The bull shark

You think of a bull as a large, stocky, aggressive animal – the bull shark (Carcharhinus leucas) is no different. It’s as wide as an oil drum, unpredictable and highly aggressive. It has a broad, flat snout that tends to head-butt prey before attacking, much like a bull charging at the red muleta of a matador (bullfighter). Females can grow to 2.4 meters and typically weigh 130 kg, males are slightly smaller making them a particularly small species. They are from the same family as the oceanic whitetip shark (Carcharhinus longimanus) and tiger shark (Galeocerdo cuvier) – Carcharhinidae. This family contains migratory, live-bearing sharks that live in warm seas (or fresh water in this case).

Carcharhinus leucasNational Geographic

They are an exceptional species of shark due to their ability to live in saltwater and freshwater. They are so common in freshwater that they have inherited a nickname, the Zambezi shark after the Zambezi river in Africa. In order for sharks to survive in the sea they have an adaptation that allows them to store metabolic wastes such as urea and trimethylamine oxide (TMAO). Generally saltwater is more salty than the water within fish, this results in fish continuously loosing freshwater through osmosis. Because of the shark’s adaptation of storing urea and TMAO, the water within sharks is slightly more salty than seawater they live in, any excess salt is removed from the bloodstream through urination. Therefore freshwater is not continuously lost in sharks like it is in fish.

But how do bull sharks live in freshwater?

Bull sharks are able to adapt to different salinities by changing their process of osmoregulation. Their kidneys are capable of gradually adjusting to suit the salinity of water they are in. As the bull shark enters freshwater, their kidneys stop removing as much salt and instead remove more urea from the bloodstream via urination. This is basically reversing the normal marine shark of method of osmoregulation and allows bull sharks to live in freshwater, which makes them a pretty extreme marine (or not so marine) animal!

This video shows bull sharks living in a lake at a golf course:

Bull shark attacks

Due to the bull shark nature of living in shallow coastal waters and freshwater this puts them in the same place that human carry out aqua activities like fishing, swimming, diving, etc and thus contact between humans and bull sharks is likely. They are considered one of the most dangerous sharks because of this and their extreme aggression. According to the International Shark Attack File (ISAF) bull sharks are ranked third with 100 total attacks, 73 of these been non-fatal and 27 fatal. Tiger sharks are second with 111 total attacks and great white sharks are first with 314 total recorded attacks.

This video shows an attack on a spear fisherman by a bull shark, it is clear how aggressive bull sharks are:

The problem bull sharks face

Bull sharks are currently listed as near threatened on the IUNC’s Red List meaning they’re not particularly endangered. However, an ever growing problem that all sharks face is shark finning. Shark finning is the process of removing the shark’s fins which are used in eastern countries such as china for a delicacy called shark fin soup. After the removal of the sharks fins, they are usually tossed back into the sea, and because of their lack of fins they are unable to swim or control buoyancy, meaning they drown. Shark finning is a particularly lucrative business with a single whale shark pectoral fin selling for up to $20,000. On average 73 million sharks are killed by humans each year, including the bull shark species versus the 6 humans that are killed by sharks per year These numbers are extremely high and efforts need to be taken to reduce the number of sharks killed.

A mass of sharks that have been finned and left to rot – Wikimedia

Although bull sharks are large, aggressive creatures that have attacked humans up to 100 times, we need to remember that when we take part in activities in water that we are in their natural habitat. Bull sharks (and other species) only attack humans because of their natural instincts, they don’t deserve to be killed for soup nor do they need to be seen as blood thirsty killers. Sharks are misunderstood predators that need humans respect and views to change.


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