What was even greater that their use for human food fish are a vital source of what?

Please provide me with the context or the sentence you are referring to. I need more information to understand what "their" refers to and what "they" are used for.

For example, are you talking about:

* A specific type of fish?

* A particular resource?

* Something else entirely?

Once I have more context, I can help you understand what is "even greater" than their use for human food.