Did you ever wonder if a "shoal" of fish is just a cooler expression for "school"? Many divers, myself included, use the words as synonyms. But are they?
No, there is a distinction, and it lies in the organisation of the group. You can differentiate three types: 
When fish are hanging around in one location, it's called aggregation. There can be one or more species present, and they come in different sizes, shapes and ages. There's no higher purpose for the gathering.
When the fish are more organised and stay together for social reasons, we speak of a shoal of fish. There can still be different species, but each adjusts to others' reactions by staying close to each other or moving along with the group.
A school of fish contains only one species in the same size, shape and age. They swim close together and move as a whole, synchronising their motions. Now you probably imagine a swirling tornado of sardines. Still, it can also be a group of trevallies swimming at the same speed in the same direction.
So these are general expressions for a group of fish. Stay tuned for our next blog about collective nouns!
