Why Are Fish Aggressive? Why do some fish show more aggression than others? Well, it's all about survival in the deep blue. These fish are ambush predators, their aggression stemming from the primal need to feed and defend their territory. The most aggressive ones, often with the most and biggest teeth, are usually the ones that eat other fish. Once they've claimed a territory or a nest, they fiercely defend it, protecting their offspring and their territory from potential intruders. So, fish ar