Bull Sharks: The Shark That Swims Up Rivers

Diving with bull sharks

By William Drumm, underwater photographer and ocean ambassador

There is a shark that swims up the Mississippi River. Not metaphorically. In 1937 two fishermen at Alton, Illinois, roughly 1,700 river miles from the sea, hauled a five foot bull shark out of a trap baited with chicken guts and put it on display in the local fish market.

That is the thing to understand about bull sharks. They are not built like the other big sharks and they do not live like them. They are stocky, blunt, adaptable and almost completely indifferent to the boundary between the ocean and the land, which is the single most useful trait a large predator could possibly have.

What Is a Bull Shark?

Carcharhinus leucas is a mid sized requiem shark of warm coastal water worldwide. What sets it apart is not size or speed but tolerance. Bull sharks live in the shallow, murky, brackish, unglamorous water that most large sharks avoid, and they are the only big shark that regularly pushes deep into fresh water.

They are named for the build. A bull shark is short, thick, deep bodied and blunt headed, and it moves with a slow, heavy, deliberate quality. Next to a reef shark it looks like a different class of animal.

How Big Do Bull Sharks Get?

Females are the larger sex, running roughly 7 to 11 feet, and a very large one will weigh around 500 pounds. Males are noticeably smaller.

The numbers undersell them. Bull sharks are built like a fist. A ten foot bull shark has far more mass than a ten foot shark of most other species, and in the water the impression is of density rather than length.

The Freshwater Trick

This is the thing bull sharks are famous for, and it is genuinely strange.

Sharks are, as a rule, locked into salt water by their own chemistry. They hold a high concentration of salts and urea in their blood, close to that of seawater, which keeps them in balance with the ocean around them. Move that animal into fresh water and osmosis starts pushing water into it. It swells. It dies.

Bull sharks get around this by actively managing it. They dump excess salt through the kidneys and through a specialised organ called the rectal gland, they lower the urea in their blood, and they urinate enormously more in fresh water than in salt. It is metabolically expensive and they do it anyway, because the reward is a river system with no competition in it.

Where they have actually turned up

• The Mississippi. The 1937 Alton shark is the famous one, confirmed by biologists from photographs. More recent work has documented bull sharks reaching the St. Louis area. They are visitors, not residents.
• Lake Nicaragua. The best story. These sharks were once thought to be a separate landlocked freshwater species. They are not. They swim about 120 miles up the San Juan River from the Caribbean, through rapids, and back out. Tagging in the 1970s settled it.
• The Amazon. Recorded well over 2,000 miles upriver, in Peru.
• The Chesapeake, the Potomac, Florida rivers, Australian creeks. Routine. Bull sharks use estuaries and river mouths as nurseries, so juveniles in brackish water are entirely normal.
• Australian golf courses. Yes, really. Bull sharks were stranded in a flooded course lake at Carbrook, near Brisbane, and lived there for years.

Are there bull sharks in the Great Lakes?

No. This is the most persistent bull shark myth on the internet and there is nothing behind it. No verified capture, no established population, and every viral photograph has been traced back to somewhere else.

The physical barriers settle it. Bull sharks do get a long way up the Mississippi, but between the Gulf and Lake Michigan sit a chain of locks, dams and an electric barrier on the Illinois River built specifically to stop fish moving through. A bull shark is a remarkable animal. It is not remarkable enough to lock through a dam system.

Are Bull Sharks Dangerous?

Bull sharks are one of three species, with great whites and tiger sharks, most often involved in bites on people. That is a real fact and it deserves a real explanation, because the explanation is not that they are vicious.

It is overlap. Bull sharks live in shallow, murky, warm, coastal and river water. That is also, precisely, the water human beings swim in. Poor visibility, a lot of people, a large opportunistic predator, and the arithmetic does the rest. Most of the encounters are cases of mistaken identity in water where nothing can see anything.

On a dive, in clear water, they behave nothing like the reputation. They come in slow and wide, they hold their distance, they look at you, and they leave. They are cautious animals. Every bull shark on this page was photographed off Playa del Carmen, on Mexico’s Caribbean coast, where I have spent a great deal of time on the sand with big pregnant females and never once felt hunted.

Bull Shark vs Great White vs Tiger Shark

Bull shark vs great white. Great whites are far bigger, up to about 20 feet, with a pointed conical snout and a black eye, and they live in cool and temperate seas. Bull sharks top out near 11 feet, are stocky and blunt with small eyes, live in warm water, and go into rivers, which no great white does. If you are looking at a shark in a river, it is not a great white.

Bull shark vs tiger shark. Tigers are longer, up to 14 feet or more, with a broad square head and dark vertical bars on the flank. Bulls have no markings. Tigers eat literally anything. Bulls are more conventionally carnivorous.

Bull shark vs lemon shark. Lemons are yellow brown, with two dorsal fins of almost equal size, which is the quickest tell in the water.

Bite Force, Teeth and Diet

Pound for pound, bull sharks have one of the strongest bites ever measured in a shark, well over a thousand pounds of force. The teeth are broad, triangular and serrated in the upper jaw for cutting, narrower and more pointed in the lower for holding.

The diet is mostly bony fish and other sharks, including, cheerfully, smaller bull sharks. They also take rays, turtles and crustaceans. They are generalists, which is exactly what you would expect of an animal whose whole strategy is to live where nothing else can.

Where Bull Sharks Live

Warm coastal water worldwide, roughly between 40 degrees north and 40 degrees south. The Gulf of Mexico and the Caribbean, both coasts of the Americas, west Africa, the Indian Ocean, Southeast Asia, northern Australia, and up every warm river mouth in between.

They are a shallow water animal. Most bull sharks live in under 100 feet of water, which again puts them exactly where people are.

Diving With Bull Sharks

The most reliable bull shark diving in the world is off Playa del Carmen, on Mexico’s Caribbean coast, and it is where I shoot them. From roughly November through March, large pregnant females move in and gather on the sand in about 25 metres of water, and the dives are run as a proper, briefed, managed encounter. It is the closest thing there is to a guarantee with a large shark.

Beqa Lagoon in Fiji is the other famous one, run as a baited dive with a large number of bulls and a very experienced team. Bull sharks also turn up off Florida, in the Bahamas, and along parts of the South African coast.

You want Advanced Open Water for the depth, a briefing you take seriously, and no gloves that look like fish. And you want to be still. Bull sharks read movement, and a diver who stays low and calm on the sand will get passes that a diver who fidgets will not. On my best day in Playa I had a female come through so close and so often that I ran out of frames before she ran out of interest.

The Shark We Get Wrong

Bull sharks are listed as Vulnerable. They are fished hard, taken for their fins, and killed as bycatch, and they have a particular problem the open ocean sharks do not: they raise their young in estuaries and river mouths. Nurseries are where we build ports, cities, marinas and runoff.

So the animal that adapted brilliantly to living alongside us is the one most exposed by it. And because of the reputation, it is the shark people are least inclined to care about losing.

That is the part I would like to change, and it is why I keep going back down there with a camera. A bull shark on the sand is not a monster. It is a big, careful, slow moving animal doing exactly what it has done for millions of years, and it is far more interesting than the story we have told about it.

Photographing Bull Sharks

Stay on the sand and stay low. A bull shark will not approach a diver who is up in the water column waving a camera, but it will come remarkably close to one who is settled on the bottom and doing nothing.

Shoot wide and shoot close. The temptation with a big shark is to reach for reach, and it never works, because water kills contrast faster than you think. Get within a couple of metres, get the eye, and let the body fill the frame.

And wait. The first pass is never the one. Bull sharks circle, assess, and come in tighter each time, and the frame you want is on the fourth or fifth pass when the animal has decided you are furniture.

More on the craft in how to become an underwater photographer, and if you want the other end of the shark spectrum, there is the whale shark guide. Prints are in the shop, and a portion of every sale supports marine conservation.

Related Images

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Frequently Asked Questions About Bull Sharks

Can bull sharks live in fresh water?

Yes, and they are close to unique in that. Bull sharks can move between salt and fresh water and stay in rivers and lakes for long periods. They do it by adjusting how much salt they retain, dumping the excess through the kidneys and a specialised organ called the rectal gland, and by changing how much urine they produce. No other large shark manages this to anything like the same degree.

Are there bull sharks in the Great Lakes?

No. This is the internet’s favourite bull shark story and there is no documented case. Every viral photograph has been debunked. Bull sharks do get a long way up the Mississippi, but the locks, the dams and the electric barrier on the Illinois River make the journey into the Great Lakes effectively impossible. There is no established population and no verified capture.

Have bull sharks been found in the Mississippi River?

Yes, and much further up than people expect. In 1937, two commercial fishermen at Alton, Illinois, roughly 1,700 river miles from the sea, trapped a five foot, 84 pound bull shark. Biologists later confirmed the identification from photographs. More recent work has documented bull sharks reaching the St. Louis area. They are rare visitors, not residents.

Are there sharks in Lake Nicaragua?

Yes, and they are bull sharks. For a long time they were thought to be a landlocked freshwater species of their own. They are not. They swim roughly 120 miles up the San Juan River from the Caribbean, leaping the rapids on the way, and back out again. Tagging in the 1970s confirmed it.

How big do bull sharks get?

Females are the bigger sex and typically run 7 to 11 feet, with a very large one around 500 pounds. Males are smaller. They are stocky and heavily built rather than long, which is where the name comes from, and a big female bull shark in the water looks considerably more solid than her length suggests.

Are bull sharks dangerous?

Bull sharks are, with great whites and tiger sharks, one of the three species most often involved in bites on humans. That is largely about overlap rather than temperament: bull sharks use the shallow, murky, coastal and river water that people swim in, so the encounters happen. On a dive, in clear water, they are cautious and deliberate animals that keep their distance. Treat them with respect, not fear.

Why are bull sharks so aggressive?

The reputation comes from where they live rather than from a special hostility. Murky, shallow water means poor visibility and mistaken identity, and bull sharks hunt in exactly the places people wade and swim. They are also bold, robust and highly territorial in a way that reef sharks are not. But the idea that they are wired to attack is not supported by how they behave around divers.

What is the difference between a bull shark and a great white?

Size and shape, mostly. Great whites are much larger, up to 20 feet, with a conical snout and a black eye. Bull sharks top out around 11 feet, are stocky and blunt nosed, and have small eyes. Great whites are cold and temperate water animals. Bull sharks are tropical and subtropical, and they go into fresh water, which great whites do not.

Where can you dive with bull sharks?

The most reliable place in the world is Playa del Carmen, on Mexico’s Caribbean coast, where large pregnant females gather on the sand from roughly November through March. It is where I photograph them. Fiji’s Beqa Lagoon is the other well known site, run as a baited dive. Bull sharks are also seen regularly off Florida, in the Bahamas and along parts of the South African coast.

What is a bull shark’s bite force?

Bull sharks have one of the strongest bites recorded in any shark relative to their size, measured at well over a thousand pounds of force. In practical terms it means very little to a diver. It matters to a sea turtle.

What do bull sharks eat?

Mostly bony fish and other sharks, including smaller bull sharks. They also take rays, sea turtles, crustaceans and, in river systems, whatever is available. They are generalists, which is a large part of why they thrive in water no other big shark will tolerate.

Are bull sharks endangered?

They are listed as Vulnerable. They are heavily fished, both deliberately and as bycatch, they are taken for fins, and because they use estuaries and river mouths as nurseries they are unusually exposed to coastal development and pollution. A shark that raises its young where we build our cities is in a difficult position.