Question

What are the benefits of shoaling and schooling in a marine environment?

Answer 1

Schooling and shoaling is a kind of collective animal behaviour by fish.
Any group of fish that stay together for social reasons is said to be shoaling, and if the shoal is swimming in the same direction together, it is schooling.[1]p365 About one quarter of fish shoal all their lives, and about one half of fish shoal for part of their lives.[2]
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).
Any shoal member which stands out in appearance may be targeted by predators. This may explain why fish prefer to shoal with individuals that resemble themselves. This is called the oddity effect.
http://simple.wikipedia.org/wiki/Shoaling_and_schooling