“What’s the difference between a lawyer and a catfish?” runs a popular lawyer joke. “One is a scum-sucking bottom-dweller and the other is a fish.”
 
The joke was probably coined—or at least popularized—in the film The Doctor (1991), with a script by Robert Caswell. The joke was further popularized by an Associated Press story about lawyer jokes in 1992.
 
   
The Internet Movie Database
The Doctor (1991)
Director: Randa Haines
Writers: Ed Rosenbaum (book), Robert Caswell (screenplay)
Stars: William Hurt, Christine Lahti, Elizabeth Perkins
 
2 August 1991, Kansas City (MO) Star, “Prognosis is good for `The Doctor’: William Hurt is great as physician who gets a taste of real medicine” by Robert W. Butler, pg. H4:
(Q: “What’s the difference between a catfish and a lawyer? ” A: “One is a scum- sucking bottom-dweller and the other is a fish.”)
 
Google News Archive
23 August 1991, The Free Lance-Star (Fredericksburg, VA), “Litigation reform,” pg. 14, col. 1:
What’s the difference between a catfish and a lawyer, one surgeon asks another in the current film “The Doctor”? One’s a scum-sucking bottom-dweller; the other is a fish.
 
Google News Archive
22 May 1992, Kentucky New Era (Hopkinsville, KY), “Lawyers say the joke’s always on them, and they’re losing their sense of humor” by Larry McShane (Associated Press Writer) pg. 10A, col. 4:
Q: What’s the difference between a lawyer and a catfish?
A: One is a garbage-eating bottom-dweller; the other is a fish.
 
11 July 1993, Los Angeles (CA) Times, “Make Lawyer Jokes a Crime? Maybe Just Bad Ones” by Robin Abcarian, pg. E1:
What’s the difference between a catfish and a lawyer? One is a bottom-dwelling, garbage-eating scavenger, and the other is a fish.
 
Google Books
Lowering the Bar:
Lawyer Jokes and Legal Culture
By Marc Galanter
Madison, WI: The University o Wisconsin Press
2010
Pg. 203:
What is the difference between a catfish and a lawyer? One is a scum-sucking, bottom-feeding scavenger. The other is a fish!
 
Google Books
The People V. Bush:
One Lawyer’s Campaign to Bring the President to Justice and the National Grassroots Movement She Encounters Along the Way
By Charlotte Dennett
White River Junction, VT: Chelsea Green Publishing Company
2010
Pg. 127:
“What’s the difference between a lawyer and a catfish?” goes one typical lawyer joke. ‘Answer: One is a slimy, bottom-dwelling scum sucker. The other is a fish.”