when they’re about to eat you.
“The clip features three sounds. The first is a “bark” produced in what the researchers called a “frontal display”, meaning where two fish swam quickly towards each other and stayed still, aggressively intimidating and staring at each other. The second is a “drum beat” produced by the largest fish in the group when circling the shoal, mostly when there was competition for food. The third “croak” was generally associated with a piranha chasing and biting another fish.”
via The Cabinet of Freshwater Curiosities, click the pic for more on the Red-Bellied Piranha.