, it's too shallow for them," he says.

How many bull sharks are out there?

Dr Gustafson says it depends on the time of year. For the majority of the year, there is likely to be a very small population in river systems across the country.

But from December to January, it's pupping season for the females, so there will be larger female sharks entering rivers that will skew the population.

By the time it reaches the end of February, most of the large animals have migrated out of the rivers and spend their time on the coastline, so the population actually changes.

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"The number of sharks [per river] is not in the thousands, it's probably not even in the hundreds. It's going to be a lot less than that," Dr Gustafson says.

What do they eat?

The bull shark's strategy of breeding means sharks can be found in the upper areas of rivers for the start of their lives.

The mothers lay their pups upstream, before eventually migrating towards the coast as they grow bigger.

"Basically they'll be small, they're born at about 0.3 to 0.5 metres," Dr Gustafson said.

"They can't actually eat hard substances [as juveniles] so they'll be eating things like squid and soft shell animals, fish.

"Then as they migrate they'll get into crabs. And as the adults come through they'll be looking for mullet and tailor.

"So as they migrate out, they change their diet."

Are river shark attacks common?

Nope. In fact, they are very uncommon.

"On the Gold Coast, where we've got a lot of canals and rivers all connected [and] we've got a huge population on the water, we haven't had any incidents for quite a long time," Dr Gustafson said.

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"And with the Swan [River] as well, there hasn't been a recorded [fatal incident] in nearly 100 years."

Are bull sharks aggressive?

According to Dr Gustafson, bull sharks are thought to be slightly more aggressive than other sharks.

"The bull shark is actually said to be the most dangerous of all sharks, including white sharks, but mostly because of its proximity to people," he said.

"And that's due to where it lives and where we've chosen to live … so the chances of encountering a bull shark are very high."

But Dr Gustafson said after years of studying the animal, he had never felt unsafe around them.

"I've handled 4-metre sharks down to half a metre, and they're not overly aggressive," he said.

"If the water is quite murky and you startle an animal, they'll end up doing a test bite to find out what you are.

"So they don't actually attack us, they are more curious — just the bite is quite strong."

Should I avoid my local river?

Dr Gustafson says the most important thing to remember after yesterday's Swan River attack is that while bull sharks are in rivers, people have been living normal lives among them for decades.

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"It's just a random, one-off incident," he said.

"If we have a long trend that's pretty rapid, then we'd have to think about different measures, but at this stage it's just a one-off, unfortunate incident."

If you encounter a bull shark and you get bitten, the advice is obvious — you should attend to the wound immediately.

Dr Gustafson says devices that emit electromagnetic pulses can also help deter sharks.

And if you're thinking about going into the river for a dip after heavy rain? You might want to think again.

"If there's been a big downpour, when the water's freshened up a little bit and it's murky, bull sharks tend to move up into rivers during those times," Dr Gustafson says.

"So after big storm events, don't go into the water until the water's cleared."

Finally, Dr Gustafson says we don't need to be afraid of bull sharks or demonise them.

"The more that we learn about their movements and environmental variables that cause them to come around, we can actually coexist with them quite easily like we have been doing for quite a long time," he said.

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