Lake Full of Sharks!

For most sharks, getting stuck in a lake would be a death sentence. That’s because they’re adapted to salty marine environments. Sodium (Na) is one of the main elements that make seawater salty. Animals’ bodies also contain this element. The right amount of sodium is essential for nerves and muscles to function properly.

Levels of sodium and water in a shark’s body are affected by a process called diffusion. Diffusion occurs when substances move from an area of higher concentration to an area of lower concentration. These areas may be separated by a thin layer of tissue called a membrane, which the substances pass through. In a shark, these membranes include the tissues that line the mouth and make up a shark’s gills, which absorb oxygen (see Shark Anatomy).

In the ocean, the concentration of salts in sharks’ bodies is similar to that of the surrounding seawater. But plop an oceanic shark like a great white or a hammerhead into a freshwater lake, and sodium will diffuse out of its body into the much less salty water. As a result, the shark “would suffer enormous loss of sodium,” explains Gausmann—too much for the animal to survive. And while sodium rushed out of the shark’s body, water would uncontrollably rush in. “Finally, the body would burst,” says Gausmann. So why didn’t the sharks stuck in the golf course’s freshwater lake explode?

Most sharks would die if they got stuck in a lake. That’s because they’re adapted to salty marine environments. Sodium (Na) is one of the main elements that make seawater salty. Animals’ bodies also contain this element. Nerves and muscles need the right amount of sodium to function properly.

A process called diffusion affects levels of sodium and water in a shark’s body. In diffusion, substances move from an area of higher concentration to an area of lower concentration. A thin layer of tissue may separate these areas. The substances pass through this membrane. In a shark, these membranes include the tissues that line the mouth. Membranes also make up a shark’s gills, which absorb oxygen (see Shark Anatomy).

In the ocean, the concentrations of salts in sharks’ bodies and in the seawater are similar. What if you took a great white or hammerhead out of the ocean and plopped it in fresh water? Sodium would diffuse out of its body into the much less salty water. As a result, the shark “would suffer enormous loss of sodium,” explains Gausmann. The animal would lose too much sodium to survive. As sodium rushed out of the shark’s body, water would keep rushing in. “Finally, the body would burst,” says Gausmann. But the sharks stuck in the golf course’s freshwater lake didn’t explode. Why not?

Bull sharks have an ability that’s rare among shark species. They can osmoregulate, or maintain their body’s salt balance, in fresh water. All sharks’ gills absorb oxygen from the surrounding water. They also absorb or release salts and water. In fresh water, the cells that make up a bull shark’s gills can adjust their functioning to prevent the loss of too much salt.

Meanwhile, the bull shark’s kidneys kick into high gear. These organs filter waste from the animal’s blood and remove the excess water that’s constantly leaking in. The resulting urine is excreted through an opening on the shark’s underside called the cloaca. A bull shark in a freshwater environment will pee out about 20 times as much fluid as it does in salt water! Its kidneys also adjust to help hold on to salts that would otherwise be lost.

Baby bull sharks take advantage of this ability to hang out in rivers, where most other shark species wouldn’t be able to survive. That helps the pups avoid larger sharks that might try to make a meal out of them. In 2023, when Gausmann investigated the history of the sharks at Carbrook, he found that they’d lived in fresh water much longer than juvenile bull sharks typically do. In fact, they appear to have set a world record, surviving at least 17 years in the lake!

Bull sharks have an ability that’s rare for sharks. They can osmoregulate in fresh water. That means they can maintain their body’s salt balance. All sharks’ gills absorb oxygen from the surrounding water. They also absorb or release salts and water. In fresh water, the cells of a bull shark’s gills can adjust the way they function. This prevents the loss of too much salt.

Meanwhile, the bull shark’s kidneys work harder. These organs filter waste from the animal’s blood. In a lake, excess water leaks in. The kidneys remove it, too. The result is urine that is excreted through the cloaca. That’s an opening on the shark’s underside. In fresh water, a bull shark pees out about 20 times as much fluid as it does in salt water! Its kidneys also adjust to help hold on to salts. If they didn’t, the salts would be lost.

Baby bull sharks use this ability to hang out in rivers. Most other shark species can’t survive there. That helps the pups avoid larger sharks that might try to eat them. In 2023, Gausmann studied the history of the sharks at Carbrook. He found that they’d lived in fresh water much longer than young bull sharks usually do. In fact, they appear to have set a world record. They survived at least 17 years in the lake!

During that time, golf course staff and players came to love the animals. The club even adopted a logo in the shape of a shark. The manager occasionally tossed meat into the lake to attract the sharks to the surface for interested visitors. But Gausmann estimates that the lake’s fish alone would have kept the six or so sharks well fed.

Today the Carbrook sharks’ fate remains unclear. There hasn’t been a shark sighting at the club since 2015. At least one of the sharks died from illegal fishing in the lake, the club’s staff recalls. The others may have been swept back out to the river and made their way to the sea during subsequent floods.

Even though the sharks are probably gone, it’s possible that future floods could bring new ones to the lake. Gausmann hopes that if sharks ever do return, they can be tagged and studied by scientists during their residence at the golf course. And he recommends keeping bull sharks’ amazing adaptability in mind when considering a swim in your favorite watering hole—particularly after extreme weather!

Golf course staff and players came to love the animals during that time. The club even adopted a logo in the shape of a shark. Sometimes, the manager tossed meat into the lake to attract the sharks. Then visitors could watch them come to the surface. But Gausmann estimates that the lake’s fish were enough to keep the six or so sharks well fed.

Today the Carbrook sharks’ fate is unclear. No one has seen a shark at the club since 2015. At least one shark died from illegal fishing in the lake, the club’s staff recalls. Later floods may have swept the others back out to the river. Then they may have traveled to the sea.

The sharks are probably gone. But future floods might bring new ones to the lake. If sharks ever return, Gausmann hopes that scientists can tag and study them during their time at the golf course. And he says to keep bull sharks’ amazing adaptability in mind. Remember it when you consider a swim in your favorite watering hole—especially after extreme weather!