We have a number of catfish songs in the Hook and Line thread. I suppose Robin's song is derived from this one:

Catfish swimming down the river,
Nigger threw out his line.
Catfish said to the nigger,
"Aha, you didn't ketch me that time"

I gather from the thread that the song comes from the 1850s, when standards for polite speech were different.

-Joe-