Hi changeling,
As you know, the passive does not state who did something and it can lead to some awkward sentences.
"It was learned how" is one of those awkward sentences. "A long time ago, people learned how to farm this high-quality fish, and now artificial breeding is wide-spread in many countries."
I don't think you want to say "rearing" conditions and "a certain season" is quite vague. Can you try those two parts again?