In Norway there was a time when, though live sardines were worth much more than dead ones, only one ship was able to bring live sardines home. The rest brought dead sardines, or sardines mushy from lack of physical activity. When the captain of the ship bringing in live fish died, it was discovered that he kept a single catfish in the tank with the sardines. The catfish kept the sardines on their toes, and though it ate a few, the rest arrived in port strong and fresh.
This is the story from where we derive the term Catfishing.
Catfish is a 2010 documentary about one man's experience getting catfished that first introduced the term to the public eye.