Behavior of Rio Sao Francisco Piranhas in Response to Changes in Space and Density

Previous observations of Rio Sao Francisco piranhas (Pygocentrus piraya) at Cleveland Metroparks Zoo revealed frequent aggressive interactions, which were associated with the injury and death of some individuals. Fishes are known to increase aggressive behavior when available space is limited. We compared the behavior of the piranhas longitudinally across three different space/density conditions: (1) in a group held in a small aquarium before the death of one individual, (2) in the same group after the death of one individual, (3) and in one individual of the group after it was later moved into a larger, multispecies aquarium. In the smaller aquarium, individuals maintained stable positions in the tank and remained inactive except to behave aggressively toward each other, retreat, or feed when food was offered. After the death of one individual, aggression decreased significantly but so did overall activity, including swimming behavior. After one individual was moved to a larger aquarium, it exhibited a striking increase in the amount of time spent swimming. Our data suggest that the well-being of P. piraya, and possibly other species of piranhas, might be better in large aquariums than in the small single-species exhibits often used in zoos and public aquariums. This study takes an important step in addressing the psychological and social well-being of bony fishes, an underrepresented area of research in zoos and aquariums.