How can bull sharks can survive in freshwater?

Answer 1

Salt water fish are taking in saltwater constantly, drinking the water and excreting salt. That is why most cannot survive in freshwater; they are still excreting the same amount of salt but not taking in the same amount. This causes their condition to steadily deteriorate, usually resulting in death.

Most Saltwater Fish cannot survive in freshwater, however, there are fish that can survive in fresh, brackish (an even mixture of salt and fresh water), and salt water. They biologically adjust to which ever water they happen to be in at the time. Some sharks like the great white, tiger and, yes, the bull shark, fall under the category of fish that can survive in all water conditions.

Answer 2

Bull sharks can live in both ocean waters and also in fresh water rivers, estuaries, and lakes for some time. They can do this because of their own special ability to adapt the process of 'osmoregulation'. Osmoregulation is the ability of an organism to maintain a constant concentration of water in its body even when its outside environment would normally cause it to loose or gain water. Freshwater and saltwater fish both osmoregulate. But bull sharks can adapt their osmoregulatory processes to survive in a broad range of water salinities, from the salt water of the ocean to the fresh water of a lake.