Complex sentence: The oyster industry in San Francisco Bay was at its height around the turn of the 20th century, it reached a secondary peak by 1911 and then faded away because of polluted conditions of the bay.
Split into simpler sentences: The oyster industry in San Francisco Bay was at its height around the turn of the 20th century. It reached a secondary peak by 1911 and then faded away because of polluted conditions of the bay.