Mermaids have been part of the folklore of many cultures in the world, but the truth is that there is no fiction behind them. Mermaids are indeed mutations of women that have the lower body of a fish. These creatures are exactly like regular women from the waist up, they served the purpose of distracting pirate ships while the law enforcement in the area caught up to the pirate ships.
There are several species of mermaids, but they have all been secluded to special areas in the depths of the ocean to avoid any interaction with humans. It would be impossible to keep them safe if humans learned about their existence and they would subject them to all kinds of terrible studies to find out how they came to exist. Human technology is incapable of understanding the genetic code mutations that we are able to manipulate.