The title refers to the sardine-size teleost fish found only off the coast of California. At very high tides, the females come up on sandy beaches and dig their tails into the sand to lay their eggs and seem to invade the land (unlike the foreign invaders that the Clampetts think they are).
Jerry Brutsche, who plays the surfer captured by Elly May, is the stunt man who always doubled for Irene Ryan (Granny) whenever the action got extremely physical.