Pacu is a freshwater fish from South America found in most rivers and streams in the Amazon andOrinoco, but it has also been reported as far away as Papua New Guinea, where it was artificially introduced using the local fishing industry, and even in the Øresund between Denmark and Sweden, and even more surprisingly in the Seine in Paris (source).
The pacu is affiliated to the carnivorous piranha, both sharing the same Serrasalminae subfamily, even though they have different eating habits. The piranha is a carnivorous species while the pacu is omnivorous with a vegetarian tendency. The difference can be seen in their teeth. The piranha has sharp, pointed teeth, while the pacu has squarer, straighter, and strangely resemble those of humans.
Pacus use their teeth mainly to crush nuts and fruits, but they sometimes eat other fish and invertebrates. They usually eat nuts and floating fruits that fall from trees in the Amazon, and on several occasions it has been reported that they have attacked the testicles of swimmers mistaking them for nuts. This earned them the nickname "testicle eaters" after they castrated some local fishermen in Papua New Guinea. So when the fish were spotted in waters in Denmark and later in the states of Washington, New Jersey and Illinois last year, mild panic ensued.
While they are not aggressive carnivores like piranhas, their jaw strength can be dangerous. One child had to be operated on after a pacu bit his finger at the Edinburgh Butterfly and Insect World in Scotland. In fact, pacu can eat anything. It became a carnivore because of the lack of food in the Amazon rivers.
It is for example legal to own a pacu in an aquarium in the United States, it is also easy to breed them. The problem is that people don't realize that pacu can grow up to 1 meter 20. Much too big for their aquarium, people release them into lakes and rivers near their homes. Hence their presence in strange places for their species like in Denmark.