What is the story of the golden carp in Bless Me Ultima?

Samuel takes Antonio to the river after school gets out before Summer break, and they go fishing. During their fishing, Samuel tells Antonio about a pagan god; whom which ruled over all carp in the river. This god was golden and no other carp could defeat it. All the carp were once human, but a different god turned them into carp as a punishment. The legend says it is bad luck to catch any of the carp, for they are people favored by a god.