Complex sentence: The three most common kinds of fish in the Horr's Island diet, catfish, threadfin herring and pinfish, were caught while immature, and the time of year they were caught is indicated by the average size of a group of harvested fish.
Split into simpler sentences: The three most common kinds of fish in the Horr's Island diet, catfish, threadfin herring and pinfish, were caught while immature. The time of year when they were caught is indicated by the average size of a group of harvested fish.