Werner Rebensteiner and Manfred Biss regularly fish on the March river. “A piranha is not something you’d normally find in this river, you’d expect them in Africa or somewhere like that,” said Rebensteiner.
The pair took a photo of the fish but although Daniel Abed-Navandi, curator of Vienna aquarium, initially thought that it was a red-bellied piranha he said that on closer inspection it appeared to be a pacu – which comes from the same family but is vegetarian and has different teeth.
It’s not clear how and when the pacu came to be in the March river. "Probably someone bred it in a home aquarium, and at some point it grew too big. At this point its owner must have put it in the river – probably a few months ago,” Abed-Navandi said.
Pet stores sell pacu as small as five to eight cm long but often neglect to warn customers that fish growth is not inhibited by tank size. Pacus are tropical fish and will die in cold weather.
While pacus are not aggressive carnivores like the piranha, their strong jaws, used primarily for eating seeds and nuts, can still be hazardous. Abed-Navandi said that pacus can mimic the colouring of red-bellied piranhas in order to protect themselves from predators.