Shoaling and schooling
Fish get many benefits from shoaling. These include defence against predators: if fish swim in schools, it is less likely any one of them will be eaten. Also, it may help a fish find food, and a mate. The school may even swim faster than a lone fish. Fish generally prefer larger shoals, shoalmates of their own species, shoalmates similar in size and appearance to themselves, healthy fish, and kin (when recognized).