“law enforcement officials confronted fisherman John Yates, saying he had caught several red groupers that were too small”...The fact that catching some fish that are too small is a crime is stupid to begin with.
Groupers change sex as they mature. They start out female; they become male much later, if the existing male in a population dies... in which case the largest of the females in the group becomes male. Thus, to protect the fishery and produce as many grouper as possible, you protect the smallest of them, because those are females who will produce lots of young.