The Ramban (Bereshit 1:29) says the reason Noach was allowed to eat animals is because he maintained them in the Ark. If he would not have brought them into the Ark they would all have perished, so they owed their life to him. If so, why is he allowed to eat fish and not be like Adam who the Gemara says was not allowed to eat it? They were not in the Ark!
I forgot which mefaresh I saw this in, but I think it was in the Mikraos Gedolos.
He was allowed to eat fish, as well as the animals which spontaneously generate, so as not to cast jealousy among the creations.