Police have said the 18-year-old was drunk when he jumped out of a canoe in Rosario del Yata, 400 miles north of the capital of La Paz.
A spokesman said the man bled to death after the attack.
He was killed last Thursday but news of the incident has only just emerged.
The victim is believed to have been a fisherman who knew the Yata river well.
Despite the reputation piranhas have received from Hollywood movies, fatal attacks on humans are rare
Dec 7th 2011