The Bull shark likes to live in shallow coastal waters less than 100 feet deep (30 metres), but ranges from 3–450 feet deep (1–135 metres). It commonly enters estuaries, bays, harbours, lagoons, and river mouths. It is the only known shark species which is found in freshwater, and can spend long periods of time in such environments. It is not likely that the Bull shark's entire life cycle is within a freshwater system, however. There is evidence that they can breed in freshwater, but not as regularly as they do in estuarine and marine habitats. Young Bull sharks enter estuaries with low salt contents and lagoons as readily as adults do, and use these shallow areas as nursery grounds. They can also live in waters with high salt contents; as high as 53 parts per thousand.