The use of the comma in this sentence is to set off an introductory clause, phrase, or word.
There is no list of items in this sentence. There are also no coordinating adjectives.
Furthermore, the comma does not separate two main clauses. We can tell this because "Although farmers have a number of ways to raise young fish" cannot function on its own as a sentence.
Finally, "Although farmers have a number of ways to raise young fish" cannot be removed without loss of meaning. It's important for the reader to know that there are a number of ways to raise young fish. That means the final option (a nonrestrictive modifier) is not correct.
Therefore, the best answer is the comma is used to set off an introductory clause, phrase, or word. The word "Although" is another clue that this is the right answer.
