When I was a little girl in Guyana, I remember seeing a school of them demolish a calf that was trying to cross the trench that I was fishing in. Someone picked me up and rushed me into the house so I wouldn't see the carnage. Another time, my sister fell off the side of a rowboat that was tied to the banks of a creek and by the time that my dad fished her out, you could have seen the bite marks on her big toe down to the bone. It's not worth your time eating the little ones because their so boney but some varieties like the Red Tail Piranhas do grow large enough to eat.
P.S. If you're rod fishing use a wire lead.