Brisbane residents who pay attention to the news are probably aware by now that the city’s river, even in its upper reaches, often has bull sharks of potentially dangerous size in it.
In fact, if you go to College’s Crossing, which is close to Ipswich and has a large picnic area on a stretch of the river with clean, fresh water full of swimmers in summer, there are shark warning signs permanently up. (Brisbane gets its drinking water from just beside this spot, so the water is definitely 100% fresh. For some reason I don’t understand, it’s only a relatively short distance further down from College’s Crossing that the river suddenly takes on its brown murky quality which is then maintained all the way to the mouth.)
The reason I’m talking about this is because a large bull shark appears to have been spotted this week up near College’s Crossing, which (according to this report) is 90km from the river mouth. It also says that before the Mt Crosby weir was built, they had been recorded as far inland as Lowood. That’s a long, long way further inland.
It’s also been reported recently that bull sharks jumping out of the river in the inner city stretch is not an unusual sight for the ferry captains:
“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.
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.
So, how does this shark manage to live in fresh water? I see there was a story on ABC's Catalyst show in 2003 explaining this. Here are some extracts from the transcript:
Richard Pillans: We’re about 85km from the mouth of the Brisbane River, we’ve got fresh water for at least 20 km downstream of us, and there are sharks in here, there’s a lot of sharks in here, and it is really unusual and most people in Brisbane don’t realise they’re basically in their backyard all the way through the Brisbane River. Almost all sharks will die in fresh water, but what’s really unique about the bull shark is that it’s equally at home in both salt water and fresh water. ...
The team think that there are four key organs involved - the liver, the kidney, the rectal gland and the gills. The liver of the bull shark is extra large, it’s the animal’s salt factory. It produces salt in the form of urea.
A/Prof Craig Franklin: Urea production and the liver is the major site so it has to be pretty big. If the shark is in salt water this salt is then stored in the kidney. As it swims upstream into fresh water, excess salt is excreted in the urine.
Neil: They will retain less urea in fresh water than sea water, so there’s a clear change between the two.
Narration: The shark also regulates salt levels using the rectal gland and the gills. Pure salt can be taken in or excreted through the gills depending on the outside water.
Richard Pillans: Different in structure between the two.
Narration: Incredibly they think the very structure of gills would gradually change as the shark swims from fresh to salt water.
Thus concludes today’s nature lesson.
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