A collective noun is a word we use to refer to a collection of things, animals or people as a whole. For things, we could use terms like a collection or pile. For people, it could be a family, crew or team. A school of fish is a collective noun as well. But we want to know more specific expressions for different species of fish. To create phrases like:
How cool is it to snorkel at dust and observe a battery of hunting barracudas? Or enjoying a calm morning, watching a shiver of whitetip sharks hovering over a colony of colourful oysters. But I have no intention to ever see a bed of moray eels again. Ah, just discover the expressions yourself in this list:

Army of Herrings

Bale of Turtle (or a Dole or Nest)

Consortium of Octopuses or Crabs

Bask of Crocodiles

Shiver of Sharks

Battery of Barracudas -
Go get your shirt!

Bed of Eels or Oysters

Blessing of Narwhals
Yes, you're blessed if you ever get lucky enough to see a Narwhal, not to mention a whole group.

Colony of Corals, Seals or Penguins

Bind of Salmon
The following post awaits with more collective nouns!