Complex sentence: Rotten shark is chosen instead of fresh shark meat because the fresh meat of the Greenland shark itself is poisonous when fresh due to a high content of urea and trimethylamine oxide, but may be consumed after being processed (see below).
Split into simpler sentences: Rotten shark is chosen instead of fresh shark meat because the fresh meat of the Greenland shark is poisonous. The Greenland shark itself is poisonous when fresh due to a high content of urea and trimethylamine oxide, but may be consumed after being processed (see below).