Scubaism

“Previously it was thought piranhas shoaled as it enabled them to form a cooperative hunting group,” said Professor Anne Magurran at the University of St Andrews. “However, we have found that it is primarily a defensive behaviour.” Piranhas could be attacked by dolphins, caimans and large fish, and forming a shoal is their way of avoiding being eaten, she said.

The researchers found that in shallows, the fish formed large shoals, gaining safety in numbers. “Although larger shoals are easier to spot, predators find it harder to pick off a single fish because of the ‘confusion effect’, caused by the erratic movements of the shoal.”

Using giant water tanks and models of birds of prey, the team simulated airborne attacks on different shoal sizes. They found piranhas in smaller shoals were more distressed and took longer to calm down. The nervousness was monitored by observing a structure near the gills which quivers rapidly with stress.

“Our study has given us a better understanding of the function of shoaling. Previously, it was thought piranhas shoaled as it enabled them to form a cooperative hunting group. However, we have found it is primarily a defensive behaviour,” said Professor Magurran.

Fish of reproductive age stay in the centre of the shoal for protection and the size of the shoal depends on the level of risk.

“At high water the fish swim in small shoals, as there is more space to avoid predators, and the threat is low,” Professor Magurran said. “However, when the water level drops, isolating the flood water into smaller lakes and channels, shoals can grow to over 50 fish due to the close proximity of predators.”