Bull sharks are one of the few shark species that can tolerate a wide range of salinities, a trait that allows them to venture into freshwater and brackish habitats such as rivers, estuaries and lagoons.
Bull sharks are one of the few shark species that can tolerate a wide range of salinities, a trait that allows them to venture into freshwater and brackish habitats such as rivers, estuaries and lagoons.
For nearly two decades, the Carbrook Golf Club near Brisbane, Australia, had the ultimate water hazard: a lake teeming with bull sharks.
It all started in 1996 when raging floods swept six young bull sharks from a nearby river into a 51-acre lake near the golf course’s 14th hole. Whenthe floodwaters receded, the sharks found themselves stuck, surrounded by grassy hills and curious golfers.
The sharks spent 17 years in the lake, sustaining themselves on its large stock of fish and on the occasional meat treat provided by the club’s staff. One shark was illegally fished out, while the others vanished after subsequent floods.
The sharks, according to a new study, are more than just a fluke along the fairway. In research published last month in the journal Marine and Fisheries Science, Peter Gausmann, a shark scientist and lecturer at Ruhr University Bochum in Germany, said that the cartilaginous club members of Carbrook demonstrate that bull sharks can live indefinitely in low-salinity aquatic environments.
Bull sharks can be found in warm coastal waters the world over. These stout sharks can reach 11 to 13 feet (3.3-3.9 metres) in length and weigh upward of 500 pounds (227 kilograms). They are one of the few shark species that can tolerate a wide range of salinities, a trait that allows them to venture into freshwater and brackish habitats such as rivers, estuaries and lagoons.
Unfortunately, this impressive adaptation often puts the sharks in close quarters with humans, one of the many reasons bull sharks are responsible for dozens of documented fatal attacks on humans.
A bull shark swimming in floodwaters on Australia's Gold Coast, December 2020.
If most sharks were to enter a freshwater environment, their internal salt levels would become diluted and they would die. But bull sharks have specially adapted kidneys and rectal glands that work together to recycle and retain the salt in their bodies.
Freshwater and brackish habitats provide young bull sharks a place to grow up without the threat of predation from larger sharks. Once they’ve reached maturity, however, bull sharks usually head to the sea, where larger prey and breeding opportunities abound. That the Carbrook shark population did not grow during their time on the golf course offered further evidence that the species prefers to breed in saltier environments.
While scientists have long known that bull sharks have the means to move between fresh and saltwater environments, no one knows if these sharks could spend their entire lives in freshwater.
Research suggests bull sharks can live about 30 years, and the Carbrook group survived in the golf course’s lake for 17 years. That suggests there is “no upper limit” to how long these sharks can spend in low-salinity environments, said Vincent Raoult, a postdoctoral researcher at Deakin University in Australia who was not involved in the new study.
“I think a lot of people would be scared to learn there could be bull sharks in their local pond, but the fact is, it’s pretty amazing that there are animals that are able to do this,” Raoult said.
While the idea of sharing a lake with bull sharks may be scary to some, the golfers relished the opportunity, said Scott Wagstaff, general manager of Carbrook Golf Club.
“Every single member here just loves the sharks,” Wagstaff said.
When the sharks were still around, he and other staff would toss chunks of meat into the water and marvel at the way the sharks devoured the food with their fearsome maws. “I’ve seen them jump completely out of the water and spin as they land. It was pretty cool,” Wagstaff said.
Extreme floods like the ones that washed sharks in and out of Carbrook Golf Club are becoming more common, and more bull sharks could end up marooned in lakes, lagoons and ponds. Although you are highly unlikely to encounter a bull shark in your local swimming hole, Gausmann recommends avoiding bodies of water recently affected by flooding.
“You should never bathe in stagnant bodies of water that once had a connection to the sea. You never know if sharks are living there,” he said, although people in urban areas may not need to worry, as urban floodwaters are often too toxic to sustain marine life.
And there is another lesson to take away, as shark sightings and occasional attacks on human swimmers prompt fearful reactions in some places.
“If this paper has shown anything, it is that it is possible to live side by side with sharks,” said Leonardo Guida, a shark scientist with the Australian Marine Conservation Society.
