While trying to understand Piranha, a question that comes up is that of whether they are nomadic in nature or they are concerned about their space.
So, are Piranhas territorial? In short, yes, Piranha are territorial. They travel in schools as a defense mechanism, but overcrowding, especially in an aquarium, will transfer to territorial fights. Piranha bark at other fish as a deterrent, and eventually gnash their teeth as a final warning to defend their territory. They can also swim around in a circular patrol in order to defend their space. Piranha during breeding will also pair up and defend the nest from aggressors.
When using the term Piranha, one has to be relatively careful. The Serrasalmus and Pygocentrus genus of the Piranha tribe, which contain the Red-Bellied Piranha, the black Piranha and the Sao Francisco Piranha are generally considered to be the more aggressive of the fish.
Along with aggression comes the ability to defend, as well as attack.
Thus, the Red-Bellied Piranha, which is the most aggressive and common of the Piranha is the most likely to defend its territory.
As such, the Red-Bellied Piranha will defend its area from those who would transgress their space. Like humans, the Piranhas become increasingly stressed with overcrowding.
Territorial Piranhas and Nesting Behavior
If you take the Piranha’s reputation, what comes as a surprise is that the male plays a significant role in the rearing of juvenile Piranha.
Prior to fertilization, a male and female Piranha will pair up and find a spot near foliage in order to nest. The female Piranha then lays up to 5000 eggs. The male Piranha, after fertilization, does not disappear. He becomes very territorial and will aggressively defend the nest.
The male and female Piranha will watch over the eggs until they hatch, chasing away invaders and threats. The eggs hatch in around 3 days.
Defending Their Space
Another circumstance when Piranha can get territorial is when there is a lack of space. Whilst this might be common in an aquarium, it can also happen in nature during the low season, when the dry season is lowering water levels. It’s less common in the rivers, but lakes and wetlands, where Piranha do reside can get significantly lower in volume.
Whilst Piranhas bark to show their displeasure, a series of noises are emitted using their swim bladder contracting or gnashing of their teeth. This teeth grinding sound is a final warning other fish, including other Piranha to stay away.
Normally Piranha can be quite timid and shy, and often immobile when they don’t feel the need to defend their territory. When they feel the need to defend their space, they will swim around in circles as if patrolling. They are keeping a ‘watch out’ for an area they consider theirs. It is a sign of displeasure.
Nevertheless, we can observe a great deal of Piranha behavior from aquariums and anecdotal evidence. We can see that there are several incidences when Piranha seem to display territorial behavior, namely when their space is invaded and during breeding.
In an aquarium, this means that you have to provide a large enough tank for Piranha to roam and avoid displays of dominance. Provide cover for the Piranha to retreat to. Overcrowded tanks, will provide the breeding ground for territorial fights.
You also need to provide a gravel bottom and foliage for them to nest in.