Some species of sharks also venture into fresh water.
And one of them attacked a 16-year-old girl on Saturday while she was swimming in a river in Western Australia, authorities said.
The girl was seriously injured by this shark of an unknown species in the Swan River, in Fremantle on the outskirts of Perth, according to a statement from the state government.
She was pulled from the water and pronounced dead at the scene after attempts to revive her failed, said Paul Robinson of Fremantle Police.
She would have liked to swim with dolphins
"It's still very early, what we know is that she was at the river with friends," he said at a press conference.
“They were on jet skis.
A pod of dolphins were seen nearby and the girl jumped into the water to swim with the dolphins,” he continued.
The girl's family, from Perth, are "totally devastated by the news" of the "very, very traumatic incident", he added.
According to experts, it is not common to find sharks in this part of the river, said Paul Robinson.
According to the perthnow website, WA Police received a call saying the girl had been mutilated about 2.5km from the mouth of the river.
Fisheries Minister Don Punch has expressed his condolences to the girl's family, Perthnow reports again.
"I am incredibly saddened by what happened and I cannot begin to imagine what the family and friends of the victim are going through," he said, showing all his "support".