by Margaret Dedloff
There are over 500 species of sharks, but only one is known to be able to live in both freshwater and saltwater: the bull shark.
Image of a bull shark. Image source.
Bull sharks are coastal sharks that prefer to hang out in water less than 100 feet deep. Bull sharks can be found all over the world, and in the United States, they can be found off the east coast and in the Gulf of Mexico.
Map showing where bull sharks are found. Image source.
These sharks are well known for interacting with and attacking humans due to their shallow marine habitat and their ability to swim upriver. In fact, bull sharks are a part of the “big three” species of sharks with the most attacks on humans (the other two are great white sharks and tiger sharks)!
Most sharks can’t survive in freshwater because swimming in freshwater removes salt from their bodies, which would result in their cells exploding and the shark’s death. Bull sharks have special adaptations that allow them to survive in freshwater. Bull sharks’ kidneys recycle salt and they have special glands near their tails that help them to maintain salt levels. These adaptations have enabled bull sharks to travel 2,500 miles from the ocean in the Amazon River and as far up the Mississippi River as Illinois. One reason that bull sharks venture into rivers is to give birth. Female bull sharks give birth in rivers or where the river meets the ocean, and young bull sharks often spend their early lives in rivers, which lack the large predators found in the open ocean.
A particularly strange episode of bull sharks living in freshwater began in 1996 when historic flooding occurred near Brisbane, Australia. This flooding caused a nearby river to overflow into a large area, including a large pond on Carbrook golf course. When the river overflowed, it brought six bull sharks with it. As the flood water receded, the sharks became trapped in the pond on the golf course. Golfers began reporting loud splashes and large shapes moving underwater in the pond, and some even reported spotting a dorsal fin above the water.
A bull shark in the lake at Carbrook golf course near Brisbane, Australia. Image source.
An important meeting between a Carbrook bull shark and a golfer. Image source.
The “Carbrook shark” became a legend, one that seemed much too far-fetched to be real, until the early 2000s when a local newspaper published a picture of one of the sharks. The sharks became famous in the area and brought business and media interest to Carbrook golf course. The Carbrook golf course added a bull shark to their logo and held shark feedings at special events, with as many as six bull sharks making an appearance at a wedding.
The six bull sharks were common guests on the Carbrook golf course until 2013, meaning that they lived in the pond for 17 years! It remains unknown what exactly happened to these sharks, though it is likely that with large flooding, they moved back to the river.
While it may seem silly to think that sharks par-tee-ing on a golf course had important implications for shark science, it did! Prior to this, it was believed that bull sharks could only survive for a maximum of 4 years in freshwater. The Carbrook bull sharks changed scientists’ understanding of what bull sharks are capable of, and they helped to decrease the public’s fear of sharks. If you’ve ever wondered what a shark par-tee on a golf course looks like, you can see a video here!
