Face Ripper

S5, E1: Angler Jeremy Wade investigates the story of a man who had his face ripped off in a remote Bolivian river, leading to a confrontation with one of his oldest adversaries. Jeremy's initial thought is that it was the work of piranhas, but they rarely attack unless threatened in some way. A chat with the surgeon who performed the post-mortem on the victim changes his mind. Then an interview with someone who witnessed the attack reveals it was a Red Bellied Piranha - but why the change in the creature's behaviour? Is there another predator in the river system threatening the piranhas?