What is the collective noun for a big group of krill?

There isn't a widely accepted, specific collective noun for a big group of krill.

However, since they are often found in massive swarms, you could use descriptive terms like:

* A swarm of krill

* A school of krill

* A cloud of krill

* A shoal of krill

* A mass of krill

Ultimately, the best term depends on the context and the visual you want to convey.